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Oil-well Casing and Tubing Troubles'

H. G. TEXTER*ABSTRACT

The aim of thi s paper i s to present, in one treat-

ise, all possible data known to the author concern-

ing the t y p e s of failure s and trou bles that bese t~ -

casing and tubing u s e d in completing oil wells.

Th er e will not be much origina l information.

Littl e spa ce will be taken up with the desig n or

select ion of p r o p e r siz es, grades, or weights ex-

cept insofar a s these factors may contribute to faii-

ures. 0 t h e r literature (see references) has ade-

quatelycovered th is phase of the general ~ ub j ec t . ' ,~

Casing and tu b i n g are designed to withstand

three principal mechanical force s tending to disrupt

or destroy tubular sections:

1 . Tension, from longitudinal loading.

2. Collapse, from unbalanced external pressure.

3. Bursting, from unbalanced internal pressure.

Somewhat le ss important mechanical f a c t o r s or

forces are:

4 . Last engaged thread failures, from sllock or

vibration fatigue.

5 . Leakage, through the threaded connectio ns.

6. Crushing, by sl ip s and tongs.

7. Wear, from drill pipe, wire l i ne s , t ub ing, or

rods.

8. Erosion, from high-velocity fluids.

9. Buckling, a, from m.echanical forces; b, from

internal pressure.

10. Torsion. .11. Miscellaneous troubles c a u s e d by: a , field

welding of casing; b, shot perforating casing;

c , handling.

Last , but not least, there i s the chemical or elec-

tro-chemical factor of:

12. Corros ion.

Cataloging and discussion of the various troubles

resul ting from the foregoing f a c t o r s are the sole

purposes of this paper. Collection or the informa-

tion i s the result of SO yea rs of oil-field expe rien ce

on the part of the author.

Because th e types of troubles are som ehh at the

same for both casing and tubing, the two products

will be considered concurrently. ?'he order of con-

sideration is not nec essa rily .the order of the im-

portance or the preva lenc e of the trouble.

1A. TENSION-C ASlNG

It n~iglitbe well to consider , f i r s t , the various

fact ors which make up tension loading in a st ring

ofXcasing,viz.: '

a. T h e w e i g h t o f t h e c a s c n g c t s e l f , hanging onto a

coupling (the top one for example). From thismust be subtracted the buoyancy effect of the

liquid in which the string i s suspended. Calcu-

latio ns on tension loads ordinarily neglec t buoy-

ancy and assume that the string i s hanging in

air, thus g i v i n g an additional, uncalculated

safety factor.

* Spang-Chalfant D~v rsr on of The Na t~ on al Supply Company,

Tuls a, Okla.

*p re se nt ed at the sprlng meetlng of the Southwestern Dlstrrct.

Drvrsron of Productron, New Orleans, March 1955.

'~ ef e re nc e s re at the end of the paper.

b. S ho ck l o a d s w h i l e r un n in g c a s i n g . S on~e t in~es

there is. an .unexpected, sudd en s lipp ing through

a tight spot which can build up, rllonlentarily, an

unwittingly high tension load.

c. Sometimes, in runncng c as in g, ct may be ' ' l oggy"

beca use of a tight spo t, a s in a sloughing forma-

tion or where lilter cake i s being sc ra pe d ofT. 'lo

free such a string, it may be w o r k e d up and

down and the load for the upstrokes can Le very

high beca use of h al l friction.

d. During cementing of c asi ng stri ng s, they are

very often r e c i p r o c a t e d v e r t i c u l l y . This i s tosc ra pe off mud cake and, of grea t importance, to

insure good bond between pipe and the cement

and the hall of the hole. As in c , the tension

involved in the upstrokes can be very high.

e. Te n rpe ra tu re c h an g e s after the string i s cement-

ed. Circ ula tin g cold mud might reduce the tem-

perature of the casing: so would flowing a ga s

well with bottonl-hole choke s. Co oling would

increase the tension load at the rate of approxi-

mately 207 psi per deg F'. drop in ternperat~re.~

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8 H . G . TEXTER

Fig. 1-Original Conception of a Tension Fai lur e

Ordinarily, however, there i s a temperature in -

c r e a s e which would lower the tension load. In

fact, the bottom sectio-n of the string may even

be placed in compression and tend to buckle.

f. High f luid pressure inside a string of cemented

casi ng will ,expand and shorten it and thus ap-

preciably increase the tension load.4

Design Factors in Tension

Bec aus e many of the foregoing var iables ar e un-- -known or incalculable, casing strings are designed

with a fairly high s a f e t y factor (more properly

known a s "design factor").* Th e one most univer-

sally used is 1.8, based on the minimum strength

figure of the top joints and d i s r e g a r d i n g t h e b u o y a nt

e l e c t of th e m u d .

Several large companies d r o p this figure to 1.6,

a s a fixed policy; and many long str ings have been

run successfully with a factor a s low as 1.5. One

company is going to 1.4 in certain areas.5 Smaller

* Th ts 1s repre sented as the ra tlo betw een the mnurnum break-

lng strength of the ca sin g, a s obtained from the data book s or

spe clfic atlo ns, and the ca lcul ated de ad-welght load of the

strlng. A de s ~ gn act or of 2.0, therefore, lnd ~c ate s hat the

load on the top length IS one half of the presumed strength of

the materlal.

companies, not being able to afford even one failure,

usually use a design factor of a t lea st 1.8 and even

2.0

Mechanism of Failure

Tension failure in API threaded casing usually6 b

occurs a s a jump-out." It invo lves the necking

down, or compressing, of the male (pipe) end, allow-

ing it to jump out of the female (coupling) end. All

of t his o ccurs very suddenly , but not quite instan -

taneously in the true se nse of the word.

The older theory was that all the mating threads

tended to slide away from each o t h e r , all at the

same time, because of the radial con~ponentof the

longitudinal tension force, until the male threads

were sufficiently reduced in s iz e to sl ip pas t the

tops of the female threads. T his is illustrated in

exaggerated form in Fig. 1.

Ther e i s a fal lacy in this theory of uniform slip -

page. If correct, it would mean t hat casi ng under

tension s hould begin to show leakage a s the load-

ing increased because of the increasing separation

between the mating surface s. Thi s, however, i s far

from true. Comprehen'sive tension-internal-pressure

experiments have proved very conclusively that, up

r ORIGINAL LINE OF THREAD CRESTS

THREAD CRESTS AFTER START OF NECKING DOWN

Fig . 2-Modern Conception of Tension Fa ilure by "Unzippering"

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OILWELL CASING AN D TUBING TROU BLES 9

t o t h e y i e l d s t r e n g t h o f t h e m e t a l u n d er t h e l a s t e n-

g a ge d - t h r e a d (w hi ch i s v er y c l o s e t o t h e f ai lu r e

po in t ) , t h e r e i s no more t endenc y fo r a c as i ng con -

n e c t i o n t o l e a k a t h i g h t e n s i o n l o a d s t h a n a t z e r o

load .

R e p l a c i n g t h e o l d e r th e or y, i t h a s b e e n s u g g e s t e dtha t t he f a i lu r e i s a s o r t o f "unz ippe ring" e f fec t i n

w h i c h t h e l a s t e n g a g e d p i p e t h r e a d s l i p s i nw a r d ly

( t o w a r d the p ipe ax i s ) f r om i t s ma t ing coup l ing

t h r e a d , t h u s t hr ow in g i n c re a s ed l o ad o n th e su c -

c e e d i n g p i p e t h re a d . I t , t h e n , a l s o s t a r t s t o sl i p ,

th rowing add i t iona l l oad on the th i rd th read ; and s o

o n u n t i l a l l o f t h e t h r e a d s h a v e b e e n p er m a n en t ly

compres s ed in to a s ma l l e r e f f ec t ive d i ame te r and

t h e c o u p li n g t h r e a d s c a n p a s s o v e r th em . T h e j oi nt

i s t h e n p a r t e d , g i v i n g t h e c h a r a c t e r i s t i c ju m p- ou t

type of c as i ng t en s ion f a i lu r e.

Al l of t h i s " unzipper ing" happ ens s o ve ry r ap id lyt h a t t h e f a i l u re s e e m s t o t a k e p l a c e i n s t a n ta n e o u s l y .

In a t e n s i o n - t e s t i n g m a c h i n e i t o c c u r s w i th a l oud

report.

An a t t emp t is m a d e t o i l l u s t r a t e t h i s l a t t e r t he o r y

o f cas ing - jo in t f a i lu r e in F ig . 2 . C ons id e ra t i on o f i t

wi l l r evea l how th i s theo ry f i t s t he f ac t s , v i z . , t h a t

c a s i n g j o i n t s which a r e l eak - tigh t a t no t en s i on

load wi l l no t 1 e a k u n d e r t e n s i o n u n t il t h e t e n s i o n

l o a d a p p r o a c h e s t h e y i e l d i n g l o a d o f t h e c r i t i c a 1

s e c t i o n .

If t h e s e c o n d t he o ry i s t h e c o r r e ct o n e , i t i s ob-

v i o u s t h a t t h e j o i n t s t r e n g t h of A P I c a s i n g s h o u l dapp rox ima te the y i e ld s t r en g th of the me ta l unde r

t h e l a s t e n g a g e d t h r ea d . T h i s i s r o ug h ly c o r r ec t .

F o r e x a m p l e , t h e a r e a u nd e r t h e l a s t e n g a g e d t h re a d

fo r 5% - in . OD, 23-lb, N-80 i s 5. 41 4 s q in. T h i s

t i m e s 8 0 , 00 0 , t h e m i n i m u m y i e l d f o r N -8 0, i s

433,000 l b, w hi c h c l o s e l y a g r e e s w it h t h e pu b-

l ishe d* minimum jo in t -s t rength f igure of 440 , 000 lb

fo r long - th r ead cas ing . B y the o lde r theo ry , t he co -

e f fi c ien t of f r i c t ion o f the th r ead s u r f ac es , va r ious ly

l u b r i c a te d , i s i n v o l v e d ; a n d t h e r e i s n o c o r r el a ti o n

*Obtained by e study of hundreds of actual pulling t e s t s .

Fig. 3-Necked-down Tension Fai lure

b e t w e e n a t t e m p t e d c a l c u l a t i o n s a n d a c t u a l p u l li n g -

t e s t f igu res.

R eg ard le s s o f the exac t mechan i s m o f f a i lu r e , t he

r e s u l t i s a necked -down p ipe th r ead ( and occa s ion -

a l l y an expanded coup l ing ) wh ich , t o the cas u a l ob -

s e rv e r, l o o ks u n de fo rm ed . L e a d , t a p e r , a n d s i z e

r ead ings by gag es wi l l tquick ly d i s pe l t h i s i l l u s ion .

In f ac t , l ay ing a s t e e l s c a l e l o n g i tu d i na l ly o v e r t h e

t h r e a d s w i l l c l e a r l y d i s c l o s e t h e n e c ki ng - do w n e f-

f e c t ( s e e F i g . 3 ).

A c t u a l l y t h e c o u p l i n g i s e x p a n d i n g w h il e t h e p i p e

i s c o n t r a c t in g ; b u t , o r d i n ar i ly , t h e c o u p l i n g i s of

g r e a t e r e ff ec ti ve w a ll t h i c k n es s s o t h a t i t i s n ot

pe rmanen t ly de fo rmed. Th us , on ly the p ipe e nd re -

ma i ns necked down ( s e e F ig . 4 ).

N o t e i n F i g . 4 how th e top s of s om e of the th r ead s

w e re t u r n e d o v e r i n t h e d i re c t io n of t h e sc r a p i n g

a c t i o n a s t h e y s l i d fr om e n g a g e m e n t w i t h t h e c ou -

p l i n g t h r e a d s. T h i s i s v e ry c h a r a c t e r i s t i c o f a j um p-

o u t a n d i s c o n c l u s i v e e v i d e n c e of a normal casir ig-

jo in t f a i lu r e in t en s ion .

\

Fig. 4-Section from Necke ddown Tension Fail ure

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10 H. G . TEXTER

sFig. 5-Occasional Mode of Tension Failu re

T h e c o n v e r s e of a tension f a i l u r e wo u ld b e a corn-

p r e s s i o n f a i l u r e. A g a i n t h e r e w o u.1d b e a s u d d e nL 6

j u mp i ng w o f t h e m a l e t h r e a d s w i th r e s p e c t t o t h e

f em a le , e x c e p t t h a t i n t h i s c a s e t h e 'c o n n ec t io n

wo u ld n o t b e p a r t e d .

S o m e t i m e s , i n t en s i o n , A P I j o in t e d c a s i n g p a r t sb y b r e ak i n g a t t h e l a s t e n g a g e d t h re a d ( s e e F i g . 5).

T h i s i s p a r ti c u l a r ly t r u e of s m a l l -d i a m et e r h e a v y -

w a ll pi pe . I t s i m p 1 y i n d i c a t e s t h a t t h e u l t i m a t e

s t r e n g t h o f t h e m e t a l u n d er t h e l a s t e n g a g e d t h r e a d

w a s r e a c h e d s l i g h t l y b e f or e t h e ju m pi ng -o ut l o a d

w a s r e a c h e d . I t i s a r e l a t i v e l y r a r e fo rm o f c a s i n g

t e n s i o n f a i l u re . I t w a s m o re c o mm o n i n t h e d a y s o f

t h e o l d A P I s h a rp - V t h r e a d a n d s e l d o m o c c u r s in

th e p r e se n t ro un d- V th r e a d d e s ig n .

H i gh E f f i c i e n c y J o i n t s

With 1 0 0 - p e r c e n t e f f i c i en c y jo in t s , wh e r e t h e j o in t

s t r e n g t h is g r e a t e r t h a n th a t of t h e b o d y of t h e p ip e ,

a t e n s i o n f a i l u r e o c c u r s , o f c o u r s e , i n t h e b od y of

t h e p i p e , a s i l lu s t ra t e d i n F i g . 6. T h e p ip e h a s

n e c k e d d o w n an d t h e w a l l h a s r e d u c e d , as i n t h e

c a s e o f a n y d e s t r u c t i v e t e n s i o n f a i l u r e of a t u b u la r

s e c t i o n .

Jo in t s o f h ig h e f f i c i e n c y m a y b e m a d e b y u p se t -

t i n g t h e e n d s of A P I c a s i n g o r b y w ork h a r de n i ng o r

in d u c t io n h a r d e n in g th e e n d s . C e r t a i n p a t e n t e d ,

s q u a r e - t hr e a d , n o n- A P I d e s i g n s a p p r o a c h o r r e a c h

100-percen t e f f ic iency and would fa i l by permanen t-

l y s t r e t c h in g o r p a r t i n g i n t h e b o dy o f t h e p ip e b e -

f o re f a i l i n g a t t h e c o n n e c t i o n .

M i l l D e f e c t s

T h e r e h a p p e n to b e v e r y fe w m i l l d e f e c t s c o n d u-

c i v e t o t e n s i o n f a i l u r e s . F o r e x a m p l e , po o r l o n gi -

t u d i n a l w e l d s ( in t h e c a s e of w e l d e d c a s i n g ) o r ex -

c e p t io n a l ly d e e p se a m s , wh ic h m ig h t f a i l u n d e r v e r y

lo w in t e r n a l p r e s su r e , wo u ld h a v e n o m e a su r a b l e e f -

Fi g. 6-Tension Failure in Body of Pip e

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OIL-FELL CASING AND TUBING TROUBLES 1 1

f e c t o n t h e t e n s i l e s t re n g t h of a t ub e. T h e s a m e

wou ld be t r ue fo r ve ry deep p i t s o r o the r s u r f ac e

b l e m i s h e s .

E v e n p o o rl y c u t t h r e a d s , a l th o u g h t h e y m i g h t f a i l

t o make a f lu id s ea l , a r e s e ldom bad enough to r e -

du ce jo in t s t rengt h . Insuf f ic ien t ly made-up connec-t ions , ve ry l i t t l e beyond hand t igh t , s how s u rp r i s -

ing ly h igh jo in t s t r en g th unde r pu l l - ou t t e s t s .

? ' her e i s one mi ll de f ec t , however , t ha t can we l l

b e f a t a l , n o t on l y i n t e n s i o n b u t a l s o i n c o l l a p s e o r

b u rs ti ng . T h i s i s a l o c al l y ha rd en ed a r e a , f rom

s t i c k in g and s l ipp ing in mi ll r o l l s o r f rom acc iden t -

a l co o l ing by a wa te r s p r ay . Th e manufac tu re r a l -

w a y s t r i e s t o d i s c a r d s u c h l e n g t h s , b u t o c c a s i o n a l

o n e s h a v e b e e n s h i p p e d , a s d e s c r i b e d f o ll ow i ng .

A t ens ion f a i lu r e occu r r ed in a comp le ted we l l i n

Wyoming here a water -quenc hed leng th of h igh-

c a r b o n c a s i n g h a d b e e n r u n. T h e f a i l u re o c c u r re da b o u t 2 months a f t e r t he we l l had bee n pu t on pro-

duct ion , s om e 1 ,30 0 f t from th e top , in th e body of

the p ipe . I t wa s r ecove red and exam ina t ion s howed

a ne twork o f f ine quench ing c r a ck s a round the f a i l -

u r e po in t . Har dnes s r ea d ing s con fi rmed the o r ig in

of the cracks . E ' ig . 7 i l l u s t r a t e s t h e a p p e a r a n c e o f

th is type of defect (magnif ied) .

A s a p r e c a u t i o n a g a i n s t l o c a l l y ha r d e n e d a r e a s ,

t h e u s e r c a n a n d s h o u l d h a v e h i s c a s i n g i n t e rn a l ly

t e s t e d t o a h i gh p r e s s u r e , e i t h e r a t t h e m i ll o r o n

t h e r a c k s . T h e API ha s now s e t up new manda to ry

( fo r N-80) an d a l t e rna t i ve hyd ros ta t i c t e s t p r e s s u resf o r c a s i n g a n d t u b i n g b a s e d o n 8 0 p e r c e n t o f t h e

specif ied minimum y i e 1 d s t r eng th o f the va r ious

i te m s. ' A t t h e s e hi gh p r e s s u r e s , e s p e c i a l l y if t h e

l e n g t h s are pounded wi th a hammer o r r a t t l ed wi th

a n a i r h a m m e r n h i l e u n d e r t e s t , a n y a c c i d e n t a l l y

cold-worked or water-quenched areas are almost

certain to fail.

T e n s i o n F a i l u r e f ro m F i e l d W e ld in g

Fig . 8a a n d 8 b i l l u s t r a t e a t y p e o f t e n s i o n f a i l u r e

r e s u l t i n g f r o m q u e n c h i n g c r a c k s b e c a u s e o f a very

improper wel d ing pra ct ice . A 7 ,500-f t s t r in g of 6-in.

OL), 17-lb, N-80 non-API ca s in g had been s c r ew edi n t h e h a n g e r of a c a s i n g h e a d a n d a h e a v y b e a d of

weld had been run around the bot tom of the han ger

t o g i v e a p o s i t i v e s e a l a g a i n s t l e a k a g e b e tw e e n i t

a n d t h e c a s i n g . T h e r e i n w a s t h e e r ro r b e c a u s e t h e

a n a l y s i s o f N- 80 c a s i n g or d in a r il y i s s u c h t h a t i t i s

p r a c t i c a l l y i m p o s s i b l e t o w e l d w i t h o u t d a n g e ro u s

e rnbr i tt l emen t . So a l i t t l e ove r a n hou r a f t e r we ld ing ,

w h i l e c i r c u l a t i n g f or c e m e nt i ng , t h e c a s i n g s u d d e n-

l y p a r t e d r i g h t a t t h e w e l d a n d d r o p p e d t h e s t r i n g.

I n t h e p h o t og r a ph t h e h a n ge r m e t a l i s a t t h e r i g h t,

t h e p i p e a t t h e 1 e f t , a n d t h e w e ld m e t a l is i n be -

t w ee n . T h i s i s a l l s h o w n m o re c l e a r l y i n t h e s k e t c h .

Fig . 7-Magnified Quenching Cracks

F i r t h h a r d n e s s r e a d i n g s, a p p ro x i m at e ly e q u i v a l e n t

to Br inel l , ahow the wide var ia t ion f rom the or ig inal

p i p e m e t a l h a r d n e s s ( a b o u t 2 68 ) t o t h e v e r y h i gh

h a r d n e s s f i g u r e of '5 49 i n t h e da rk a r ea . T h i s i s

w h e r e t h e h e a t e d a r e a h a d b e e n m o s t d r a s t i c a l l y

quenched by the coo l ing e f f ec t o f t he ad jacen t co ld

p ipe me ta l .

N o t e t h e q u e n c h i n g c r a c k a n d t h e f a i l u r e p o i n t a t

the bo t tom o f the p ipe s ec t ion , a t abou t the th r ead

runout . No doub t the re were a number of quenchin g

c r a c k s i n v a r i o u s p a r t s of t h e p i p e s e c t i o n , b u t o n l y

one happened to s how up in th i s pa r t i cu Ia r pho to -

graph.

T h i s t r o ub l e o c c ur r e d i n t h e M a g no l ia F i e l d i n

A r k a n s a s i n 1939. S i n c e t h a t t i m e t h e d a n g e r o f e m-

b r it t le m e n t b y w e l d i n g i s s o w e ll - un d e rs t oo d t h a t

t h a t t h e p r a c ti c e h a s b e e n a b a nd o n e d e n t ir e ly if

t h e re i s g o i ng t o b e a n y t e n s i l e s t r e s s w h a t s o e v e r

o n t h e r e s u l ti n g w e l d. O n l y p r e s s u r e b e a d s a r e e v e r

r un , a n d e v e n t h e s e a r e n o t c o n s i d e r e d v e ry g o od

p r a c t i c e . R e l i a b l e s e a l s a r e m or e s a f e l y m a de w i th

pack ing .

P r e v e n t i o n o f T e n s i o n F a i l u r e s

S e l e c t i n g a p r o p er d e s i g n f a c t o r i s , of c o u r s e , t h e

f i rs t s t e p i n a v o i d i n g t e n s i o n f a i l u r e s o f c a s i n g .

S o m e c a r e i n s t a b b i n g a n d m a k in g up t h e t h r e a d e d

c o n n e c t i o n s s h o u l d b e o b s e r v e d , b u t t h i s i s pr im a ri -

ly to p r e v e n .t l eak age . On ly g ro s s mis -hand l ing ,

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12 H. G. TEXTER

Fi g. 80-F ailur e Because of Improper

Welding Practice

START

O FCRACKWHICH

RESULTED

INFAILURE -

ig. 8b-Diagram of Fig. 80

s u c h a s v er y b a d g a l l i n g o f t h e t h r e a d s , c o u l d r e-

s u l t i n t e n s i o n f a il u r es .

A s s t a t e d i n th e 4 t h Ed it io n of r iP1 RP 5C l :

" C a s i n g s t r i n g s s h o u l d b e p i c k e d up a n d l o w e r e d

c a r e f u l l y , a n d c a r e e x e r c i s e d i n s e t t i n g s l i p s t o

a v o i d s h o c k l oa ds ." ' A l s o , if t h e r u n n i ng o f as t r i n g o f c a s i n g i s e v e r i n t e r ru p t e d , t o m a k e ma-

c h i n er y r e p a i r s f o r e x a m p l e , t h e s t r i n g s h o u l d b e

"w orke d" up a nd dow n o r mud sho u ld be c i r c u l a t e d ,

o r b o th , t o p r e v e n t s t i c k i n g . O t h e r w i s e t h e s t r i n g

m a y s t i c k t o s u c h a n e x t e n t t h a t i t c o u ld b e p ar te d

u p o n r e s u m i n g r un n in g . T h i s h a s a c t u a l l y h a p p e n e d

i n c a s e s b ro u g h t t o t h e a u t h or ' s a t t e n t i o n .

A n o t h e r p r e c a u t i o n a g a i n s t t e n s i o n f a i l u r e s w h i l e

r u nn i ng c a s i n g i s th e u s e o f s li p - ty p e e l e v a t o r s a s

opp ose d t o t he o lde r "co l l a r t ype " w hic h p i c k up

o n t h e b e a r i n g f a c e of t h e c o u p l i n g . T h e d a n g e r i s

n o t b e c a u s e o f a n y i n h e r e n t f a u l t i n t h e c o l l a r- t y pee l e v a t o r , b u t b e c a u s e o f p o s s i b l e w e a r of t h e l i f ti n g-f a c e o r b e c a u s e o f a d e f e c t i v e ( s h a r p ) b e a ri n g f a c e

o f t h e c o u p l i n g , a l l o w i n g t h e l o a d t o r e s t u n e v e n ly

o n t h e e l e v a t o r l i ft i n g s h ou l d e r. I h i s t e n d s t o c o ck

t h e c o u p l i n g o u t of a l i g n m e n t a n d c a n e a s i l y f l ip i t

off and drop th e s t r i ng .

I n ci d en t al ly , a b r o k e n e l e v at o r ba il h a s b e en

know n to r e su l t i n t h i s f l ipp ing o ff o f a c oup l i ng .

S uc h a n a c ci d en t wo uld n o t r e s u l t i n a dr oppe d

s t r i n g w i t h a s l i p - t y p e e l e v a t o r .

F o r t h e s e r e a s o n s t h e r e i s t h e A P I r ec om m e nd a-

t i o n t h a t o n l y s li p - ty p e e l e v a t o r s b e u s e d i n r u nn i ngl o n g o r i m p o r ta n t s t r i n g s of c a s i n g .

T e n s i o n f a i l u r e s , a s m e n ti o n ed p r e v i o u s l y, c o u l d

oc c ur a f t e r c e me n t ing if t he r e w e r e so me ve r y un-

u s u a l c o o l i n g e f f e c t . A l l o w a n c e s h o u l d b e m a d e f o r

t h i s , if l i ke ly t o oc c ur , w he n L and ing t he , s t r i ng i n

t h e c a s i n g h e a d . H o w e v e r , i n t h e g r e a t m a j o r it y of

c a s e s i t i s b e tt e r t o s e t t h e c a s i n g i n f u ll t e n si o n .

I f even a s m a ll am ou nt o f w e i g h t i s s l a c k ed off,-t h e r e i s d a n g e r o f p l a c i n g s o m e of t h e u n c e n i e n t ed

s e c t i o n i n c o n ~ p r e s s i o n w it h a t t e n d a n t d a n g e r of

b u c k l in g . T h i s w i l l b e d i s c u s s e d f u rt h e r h e r e i n .

C o m p r e s s io n F a i l u r e sC o m p r e s s i o n f a i l u r e s a r e v e r y r a r e ( o r r a r e l y re -

p o r t ed ) , b u t t h e a u t h o r k n o w s o f o n e c l e a r l y e s t a b -

l i s h e d c a s e , i n 1 9 2 6 , w h e r e a s t r i n g o f 5 '4 -i n. OD,2 0-lb s e a m l e s s c a s i n g w a s s e t d ow n o n b otto m s o

h a r d t h a t a p i p e e n d j um p e d s e v e r a l t h r e a d s i n t o

t h e c o u p li n g. L a t e r , w h e n t h e s t r i n g w a s r e c o v e r e d ,

t h i s c o n n e c t i o n p a r t e d a t lo w t e n s i l e l o a d . H o w e v e r ,

t h e re w a s s o l i t t l e e v i d e n c e o f t h e p i p e b e i n g n e c k -

e d d o w n , o n c a s u a l e x a m i n a t i o n , t h a t t h e o p e r a t o r

t h r e a t e n e d s u i t a g a i n s t t h e m a n u f a ct u r er , c l a i m i n g

t h a t h e ha d n o t d u ll ed b e y o n d t h e w e ig h t of t h e

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OIL-WELL CASING PLNDTUBING TROUBLES 13

s t r i n g i n c o m in g o u t of t h e h o l e. I n t h i s r e s p e c t , o f

c o u r s e , h e w a s r i g h t; b u t h e d i d n o t k no w, o r d i d n o t

w a n t t o a d m it , t h a t t h e s t r i n g p r e v i o u s l y h a d b e e n

dropped.

1B. T E NSI ON- T UBI NGT h e m e c h a n i s m of t u b i n g t e n s i o n f a i l u r e p a r a l l e l s

t h a t of c a s i n g .

S i n c e s o m e 9 0 p e r c e n t of a l l o i l - w e ll t u b in g i s

u p s e t d e s i g n , p r a c t i c a ll y a l l f a i l u r e s a r e i n t h e b o dy

o f t h e p i p e . H o w e ve r, a n y t e n s i o n f a i l u r e s a r e r a r e

b e c a u s e t u b i n g i s a l m o s t a l w a y s r u n i n s i d e of c a s -

i n g a n d , e x c e p t f o r o c c a s i o n a l t r o u b le o f u n s e a t i n g

p a c k e r s , i t s e 1 d o m b e c o m e s s t u c k a n d t h e re f o re

n e e d s n o t b e p u l le d o n .

A s w i th u p s e t c a s i n g , u p s e t t u b i n g w il l s t r e t c h

b e f o re f a i li n g . T h e a u t h o r o n c e o b s e r v e d a n o p e r a -

to r pu l l i ng on a f r oz e n s t r i ng o f 3 'L -in . t ub ing in a n

8 , 20 0 -f t w e ll from w h i ch t h r e e l e n g t h s w e re l a i d

down be f or e f a i l u r e oc c ur r e d . When the s t r i ng wa s

f i n al l y f r ee d a n d l a i d d o w n, t h e t o p 2 0 t o 3 0 l e n g t h s

a c tu a l ly t a l l i e d f rom a n inc h to a f oo t l onge r t ha n

w h e n t a l l i e d i n t o t h e w e l l , a n a v e r a g e e l o n g a t i o n of

1 0 in . p e r l e n g th .

F'ig. 9 i s a n i d e a l e x a m p l e o f a p u r e t e n s i o n f a il -

u r e i n u p s e t t u b in g . I t w a s t h e t o p l e n g t h of 1 2 , 6 0 0

f t of 2=/, - in . OD, 4 .70- lb , N-80 se t in a sou th Loui-

s i a n a w e ll . W h i le p u l li n g u p t o u n s e a t a "s t inger"

fro m t h e p a c k e r , t h i s l e n g t h p a r t e d .

T h e s h o r t e r s e c t i o n i s t h e t o p o f th e f a i l u r e an ds h o w s t h e s l i p m a r k s of t h e s p i d e r u s e d i n s e t t i n g

t h e s t r in g . T h e o t h e r s e c t i o n s h o w s o v e r s h o t m a rk s

m a d e i n f i s h n g f o r t h e l o w e r p a r t of t h e f a i l u re

w h i c h h a d f a l l e n b a c k i n t o t h e w e ll .

Note t he ve r y nor ma l ne c k in g down, e v ide n c e d by

t h e t h i n n e d w a l l a t t h e b re a k. N o t e a l s o h o w t h e in -

t e r n a l p l a s t i c c o a t i n g o f t h e l e n g t h h a d b e e n c r a c k -

e d o f f b y t h e e x t r e m e s t r e t c h i n g b e f o re f a i l u re .

N o n - u p s et t u b i n g m a y f a i l by j u m p in g o u t o f i t s

c o u p l in g , b u t i t i s m o re l i k e l y t o f a i l b y p a r t i n g a t

t h e l a s t e n g a g e d t h r ea d , a s s o m e t i m e s h a p p e n s w it h

c as in g ( s e e F ig . 5). F'or both externa l -upse t andn o n - u p se t , t e n s i o n f a i l u r e s i n t u b i n g a r e s o in f re -

q u e n t as t o b e p r a c t i c a l l y n e g l ig i b l e.

T o P r e v en t T e n s i o n F a i l u r e s

I n g e n e r al , t h e s a m e c o n s i d e r a t io n s w i th r e s p e c t

t o p r e v e n t io n o f t e n s i o n f a i l u r e s a p p l y t o t u b in g a s

a p p l y t o c a s i n g .

U s e u p s e t t u b i ng e x c e p t i n v e ry s h a l l o w a r e a s .

l ' h i s i s a l s o n e c e s s a r y t o a v o i d l a s t e n g a g e d t hr ea d

f a i l u r e s .

Se l e c t p r ope r s t e e l g r a de . Wi th we l l s now produc -

i n g f ro m d e p t h s e x t e n d i n g t o 1 8 , 0 0 0 f t, it i s b ec om -

i n g n e c e s s a r y t o a d o pt a s t e e l g r a d e a b o v e N -80.

Fig. 9-Upset-tubing Tension Failure

I n l i e u of h i g h e r g r a d e , t h e r e i s t h e e x p e d i e n t o f

r unn ing c om bina t ion s t r i n g s of two d i a m e t e r s in

w h i c h t h e u p p e r p o r t io n i s of g r e a t e r w a l l s e c t i o n .

T h u s t h e u p p e r p a r t m i g h t b e 2 % -i n. OD, 0.440-in.

wa l l and the low er pa r t 2'4-in . OD, 0 .190- in . (API)w a l l . T h e I D of t h e 2 7/,-in. i s t h e n t h e s a m e a s fo r

t h e 2)/,-in. T h i s p a r t i c u l a r c o m b i n a t io n h a s a c t u a l l y

be e n r un a nd i s now be ing o f f er e d to t he i ndus t r y .

2A. COLLAPSE-CASINGM e c ha n is m o f C o l l a p s e

T r u e h y d r o s t a t i c c o l l a p s e w i l l f l a t t e n a l e ng th o f

c a s i n g p r a c t i c a ll y w al -l t o w a ll . T h e r e s u l t i s v er y

of t e n a t r ough , as i n F i g . 1 0 a a n d F i g . l o b ; b u t m a y

a l s o b e i n t h e f o r m o f ri bb o n, F ig . l l a , I l b , a nd

l l c . As i n d i c a t e d , t h e f l a t t e n i n g u s u a l l y s t o p s a d -

j a c e n t t o t he j o in t be c a us e of t he re in f o r c ing e ff e c t

o f t h e r e l a t i v e l y h e a v y c o u p l i n g o r jo i n t s e c t i o n .B e i n g a n a 1 m o s t i n s t a n t a n e o u s s o r t o f f a i l u r e ,

t h e f a c to r s de c id i ng whe the r t h e fi nal fo rm of a c o l -

l a p s e w i ll b e a t r ough o r a r i bbon a r e unknown, bu t

p r o b a b l y i n v o l v e c e r t a i n c o n d i t i o n s of u n s t a b l e

equi l ibr ium.

I n a l a b o r a t o r y c o l l a p s e c h a m b e r t h e p i p e l e n g t h s

u n d er t e s t a r e o n ly p a r ti a ll y c o l l a p s e d , a s i n F i g.

12. T h e s y s t e m u s e d i n t e s t i n g c a n d e v e lo p v er y

h i gh p r e s s u r e , b u t i s v e r y l i m i t e d a s t o t o t a l e n e r g y .

T he r e f o r e , a f t e r p r e l im ina r y f l a t t e n ing de f o r m a t ion

o c c u r s , t h e p r e s s u r e i n t h e s y s t e m d r o p s to z e r o a t

o n c e a s t h e r e i s n o f o ll o wi n g- u p v o l u m e to c o m p l e tet h e f l at t e ni n g .

I n a w el l t he r e i s e n o u g h e ne rg y , a s th e flu id

a b o v e t h e c o l la p s e p o in t d e sc e nd s , t o b ri cg t h e

w a l l s o f t h e p i p e c o m p l e te l y to g e th e r. T h i s i s w h a t

i s m e an t b y a t r u e h y d r o s t u t i c c o l l a p s e . A s i t o c-

c u r s m o s t o f t e n i n d e e p w e l l s , i t i s very s e l d o m r e-

c o v e r e d f o r s t u d y o r f o r ph o t o g r ap h i n g .

N o n - h y d r o s t a t i c C o l l a p s e

O c c a s i o n a l l y c o l l a p s e n l ay b e t h e r e s u l t o f d ia -

s t r o p h i c s h i f t i n g o f f o r m at i o n s c a u s e d b y a n e a r t h -

q u a k e o r f au lti ng . E x a m p l es o f t h e 1 a t e r a r e re-

p o r t e d i n C a l i f o r n i a i n c e r t a i n a r e a s .

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14 H. G . TEXTER

Recently, in North Dakota, serious trouble has

been encountered in some 1 4 or more wells in which

the oil st rings , 5'4-in. OD or 7-in. OD, are either

p a r t i a1 1y collapsed or bent out of straight. This

trouble has occurred as soon as 3 months after the

wel ls are completed and i s located in the t h i c kCharles salt section which runs from 7,500 to 8,200

ft in depth. (Production is found at 8,400 to 8,500

ft.)

Fig. 13a is a photograph of a section of 5'4-in.,

17-lb, 5-55 casing recovered from one of the North

Dakota wells by w a s h n g over and fishing. Both

casing and tubing, which was inside, were recover-

ed together. Very evidently the c asing i s not col-

lapsed wall to wall, as in the case of hydrostatic

collapse. It can better be descr.ibed as "crushed."

Among the theories which have been proposed toexplain those failures, one can mention the follow-

ing:

1. Large bodies of loose rocks from overhanging

formations falling against the casing in large,

washed-out cavit ies. Th is theory i s not very

plausible in view of the limited velocity which

rocks may reach when falling in liquid and be-

cau se of their limited mass.

2. Tectonic, glacial-like flow of the e n t i r e sa lt

section, as in some California forinations. This

theory i s stil l l es s plausible in view of the fa ct

that North Dakota formations are nearly horizon-

tal.

3. The most plausible explanation to date is that

salt behaves plastically and flows, tending to

close up the hole. If the annular space around

the casing were completely filled with cement,

then the salt could not flow. In this case, how-

ever, the plastic behavior of salt results in a

progressive inc rease of pressure of the sa lt on

the casin g t h r o u h the cement. Th e ultimate

value of this pressure could be e q u a 1 to the

overburden pres sure, i.e., approximately1

psiper foot of depth, which i s a b o u t double the

pressure a gainst which the str ings generally are

designed for collapse. Therefore, long before

the ultimate pressure i s r e a c h e d the cas ing

st ar ts deforming, which temporarily rel ie ves the

pressure. Only after a certain period of time,

the pressure builds up again and collapsing re-

sumes. In other words, colla pse resul ting from

sa lt flow i s a slow process as opposed to col-

lapse caused by fluid pressure which almost in-

I stHntaneously fla ttens 'the cas ing wall to wall.

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OIL-WELL CASING AND TUBING TROUBLES 15

Fig . lla-Casi ng Ribbon-type Collapse Fig. 11b-Sections from Ribbon Collapse

\

Fig. 1 c-Ar tist 's Conception of Ribbon-type Collapse

Fi g. 12-Partial Collapse (from Collapse Chamber)

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16 11. G. TEXTEK

Fig . 130-Casing C r u s h e d b y Salt F low

Fig. 13b-Casing Bent by Salt Flowl 'his last hypothesis is well borne out by an in-

teresting experience in one deep North Dakota

wildcat. A strin g of 9%-in. casin g had been se t

through the Charles sal t w h e r e i t was much

deep er than usual-8,704 to 8,77 9 ft. After some

43 days of dr il ling ins ide, a c a l i p e r survey

showed there were 5 slightly constricted spots

where the inside diameter was '4 in. to '4 in.

below the original inside diameter. These con-

stric tions were r o 1 1 e d out, but some 44 da ys

later at least one new constriction was found

right at the location of the greatest original de-

foniat ion . So i t s e e m e d tha t something bas

slowly pressing against the pipe.

'I he third theory sug,gests that if the a n nu 1 a r

spa ce uetbbeen the c asing and the sal t ae re sol idly

filled v.it1i cemen t, then d es igni ng he portion of the

string tl~rough sa lt strong enough to withstand the

overburden pressure should prevent the trouble. Out

channelling makes it in~probable that the annular

space bi l l be completely filled ~ i t h enient if the

hole is washed out. ? herefore, in addition to using

a stronger casing and niaking a careful, complete

cement job, it is necessary to drill \\ith fully salt-

satu rated niud to minimize washing out. Th es e are

indeed the practices which were recently adopted in

d r 1 l i n g new hol es and in which it i s hoped no

trouble \\ill ever develop.

Unfortunately, early wells in the Williston Basin

!\ere not drilled thi s way through the sa lt sect io ns

and the holes are washed out in a very irregular and

asy~i~et r ica l ay. In these wells the annular space

i s not entirel y filled with cemen t. 'l'he slowly Ilow-

ing sal t does not contact the casing at the s an1 e

time all the bray around and all the way along its

length. Therefore, in addition to collapsing the cas-

ing i s bent out of s traig ht (se e E'ig. 13b).

Mechanical Collapse

At least one case has conie to the author's atten-

tion in which some foreign objecl seemed to have

been rolled into 5 or more different lengths whilea 9,500-ft str ing of cas ing wa s being run into a well

near Beaun~ont,1 xas, in June 1939.

The string was ?-in. OD, nlostly 26-lb. I'he evi-

dence clearly indicated that only partial (and there-

fore mechanical) colla pse was involved. 1 h e de-

formed spo ts could be reamed ou t and a total of five

were lat er recovered by pulling th e upper part of the

string.

Fig. 14 shows the third dinge, after it had been

reamed out and recovered. One can visualize \"hat

it must have looked like originally-just an ordinary

dinge. b'ig. 15 i s a diagrammatic view of the fivedinges that were recovered. 7 here i s no telling how

many more there might have been as the hole was

sidetracked after recovering the fifth partial col-

lapse.

A s a h int a s to what might have c a u s e d th is

dingeing, consider the following e x c e r p t from a

field man's report made in 1936: "The cas ing was

s e t with the us e of hand-operated sli ps. A s about

the hundredth joint w as being run, one of the sl ip s

was c a u g h t by the casin g and dragged p art way

through the table, put t ing q ui te a large dinge in the

c a s i n g ."

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OIL-WELL CASING PLND TUBING TROUBLES 17

Fig . 14-Casing Dinge (after reaming out)

Squeeze-job Collapse

l'h e pressure causing most casing coll apse fail-

ures i s simply that of the hydrostatic head of the

surrounding fluid. Either the design factor was too

low, the wrong weight or grade of casing was used,

or the pipe was defective. However, another cause

of casing c oll apse i s incorrect procedure in carry-

ing out a cementing sq ueeze job. From E'ig. 16 i t

FIRST DlNGE

711112' TO 711Wi1

7 6 27 ' TO 7 6 3 1 'BOTTOM OF 9 5 1 s " OD

CASING SET AT 76 50 '

THIRD DlNGE - 7708' -4 '\ I '\.

FIFTH DlNGE

78 25 ' TO 78 30 ' SIDE TRACKED HOLE

BOTTOM [17 " CASING - 9 5 0 0 '

Fi g. 15-Diagrammatic Vie w of a Series of Dinges

should be clear that the tubing pressure used to

force cenient through the perforations bill, to some

exten t, be applied on the o u t s i d e of the ca sing

above the packer. This can and often has caused

collapse.

An obvious remedy i s to apply some Lack pres-

sure in the annulus b e t w e e n the tubing (or drill

pipe) and the casing. This i s a s tep q u i t e often

taken and s h o u l d always be considered if the

s q u e e z e pressures are to be high. l iowever, in

ca se s of very high sque eze press ure s, upwards of

5,000 psi, the upper part of the casing string niay

not be strong enough. E'ortunately, there remains

the expedient of setting the packer a hundred or so

feet above the ~er fo ra ti on s, hus allowing some of

PACKER SET A

CONSIDERABLE DISTANCE

ABOVE SQUEEZE POINT

PACKER SET IMM EDIATE

ABOVE SQUEEZE POINT

\

PERFORATIONS

CEMENT SOUEEZ

Fi g. 16-Mechanism of Squeeze-iob Casing

Collapse

the squeeze pressure to dissipate itself in the sur-

rounding forn~ations.This works successfully, but

has the slight disadvantage of leaving more cenient

in the casing to drill out.

Effect of Dinges

The most harn~fulaccident that can happen to a

piece of c asing i s a dinge. J ust a s a tall, slender

colunin can support a heavy weight a s long a s it i s

~e rf ec tl y traight, a tube can withstand heavy ex-

ternal pressure a s long as it i s perfectly round. De-

I form either one but slightly and its ability to resist

failure goes down treniendously.

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Theoretical calculations can show that a perfect-

ly round, thin-uall tube, if deformed but 1 percent

out-of-round, will have it s resist anc e to collapse

lobered in the nature of 2 5 percent. Th is .is nien-

tioned to indicate the importance of handling casin g

s o a s never to dinge it.

Crushing by sl ip s or tongs i s a deformation acci -

dent uhich may easily lead to true collapse. Un-

fortunately, even if the failed tube ever i s recover-

ed, it would be very difficult or inipossible to de-

termine that c r u s h i n g by tongs or s l ips was to

blame. l' he tong or sl ip di e marks might or might

not furnish a clue.

Eiaxial Stress Effects 9~ '

A very important factor affecting the resistance

to colla pse of c asing i s longitudinal tension and

longitudinal conipression. The former mill reduce

collapse strength; the latter uill increase it. l 'his

i s the of biaxial st re ss . It can Le cal-

culated mathematically and has been demonstrated

by many te st s involving t he effect of te nsi on or corn-

pression on the collaps e resistance of tubular sec-

tions.

The effect of tension on collapse resistance (bi-

axial s tre sse s) i s taken into account in conlLination

casing strings where heavier sections, or sections

of higher API grade, are run in the lower parts of

casing strings for better collapse strength. l 'hese

lower sections, because of adding increased ten-

sion to the upper sections, decrease their collapse

strength which must be considered in calculating

design factors. Pip e n:ills furnish engineering dat a

for this purpose which i s used almost universally

in des igning cas ing programs for deep wells.

Several years ago the author was perniitted to

review dat a on four stri ngs of casin g, all in one

area, in u 11i c h the pipe very evidently had col-

lapse d. Th e si ze \\ as 654-in. in con!bination str ing s

of several ueights and grades; and, on account of

material shortage, design factors in collapse had

been kept to a minin~um. 'o illus trate, following is

a tabulation of the d ata on Fell U.

Size, WeightDesign Factors

Length, OD , per Foot, Grade,

Ft In. Lb APImension* Collapse

TOP1,098 6% 24 J-55 1.7 4.70

3,489 6% 20 J-55 1.6 0.90

2,603 6'/, 24 J-55 3.5 0.90

Bottom

1.226 6% 24 N-80 12.7 1-04

Total depth of str ing: 8,416 f t

*B ase d on short threads and cou pl~ ngs .

Based on actual we~ghtof mud, 12.2 Ib per gal, and

corrected for effect of tensto n.

In each of the four bells, of which the toregoing

is representative, the failures occurred uh ile s u a l -

Ling: and in each case had been suabbed to some

point belou the failure point. In the case illustrated,

the pipe collapsed at 4,550 ft a f t e r having been

swabbed down to 5,250 ft. 'l'herefore, the failur e

occurred at the bottom of the 20-1L 3-55 section.

MI 6'4-in., 20-lb 3-55 has a lis ted niinin~uni col-, !

lap se strength of 3,060 psi . 1he tension load of the

24-lb 5-55 and the 24-lb N-80 sections, hanging Le-

lou it, reduces the collapse strength to 2,600 psi.

At the depth uhere collapse occurred, in 12.2-lb

mud, the external pressure uould be 4,55Uxu.634

( p s i per ft of he ig ht ) = 2,885 p s i . 'l'herefore, the de-

sign factor at the collapse point \\as only 0.9. Sin)-

ilar cond itions exis ted in each of the other three'

wells and they are, therefore, intere sting a c t u a 1

esaniples of failures resulting from the eBect oftension on col lap se. F'or each !%ell it had been re-

ported: "Casing cemented in full t e n s i o n . then

slacked of7 'a little,' to hang in head." l ' l i u s the

str ing s probably were in full tension, to all intent s

and purposes.

Fig. 17 illustrat es an extremely interesting cas-

ing failure involving biaxial stresses. The peculiar

necking doun occurred in a length of 5'1;-in. 011,

14-lb, H-40 API casing near the top of a 7.800-ft

well in California.

The joint strength being inadequate (design fac-

tor of only 1.1) , there had been a junip-out t ens ionfailure while working the string for cementing. 1'0

reconiplete the string, a well-knoun t,ype of

casing bowl u a s loner ed over the top length a t the

jump-out point and sorile tensi on e x e r t e d . l ' l ~ i s

serves to set slips uhich cause lead rings to pack

tightly around the pipe and serve as a fluid seal.

After thus recompleting the string, it was dis-

covered that tools nould not pass, and the necked-

down secti on shown in the photograph b a s recover-- .ed and studied. l 'h e obvious conclusion h a s that a

conibination of tension and collapse f o r c e s had

operated according to well-knoun laws of biaxial

stresses, with the resulting perfect example of a

tension-collapse failure. The collapsing force u a s

applied by the lead rings uhich, under con~ press ion,

floued and reacted exactly like a liquid.

hei the r of the for ces was very high. The tension

must have been b e l o ~he ten si le strength of 5'/;-in.,

14-lb, li-40 casing because no further parting oc-

curred and, therefore, the longitudinal tension was

something under 241,000 Ib ( the a v e r a g e joint

strength of the ca sing); but the result ant of the two

force s u a s enough to deform the pipe a s a combined

tension-collapse failure. iliaxial str es s calculations

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OIL-WELL CASING A ND TUBING TROUBLES 19

Fi g. 17-Casing Collapse from Biaxial Stresses

will show that, at an assunied tensile load of only

100,000 lb, the col lapse resi stance of 5'4-in. On ,

14-lb, H-40 tubing is lowered to 1,600 psi from it s

normal minimum collapse s t r e n t h of 2,440 psi .

Small wonder, then, that the lead rings could flow

s o tightly against the tube as to start the necking-

down type of col lap se illust rated. Once the co llapse

had been initiated, tension alone could continue

the deformation. (Ordinari ly, the act ion would pro-

ceed to the parting point, but did not in this case.)

It has been pointed out that because the effect of

buoyancy on a casing string puts the bottom of the

string in hydrostatic longitudinal conlpression, and

because compression increases the resistance ofcasi ng to collaps e, there i s no need to consider the

biaxial st re ss phenomenon. " nith this the author

is in disagreement, because the final tension con-

dition of a cemented strin g of casi ng i s usually un-

known, n~ainly ecause of unpredictable temperature

changes; and it is better to have sonie additional,

uncalculated safety factor. Also, since we ordinari-

ly disregard the effect of buoyancy in making ten-

sion design-factor calculations, it is just a s logical

to disregard its effect in figuring collapse design

factors.

Design Factors for Collapse-X t h most operating conipanies the accepted de-

sign factor in collapse i s 1.125. Th is i s based on

the published minimum collapse data, taking into

account the effect of tension on collapse in conili-

nation strings. It also is based on the assuniption

that there is no fluid inside the casing and that the

surrounding fluid has a hydrostatic head of 0.5 psi

per foot of height regard les s of the actual weight of

the mud.

A few con~panies~ o r ko higher design factors,

such as 1.25 and even 1.33. Sonie others drop to a

factor of 1. 0 (and even to 0.85' b e 1ow the top ofcen~ ent ), ut they usually take into account the ac-

tual hydrostatic pressure of the mud in the annulus.

?'here has been a trend toward the use of collapse

des ign factors les s than 1.0 for i n t e r m e d i a t e

strings. Th is is on the theory that the se larger-dia-

meter s t r i n g s , such as 9'/,-in. and 10%-in., will

never be b a i 1 e d down. 1'0 he author this makes

good sense, but the operator should be pretty sure

that there will Le no unexpected emptying of the

string by a thief sand or for any other cause.

Prevention of Collap se Failures of Casing

The principal step, of course, i s to use the cor-

rect weight per foot and grade of casin g for the con-

ditions in the bell, and apply the accepted design

factor. The e l i ec t o f t ens ion on co l lapse m us t never

be neglected , as has been illustrated heretofore.

Another factor which must be considered in the

relati vely new method of drilling with ga s i s the

cooling effect of expanding gas a s i t flows down the

oil string. With cooling, and with the pipe immov-

abl e at both ends, there i s a very appreciable in-

crease in tension and, therefore, a calculable de-

crease in resistance to collapse.

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20 H . G . TEXTER

Ordinarily it is hardly worth-while or economical

to try to guard against collapse from mechanical

c a u s e s such a s diastrophism (faulting or earth-

cluakes). Th e in tens it ie s of su ch phenomena are un-

known and are usuall y s o great that design factors

of 5 or 10 or even grea ter st il l would not preventfailure. Even s o l i d bars, for example, would be

sheared off by a major fo rm a t o n sl ippage or

faulting.

tiowever, the probleni has been attacked to some

extent and an interesting paper w a s presented at

API Pacific Coast District meeting in Los Angeles

in Ju ne 1953.'' It is well worth reading.

Preventing collapse froni plastic f l o ~ f s al t beds

has previously been disc ussed .

One very worth-while precaution in the prevention

of co lla pse failure s would be to avoid dingeing of

the casing in handling, either in trucking or pullinginto the derrick. F'ortunately, dinges can be seen

r e a d i 1 y and can be fe lt ~ i t hhe hand; and any

length which might have been knocked out of round

during transportation should be discarded. Passing

an API drift conipletely through each length before

stabbing is considered well worth-while in an ini-

portant casing job.

Even after conin~ercially ound dinge-free c asi ng

has been stabbed, it can be deformed by tonging.

This is not very likely with heavy-wall casing, but

it ha s happened and will, of course, materia lly re-

duce resistance to collapse. poorly fitting or ini-properly designed tongs could cause dingeing; but

it i s more often a result of over-tonging, especially

with one of the non-API shouldering types of joint,

which arrives at a rather a b r u p t end point to its

make-up.

Effect of Wear on Collapse

After cas ing i s cemented, particularly the pro-

ducing string, wear i s undoubtedly a factor in col-

la ps e failur es. l'he wear might be froni drill pipe,

especially where tungsten carbide facing on the

tool joints has rubbed a g a i n s t the inside of the

casing . Even non-hard-faced tool joints will produce

sonie b ea r effect.

l' he us e of rubber protectors, adequate in si ze

an d properly placed on the drill pipe, will effective-

ly prevent wear of the casing.

Rhere wells are drilled in with cable tools, wire-

line cutting must surely be the direct ca use of some

collapse failures. Collapsed casing i s s o seldom

recovered for investigation that we can only guess

at the serio usn ess of wear. However, several ca se s

are on record where wear from tool joints took place

in surface casing which was recovered and exam-

ined. See, for example, Fig. 40a and 40b discussed

in Sect. 7A (Near-Casing) herein .

Effect of Mill Defects

Rlill defe cts , of co urse, can resu lt in low col lap se

values , but there i s a s yet no reasonably inexpen-

sive method of checking collapse strength. Even to

spot-check by making one collapse test per 100

lengths or per carload w o u 1 d be inordinately ex-

pensiv e and time-consuniing. Right now the API is

inves tigati ng the su bje ct of checki ng perforniance

values , and there i s sonie hope that a quick method

of collap se testing shor t cylinders, sel ected from

lots of c a s n g , nlay be developed. l h i s would at

le as t give representative information.

Pendin g the developnlent of some sat isf act ory

perforniance-value t e s t in collapse the next best

thing, in the author's opinion, i s a high-pressure

internal hydrostatic test. Admittedly, this i s not

testing the pipe in the right direction, s o to spe ak,

but experience has i n d i c a t e d that if a tube will

withstand an internal pressure of somewhere around

80 percent of it s calculated yield strength, i t will

also w i t h s t an d an external pressure equal to or

greater than it s published collapse strength. Th e

only possible exception to this would be the case

of an out-of-round or d i n g e d tube, which can be

readily observed and discarded.

Some of the things which a pipe mill can do wrong

in manufacturing a piece of cas ing or tubing havelittle effect s o far a s resist ance to collapse i s con-

cerned. Th is i s true whether it be made by the butt-

weld, lap-weld, electric-weld, or sea nil ess proces s.

F'or example, a tube with a very poorly made butt

weld, which might fail at a v e r y low internal pres--sure, obviously could show a perfectly nornial col-

lapse strength. A s another example, an eccentric

walled s ean ~l es s ube, far b e y o n d the al lowable,

will almost invariably col lap se above i t s minimum

published strength figure. l'h is i s becau se eccen-

tricity in seaml ess pipe i s spiral and the thin si de

i s not in a straight line. As a result, the heavy sidereinforces the thin sid e m u c h more than might be. .imagined.

Very bad surface def ects, s uch a s deep pi ts and

deep seams, well over the API allowable (see Fig.

18a and 18b) have no appreciable effect on collaps e

resistance. Obviously this i s because their area i s

relatively small.

Out-of-roundness, like dingeing, will greatly low-

er the resis tance of casin g to collapse; but it, as

in the ca se of dingeing, can eas ily be discovered.

It would be evident in rolling the pipe on skids or

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OIL-BELL CASING AND TUBING TROUBLES 21

Fi g. 180-Bad Pi ts Which Did not Initiate Collapse

Fi g . l8b-Very Bad Seam Khich Did not Lower Collapse Strength

in dri f t ing . This ord inari ly would be a t ransporta- I

rare .

I iowever, they occasional ly do occur and a good

exan ~p le s i l l u s t ra t ed in F ' ig . 19. ? 'h is was a z3/ ,- in .

OD, 4.7-lb, 3-55 tubing sub which c ol lapsed in a

b e l l a t a depth of 10,54 0 ft . E'rom it s publis hed col-

l ap se s t reng th of 7 ,180 ps i , i t s h o u 1 d have wi th-

stood the hydrostat ic head of 9.625 lb per gal fluid

at 14,360 ft , even though the tube had barely niini-

nlum properties.

Inasnluch a s the specimen i ndic ated normal anal -

y s i s and proper wal l th ick nes s , i t n lust be assumed

that e i ther the tube had been d inged out of round or

i t w a s he ld in c o ns id er ab le t e n s i o n a s b e l l a s

being empty down to the point of collapse. From the

e l l i p s e of b i ax i a l s t re ss es , the ca l cu l a t ed t ens ion

l o a d a t th e f a 1u r e point would need to be only

34,300 l b ~ h i c hs, of course , qui te possib le .

Another possib i l i ty i s that the f lu id (mud) weight

might have been greater than the usual ly assumed Fi g. 19-Tubing Col laps e (from a well)

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2 2 l i . G. TEXTER

Fig. 20a-Casing Burst Failure-Outside View- - ---.--~ ~ - = -. . - -

Fig. 2Ob-Casing Burst-Inside View

gradient of I/, ps i pe r foot of height (9 .625 Ib per

ga l ) . l h u s a 13 .1 l b pe r ga l mud would be enough

t o h a ve c a u s ed c o l l a p se w i t h o u t an y te n si o n in-

volved.

3A. BURSTING-CASING

Mechanism of Fai lure

Usua l ly a bu rs t con s i s t s of a l ong i tud ina l sp l i t ,

some 2 t o 6 f t long, vri th no tea ring a t the end s of

th e s p 1 t , a s i n F i g . 2 0 a an d 2 0b . O c c a s i o n a l l y

t h er e i s a "Y" a t t h e e n d s o f t h e s p l i t a s t ho u gh

the ma te r i a l were s t a r t i ng t o t ea r c i rcumferen t i a l l y ,

a s in E'ig. 21.

F ig . 20a and 20b are examples of duct i le fa i lures ,

and i t i s cha rac t e r i s t i c t ha t t he ou t s ide d i amete r of

t he p i p e i s a p pr e ci ab ly s w e l l e d a l o n g t he s p l it .

T h i s i nd i ca t e s t ha t t he p ipe had expanded in a duc -

t i l e manner j u s t be fo re fa i l ing . T h i s i s we l l - il l u s -

t ra t ed i n a s i de v i ew, Fig. 22, of the fai lu re sho wn

in F ig . 20a . Inc iden ta l l y , t h i s i s an ac tua l example

of a bur st fa i lure recovere d from a h igh-pres sure

vvel l nea r l i ayn esv i l l e , Lou i s i ana , i n 1946 .

If the ca sin g ha s not been properly heat - t reated

o r b a s acc ide n ta l l y quenched o r co ld -worked ( a s by

s t ic k i ng i n th e r o ll s) , t h e s p l i t u s u a l l y w ill b e

Fig. 21-Non-ductile Type of Burst Failure

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OIL-KELL CASING AND TUBING TROUBLES 23

Fig. 22-Ductile Type of Burst Fa ilure (Side View)

rather short and there will be no appreciable swell-

ing of the un spl it portion. E'urthernlore, the pipe i s

broken transversely a t one or both end s of the sp li t

(Fig. 21). Th is i s an example of a non-duc t i le orbrittl e type of bu rst failure.

Even l e s s frequently there will be a secti on (or

several sections) actually blown out of the pipe.

?h is i s illustrated in Eig. 23. l 'his, again, is an

example of a bri t t le type of failure.

Effect of Plug Scores

A mill de fect, familiar to and understood by seani-

le s s tube manufacturers and conducive to burst fail-

ures, i s known a s an internal plug score (s ee Fig.

24). A plug score, and there may be several or none

i n any o ne tu be, i s a l ~ a y songitudinal and usually

runs the full leng th of the tube. It i s the result of

hard bits of metal which may stick to the high mill

plug during the rol ling o p e r a t i o n (r ight after

piercing). In this operation the rough pierced tube

is elongated by rolling between ro 1 1 s and over a

bullet-shaped mandrel or plug, in what is called a

"high mill" or "rolling mill," and it i s during thi s

operation that internal plug scores are forn~ed.Ordinarily plug sc or es are round-bottomed and

have so little effect on bursting strength that it is

only by accident that the bursting split will follow

a scor e, even though it i s more than 12'4 percent of

the wall thickness in depth (and therefore beyond

the API allowable tolerance). Very occasionally,

however, a sco re i s so sharp-bottomed that it is ,

effectively, a sharp notch and bill induce a burst

failure. Unfortunately, the depth of the sc ore i s not

a measu re of i ts effect. So, blindly following the

API tolerances, it i s often passed a s being harm-

les s, a s ha s the case in F'ig. 24.

Neither visual, mechanical, optical, nor magna-

flux inspection can differentiate between a harmless

sco re and a serio us one. So far as we know today,

Fig. 23-Very Britt le Typ e of Burst Failure

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Fig. 14-Internal Plug Score Burst Failure

only a high internal-pressure test, to, say, 80 per-

cent of the calculated nlinimunl yield strength, will

cull out harmful plug scor es. 'l'his i s particularly

true for tubing, a s will be discu ssed under that s ul -

ject.

Effect of Seams

Similar in effect a s a cause of burst failures are

external, and sometinles internal, s e an?s . Seams

(which niay seem an anomalous t e n ] when applied

to seamless tubes) are longitudinal discontinuities

with a left-hand spiral, which exist quite universal-

ly in casing, tubing, and o t h e r tubular products

made by the m o d e r n domestic piercing process.

l' he y are a resul t of rather complex tn ist ing , tear-

ing forces which open up longitudinal slag and gas

pockets d r a w n out by rolling the original ingots

doh n to the rounds which are to be pierced. Unfor-

tunately, steel ingots cannot be cast without some

sl ag inclusions and small gas pockets. The surface

condition of the round from which the seamless tube

i s to be produced and the mill set up are both very

important factors in producing tubes with the mini-

nlum number of outside seams.

Some seams a re s o minute they can be found only

by polis hing and magnafluxing the su rf aces of tubes .

llowever, quite a few can be seen by careful exam-

ination of the unpolished surface. Some seams are

s o utterly harmless that when bursting i s induced

the rupture will not follow the seam s at all. l h i s is

true even of se ams which have depths a s great iss

15 percent of the nall thickness, and it is partly for

this reason that 12'4 percent depth is per ~i~it tedy

the API specifications. Some seams have the ap-

pearance of being what are called hairline or tight

sea ms, and in order to be su re of their depth it i s

necessary to explore the depth by grinding and

checking the depth with a dial indicator.

Heally bad seanls will, of course, tear open under

internal p ress ure, a s illus trate d in I.'ig. 25. It must

not be believed, however, that a seam such a s thi s

one can he detected easily. In fact, experience has

shown that just the o p p o s i t e i s quite often the

case. Bad-looking seams often fail elsevlhere in the

tube, wh ereas harmless-looking o ne s may tear open

under test, a s occurred in the c as e illustrated.

So, a s in the cas e of plug score s, the only logic-

al and sens ible procedure is to tes t

all tubes, regardless of the surface appearance.

Therefore, the API ha s now s e t up new mandatory

and alternative hydrostatic-pressure tes t t a b Ie s ,

Fi g. 25-External Seam Burst Fai lur e

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OIL-WELL CASING AN D TUBIYG TROUBLES 25

Fig. 26-High Mill"0veriill" Burs t Fai lur e

and such te sti ng i s very strongly recomnlended a s

the most logical and sensible method of determin-

ing bursting (and other) perforniance-value proper-

tie s of casin g or other t u bu 1 a r niaterial. I n the

author's opinio ni3 it is utterly ridiculous to "strain

at gnats and s ~ a l l o ~an ~e ls " by discarding pipe

for surface belniishes ~ h i c h eldom affect the per-

formance values and a c c e p t good-looking tubeswhich niay fail in service because of geometry or

metallurgical deficiencies.

Other M i l l Defects

Another, but fortunately rare, type of niill defect

which can r e s u 1 t in burst iailure i s known a s a

high-mill "overfill" or "fin." 'l'his, al so , i s formed

in the rolling operation and i s caus ed by the out-

side diameter of the pierced ijillet being too large

for the groove in the high-mill rolls. Th is defect i s

rathe r difficult to de te ct by mill or field inspect ion

methods short of rnagnafluxing or n~agnagloin~r,

better st il l, by high-pressure testing. An overfill i s

a longitudinal s e c t i o n of the pierced tube which

was squeezed out between the two rolls of the high

mill during the tube's f i r s t pass through the mill

and i s then rolled down and press ed into the out-

s ide s u r f a c e i n the sec ond pa ss . An example of

suc h a defect which failed by bursting in a well i s

shown in Fig. 26 . It failed a t a k n o w n maximum

well pressu re of 1,50 0 psi, disregarding any pres-

sure surges, and undoubtedly would have failed in

a high-pressure t est. Th is would have been 6,600

psi for the size, weight, and grade involved (new

AFI mandatory te st for ?-in., 26-lb, h-80).

Other mill defects, previously mentioned under

tension failures, which might conceivably result in

burst failures, are quenched areas and work-harden-

ed areas (see Mill Defects in Sect. 1, 1-ension).

Welded casing, but seldom seamless, can unin-

tentionally contain laminated areas which have re-sulte d in burst failu res. ?'his comes from skelp (the

sheet s or strips from which welded c a s i n g i s

formed) niade from bloonis which were cropped too

close to the upper end of an ingot. While coolicg

the m o 1 t e n nietal in the top of an ingot mould

s h r i ~ ~ l i sown, leaving an open or porous depression.

Rolling the ingot into bloonis cannot weld this cav-

ity; and if not cropped and discarded , it resu lts in

bars or sheets having slag and dirt inclusions be-

tween two layer s of cle an nietal and is known a s

"laminated" steel. If such skelp is delivered to a

pipe department, t here i s no simple method of de-tecting it and a poor, although normal-looking prod-

uct may be shipped to the consunier.

Here, again, high-pressure testing can eliminate

the defective lengths. I f the lamination i s too sli ght

to fail under an 80-percent hydrostatic test and was

not revealed during a threading operation, it can be

considered capable of meeting all other performancevalues.

Design Factors-Bursting

As for tension and c ollap se, the first step i s to

observe a proper d e s i g n factor in selecting the

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26 H. G. TEXTER

weight and grade of casing. Even though tubing is

run inside, packed off a t the bottom, and the annu-

lus filled with niud, it should be assumed that the

full shut-in pressure of the well might be imposed

on the inside of the ca sing string. The packer might

fail , the t u b i n g may leak or even burst, and t hecasing would then have to withstand the full pres-

sure.

A comnlonly used design factor against burst, for

casing , i s 1.0. T hi s may seem rather low, but it i s

based on the assumption that the well i s producing

pure gas, methane for example, and will have a bot-

tom-hole pressure, in pounds per square inch, equal

to one half the depth in feet and also assumes that

gas has no weight and will have a top-hole pressure

equal to the bottom-hole pressure. l'his, of course,

i s not true and the difference between the a s s u m e d

top-hole pres sure and the actual figure is an addi-tional safety factor.

Thus, it i s quite possible to have a 10,000-ft ga s

well in which the bottom-hole pressure is 5,000 psi

and the top-hole press ure 7 0 percen t of thi s, or onl y

3,500 ~ s iconsidering the weight of the gas). So,

if the top length of c as ing were sel ec ted to have a

1.0 design factor against the bottonl pressure, neg-

lecting the gas weight, the actual design factor,

agai nst 3,500 psi , would be 1.0 + 0.7 or 1.43, which

i s quite reasonable.

Barlow's forn~ul a or calcul ating burst strength of

a tube i s incorrect except for very thin-wall tubes,

but the error is on the side of ~ a f e t y . ~n addition

to this the published i l P l Bul 5C2 figures are only

87'4 percent of the calculated internal yield-pres-

sur e figures, bas ed on published wall thick ness es,

uhich gives a st il l further safety factor. Hence, the

justification for s o low a design factor a s 1.0.

Usually only the bursting s t r e n g t h of the top

length in a string need be considered. Effectively,

there i s no fluid surrounding the top length; but a s

we go down in depth, there may be more and more

external fluid pressu re s o that the differential in-

ternal pressure rapidly beconies le ss and les s.. .I f there i s a change in section, either as to grade

or weight, near the top, then the differential pres-

sure inside the top length of the neb section should

be taken into consideration. Thi s i s very seldom

critical in an average casing string; but i f there is

no flu id surrounding the c as in g, th is fac t m ust then

b e t a k e n i n t o a c c o u n t i n f igw in g th e de s ig n f ac tor .

Pressure Testing

After designing a casin g string again st burst, the

next most important step, in the author's opinion,

i s to hydrostatically internal-pressure t e s t each

length of casing, either at the pipe mill or on the

locat ion, to 80 percent of the ca lculated nlinimum

internal-yield pressure (see new API mandatory and

alternative hydrostatic-test pressures for casing

and tubing7) before running the casing in the well.

Th is i s good practice not only for the productionstring but al so for the outer strings. It i s the sin]-

pl es t perforniance-value te st of ca sing, a s hereto-

fore noted, that has ever been devised.

In addition, for high-pressure wells many com-

panies are adopting the very worth-while practice of

testing the cemented production string before drill-

ing out the cement plug to something over the ex-

pected shut-in top-hole p r e s s u r e . l'his can very

easily be done with modern cementing pumps at a

very nominal c ost; and i t i s much better to encoun-

ter a failure at this stage than after the well has

been brought in.For example, a certain operator in Wyoming tests

the production string, usually 7-in. OD, in 13,000-ft

high-pressure gas hells to 6,000 psi mud pressure

and ho ld s thcs pressu re for 30 min. Thus they knob

that the string waslanded in good condition and any

later fa ilures niust be caused by subseque nt drill-

ing, gun-perforating, or other conipletion operat ions.

Furthermore, t hi s i s a good tes t of the joint tight-

ne ss and gives some indication as to whether the

lengths were screwed up properly.

Effect of Tong Marks

Under Sect. 2U. Col lapse, it ha s been p o i n t e dout that tongs can dinge casing and reduce the col-

lap se strength. T hi s daniage i s even more critical

with respect to bursting because of the longitudinal

notching effect of the tong dies in combination with

the dingeing. l' hi s point i s covered in a paper pre-

sent ed at t he 1947 meeting of t he API Pacific Coa st

Oistrict in Los Angeles. l4 The writer states, "The

critic al combination of notching a t the ax is of the

flattened area reduced bursting pre ssu res a s much

a s 70 percent with notch dept hs of 15 percent to 1 7

percent of the wall thickness and ovality of 3 per-

cent to 4 percent."In 1945 a mill bursting t es t was conducted on

several tong-dinged lengths o f 5'4-in., 17-lb, N-80

casing found on a pipe rack after h a v i ng been

pulled from a well. E ig. 27a illustrates the degree

of dingeing and Fig . 27 b shows that the bursting

fracture originated in the tong marks. Th is partic-

ular sample failed at 8,640 psi; hhereas the true

burst pressure, calculated on the a c t u a l (from strip

tests) ultimate strength of the metal, s h o u 1 d be

14,370 psi . Thus , the conibination of dingeing and

die n o t c hi n g made a 40-percent reduction in the

bursting strength.

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OIL-KELL CASING AND TUBING TROUBLES 27

F i g . 2 7 0 -T o n g- d in g e d C a s i n g

F i g . 2 7 b -C a s in g B u r s ts C a u s e d by T o n g D i n g e i n g

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28 H. G.TEXTEH

Biaxial Stress Effect

Under Sect. 213. Collapse, it has been pointed out

that the effect of t ension i s to decrea se the resis-

tance to collapse. Fortunately, the reverse i s true

for bursting. l'he hadai-Holmquist paper s h ow s

that, up to the yield point in tension, the effect of

tension i s to increase the resi stance of casing to

burst. Ordinarily, because t his factor usually is not

t a k e n into account in calculating safety factors

agains t bursting, t here i s an added uncalculated in-

crease in safety factor.

Unusual Burst Failure

Right here the writer would Iike to review a very

peculiar burst (?) failure of casing illustrated in

E'ig. 28. l'he circurt:stances are int ere sti ng and con-

cern a well in I'ennsylvania kvhich suddenly blew

'IPSl 'he well, being drilled with c a b l e tools, Bas

ca se d principal ly \*,it11 7-in. OLi, 26-lb, 5-55 seam-

le ss s et ins ide 85/,-in. 01). It had encountered a gas

stratulil causing various troubles and resulting in

riluch fishing inside the 7-in. string and a long peri-

od of inactivity. ?'hen, quoting fro111 an invest iga tion

report:

"G'n Augus t 15,1951, the well was bailed partial-

ly dry and c a p p e d with 1,800 psi g as pressure

showing on the ?-in. c a s n g . On August 18 the

7-in. casing burst, transferring the pressure to the

8'/,-in. s t r i n g ~ h i c h hen blew ofl at the casing

head through a 2-in. valve and uas hed out a ditch"a t ground level. After killing the g as flow, it was

found the top length of 7-in. OE, 26-lb, 3-55, just

below the 1 3 ft of 7- in., 35-lb, IV-80 (landing nip-

p le) , had b u r s t (?) and fragmented into a large

nuwber of slllall pieces each of which had to be

fished individual ly from the well bore. At the time

of our visit, they had been retrieving for 8 days

and had recovered 91 small piece s. l ' h e s e had

been weighed and it was establ ished that so far

19 ft of thi s range 2 length had been recovered."

Some 80 of these pi eces are s h o w n in E'ig. 28.

LaLoratory exan~i natio n indicated good ductility,

vrhich strongly indicated either some sort of explo-

sion or possibly hydrogen enlbrittlement. The latter

t h e o r y seemed unlikely bec aus e only ..one length

was involved, and this theory does not explain the

very excessive shattering involved.

F'urther recovery of the string showed bad wire-

line cutting; s o it was finally deduced that the fol-

loning chain of events might have occurred:

a.A

wire-line cut spo t well d o w n in the ?-in.st ring had finally broken through.

b. Gas at 1,800 psi had rushed through the open-

ing into the annulus between the 7-in. and the

8%-in. strings.

c. ?'he gas compressed the air in the annulus a s it

forced it upward and, in compressing the air,

raised it s teniverature.

d. At some point near the top of the annulus there

would be a critical mixture of air and gas, and

the "diesel effect" of rapidly conlpressing the

mixture served to se t off an explosion (or rathe ran intplosion).

Some credence is given to the phenomenon having

been an inlplosion by the fact that many of the re-

covered pieces seemed to have been Lent inwardly

ins tead of outwardly. It i s a very interest ing form of

cas ing failure and i s included a s a matter of record.

F i g . 28 -Casing Which "Blew Up" i n a Well

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OIL-WELL CASING AND TUBING TRO UBLES 29

3B . BURSTING-TUBING

M o s t c o m m e n t s o n t h i s s u b j e c t , u n d e r C a s i n g ,

apply equal ly wel l to tubing .

B u r s t i n g i s a re la t ively common form of tubing

t r ou b l e, e s p e c i a l l y i n h i g h - pr e ss u r e w e l l s , a n d c a nb e e x c e e d i n g l y s e r i o u s . F o r e x am p l e , a s t r i n g o f

23/ ,-in. tub in g fa i led by sp l i t t in g near t he top of a

13 ,500 - f t we l l i n s ou th Texas and had to f low p rac -

t i c a l l y w i d e o p e n f o r s o m e 8 m on ths b e f o r e th e

owner s da r ed t r y to s hu t in and k i l l t he we l l . Had

they s hu t in ea r l i e r , t he upwards o f 9 ,700 p s i p r e s -

s u r e w o u l d h a v e b e e n i m p o s e d o n t h e s u r r o u n d i n g

5'4-in. OD, 20-lb, N-80 s t r ing , wh ich p robab ly a l s o

w o u l d h a v e b u r s t a n d p r e s s u r e d a l l t h e h o r i z o n s i n

t h e a r e a . H a d t h i s h ap pe ne d, a f i e ld po ten t i a l ly

wor th $175 ,000 ,000 migh t have been ru ined .

F i g . 29a-Tubing Plug Score Burst Failure

A r e l a t iv e l y n e w c a u s e o f b u r s ti n g t u b in g i s i t s

u s e in hyd rau l i c f o rma t ion - f r ac tu r ing ope ra t ions .

T h e p r e s s u r e s i n v o lv e d are s o m e t i m e s q u i t e h i g h .

P l u g S c o r e s

A s i n c a s i n g , p l u g s c o r e s ma y b e a f o c a l po i n t

f o r f a il u r e. T h e w r i t er i n v e s t i g a t e d a c a s e ( i n L o u i-

s i a n a i n 1 9 4 7 ) w he r e f o u r l e n g t h s o f t u b in g f a i l e d

s u c c e s s i v e l y i n a s t r i ng of 2% - in . wh ich wa s be ing

t e s t e d u p t o 4 , 8 0 0 p s i b e f o re b e i n g o p e n e d up fo r

p roduc t ion . Al l of t he s e f a i lu r e s were in s ha rp -bo t-

tomed , bu t no t ve ry deep , p lug s c o r e s ( s e e F i g .

2 9 a) . W h e n c u t o p e n fo r e x a m in a t io n , t h e s c o r e s

were found to be w i t h i n t h e A P I a l l o w a b l e d e p t h

s pec i f i ca t ion , bu t were ve ry s ha rp -bot tomed ( s e e

photomicrograph, F ig . 29b) . The y could not have

been culled out by any mill or f ield-inspection rneth-

od shor t of h igh-pressure tes t ing .

Tong Marks

T o n g c r u s h i n g o r n o t c h i n g i s j u s t a s f a t a l f o r t ub -

i n g a s f or c a si n g . F i g . 3 0 i s a vie^ of s ome 2% - in .

OD t ub in g w hi ch f a i le d i n a f a i r l y d e e p w el l a t

N e i b e r Dome, Wyoming, in 1947. Very evidently

d e e p t o n g m a r k s w e r e d i r e c t l y r e s p o n s i b l e , as t h e

s h u t - i n p r e s s u r e w a s r e p o r t e d as be ing on ly 1 ,500

p s i .

Fig 29b (X40)-Microphotograph of Pl ug Score

in Fig. 29a

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30 H.G . TEXTER

Fig. 30-Tubing Burst Caused by Heavy Tong Marks

Preven t ion of Bur s t Fa i lu r es of Tub ing

Th e sam e p recau t ions ob ta in a s f or cas ing . Above

all, before running tubing in to h igh-pressure wel ls ,

t he l eng ths shou ld be t e s t ed to the new API high-

pres sure f igures and rat t le d wi th a i r hammers whi le

under tes t . A lso , if pract ica l , the s t r in gs should be

h igh-p ressu re t e s t e d in the w e l 1 , a s w as r ecom-

mended for casing.

4A.LAST ENGAGED THREAD FAILURES-CASING

For cas in g l a s t engaged thr ead f a i lu r es a r e p r e t ty

much a th ing of the past , but the pr inciples should

be s e t down a s a mat ter of reference.

Under Sect . 1A. Ten sion , i t wa s mentioned tha t

c a s i n g s o m e t i m e s f a i l s by fracturing at the root of

t he l a s t e n g a g e d thread. A much more common

c a u s e of t h i s ty p e of f a il u re i s no t t e n s i o n , bu t

f a t igue ; and in these c ase s , t ens ion i s of ten a min-

or or negl ig ible factor even though the resul t looks

l ike a tension fa i lure . Lack of knowledge of the

ca us es of l a s t engaged th r ead f a i lu r es l ead s to an

Mechanism of Failure

It seems to be almost purely a notch phenonienon

in which the root of a thread i s the notch an d the

fat iguing act ion i s that of bending. Th e angle of

bend i s ac tua l ly minu te , bu t i t i s bending neverthe -

l e s s .I n nicking and breaking a rod in a v ise , the bend-

i ng a c t i o n i s of h igh am pl i tude but involves very

few s t r e ss r ev er sa l s . In c as in g f a i lu r es , t he bend ing

act ion i s con ceived to be of ext remely low ampl i-

tude but of a very high number of s t r es s reversa ls .

In o ther words, t he bending ef fect i s th at of a v ibra-

t ion of a f requency approach ing that of sound wave s.

T he c au se of the v ibrat ion i s the pounding of the

unprotected dr i l l s t r ing (dr i l l p ipe, dr i l l col lars , or

k e ll y) w hi ch i s r o t a t e d i n s id e t he c a s i n g s t ri n g

whi le dr i l l ing ahe ad. Not much dr i l l ing i s done in-

s i d e a product ion s t r ing, but a great dea l i s doneins ide the ou te r s t r ings ; hence the r eason fo r t he

much gre ater f requency of la st engaged thread fa i l -

u r es in secondary s t r ing s a s compared wi th the f inal

s t r ing.

The obvious s tep for prevent ion of the t rouble i s

to dampen the blows of the dr i l l p ipe ag ain st the

ins ide of t he cas ing . T h i s i s r ead i ly accompl i shed

by the use of heavy rubber protectors on the dr i l l

p ipe, kept c lose to the tool jo in ts . The la t ter , as

a so r t of para si t i c ma ss, are the principal of fenders

in casing vibrat ion.

F ig . 31a i s a ve ry r ep resen ta t ive example of a

la st engaged thread fa i lure in a length of 8% -in . OD,

28- lb , H-40 casing f rom an Andrews County , Texas,

wel l in 1947. The fa i lure occurred 62 f t below the

su r f ace in a 4,000-f t secondary s t r ing, af ter 24 days

of dril l ing. I t was learned that a crooked kelly with

- -incor rect approach to thei r e l iminat ion. I Fi g. 3la -La st Engaged Thread Failure-Casing

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OIL-WELL CASING AN D TUBING TROUBLES 3 1

no rubber protector was being u s e d and that i ts

pounding was undoubtedly the source of the vibra-

tion resulting in the failure.

Fig. 31b shows a sketch of this particular size

and weight of cas ing , screwed into i ts coupling, and

with a line showing the point of failure vis ibl e inthe photograph. hleasured up from the point of the

pipe thread, i t i s the la st root of a thread engaged

with a coupling thread which i s the notch a t which

the minute bending st re ss was concentrated. Hence

the term "la st engaged thread failure."

Factors Involved

In the early days of drilling the Big Lake Field

in Te xa s and the Oklahoma City and Cr e s c e n t

Pools in Oklahoma, the author spent considerable

time studying last engaged thread failures and came

to the following well-demonstrated c o n c 1 u s i o n s

which should be a pplica ble al n~ os t nywhere:

a That the great majority of them occurred in 1034-

in. and 9%-in. secondary strings, s e t at 3,500

to 6,000 ft.

Fi g. 31b-Diagram of La st Engaged Thread Failure I

Fig . 32 (X100)-Photomicrograph of an Incipie nt

Last Engaged Thread Failure

b. That all failures occurred above the cement or

fre eze point.

c. That the straighter the holes, the more the fail-

ures. Crooked holes tend to damp the vibration.

d. That in practically every case rubber protectors

were not being used on the drill pipe, were bad-

ly worn, or were not kept close enough to the

tool joints.

e. That tension was either a very minor factor or

not a f a c t o r at all . In one case in Texas, 15

la st engaged thread failures were f i s h e d out

from one string of 9%-in. After the first failure

some of the pipe must necessarily have been in

compression, not in tension.

f . That the old API sharp-V thread was more sub-

ject to fatigue fracture than the present round-V

thread. See Fig. 32, a photomicrograph showing

the start of a last engaged thread crack at the

root of a thread in 9%-in. OD, 36-lb, old Grade

C-45 casing. Note the very sharp root of t h i s

thread.

g. That the grade of steel i s not a f a c t o r . All

grades and a l l makes of pipe, seamless and

welded, were involved.

Inasmuch as, in the last decade, very few cases

of last engaged thread failures in casing have come

to the author's attention, it c an confidently be as-

sumed that the difficulty has been solved-principal-

ly by insuring that ample rubber protectors are on

the drill pipe which i s used inside the outer string

and, secondarily, by the almost universal adoption

of round-V threads.

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32 H. G . TEXTER

4B . L A S T E N G A G E D T H R E A D

F A I L U R E S - T U B I N G

A s w i th c a s i n g , t h i s i s n o lo n g e r s o p r ev a l e n t a

t r o u b le a s i t w a s s o m e y e a r s a go . N e v e r t h e l e s s , i t

d o e s s t i l l e x i s t a n d t he s a m e r e a s o n in g a p p l i e s a s

t o t h e c a u s e . H o w e v e r, k i t h t u b in g t h e v i b r a t i on s

a r e c a u s e d b y p u m p in g o p e r a t i o n s i n s t e a d of b y d r i l l

p ip e . A l so , i t o c c u r s m o s t o f t e n i n n o n - u p se t t u b ing .

T h e h e a v y w a l l u n d e r t h e t h r e a d s o f u p s e t t u b in g

u s u a l l y p r e v e n t s t h i s t y p e o f f a t i g u e f ai l u re .

As w i th c a s in g , i t o c c u r r e d m u c h m o r e f r e q u e n t ly

in t h e d a y s of t h e o ld sh ar p -\ : t h r e a d s t h a n to d a y.

P r e s e n t r ou n d -to p , r ou n d- bo t to m th r e a d s n a tu r a l l y

p r e s e n t l e s s o p p o rt u n it y fo r s t r e s s c o n c e n t r a t i o n,

wh ic h i s t h e f o r e r u n n e r of a f a t i g u e c r a c k . ( Se e d i s -

c u ss io n u n d e r 4 A . Ca s in g . )

A good example o f a t y p i c a l L a s t e n g a g e d th r e a d

f a i l u r e is i n F i g . 33. T h i s i s o ld 10-V ( sh a r p ) t h r e a d

t u b in g w h i c h h a d b e e n i n s e r v i c e f or 7 m o n th s i n a

2 ,2 0 0- ft so u t h L o u i s i a n a pu m p in g we l l i n 1940.

P r e v e n t i o n of t h i s tr o u bl e i s o b v io u s : A v o i d

sh a r p - V th r e a d s a n d u se o n ly u p se t t u b in g in a l l e x -

c e p t v e ry sh a l lo w , s lo w- p um p in g, we l l s .

5.LEAKAGETHROUGHTHREADED

C O N N EC T IO N S -C A S IN G A N D T U B I N G l 5

O r d i na r il y o n e t h i n k s o f flu id l e a k a g e a s a r e s u l t

o f i n t e r n a l p r e s su r e ; i .e . , t h e f l uid i s l e a k in g fr omt h e i n s i d e o f a p ip e t o t h e o u t s id e . Ho we v e r, l e a k -

a g e i n e it h e r d i re c t io n m us t b e co n s id e r ed s i n c e

t u b ul a r s t r i n g s m u s t r e s i s t b o th i n s i d e a n d o u t s i d e

p r e s s u r e s . T h e s a m e f a c t o r s a p p ly i n e i t h e r d i r ec -

t ion.

T h r e a d S h a p e

F ig . 34 a n d 35 s h o w i n g r e a t ly e n l a rg e d s c a l e t h e

d e t a i l e d s h a p e o f API c a s in g ( o r t u b in g ) t h r e a d s ,

b o th t h e o ld sha r p -\ / a n d th e p r e se n t r o un d th r e a d s .

As in d i c a t e d , t h e m a t in g th r e a d s o f p ip e a n d c o u -

p l in g a r e d e s ig n e d to m e e t f la nk to f l an k w i th o u t a p -

~ r e c i a b l e le a ra n ce ; bu t t h er e i sa

def in i te , a l thoughs m a l l , c l e a r a n c e p u r p o s e ly l e f t b e t w e e n t h e t o p o f

e a c h t h r e a d a n d i t s m a t i n g r oo t. T h e r e a s o n f o r t h i s

c 1 e a r a n c e i s t h e i n a b i l i t y o f p ip e - th r e ad in g m a-

c h i n e s t o p r o du c e t h r e a d s s o a c c u r a t e l y, t i m e a ft e r

t im e , t h a t e a c h c r e s t w o u 1 d fi ll c omp le te ly eac h

m a t in g r o ot a s r e l i a b ly a s e a c h flan k c a n b e c u t t o

f it i t s m a t i n g fl an k. T h e c h a s e r s w h i c h a r e u s e d t o

c u t t h e t h r e a d s c o u ld b e m a de t o d o th i s a f ew

Fig.33-Last Engaged Thread Failure-Tubing I Fig. 34-Obsolete API Sh ar py Threads (Enlarged)

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Fi g. 35-Present AP I Round Threads (Enlarged)

times, but the tops of the thread chasers of the die

head wear much more rapidly than the roots (or theflanks), so that the r e s u l t a n t roots in the pipe

threads would soon have a larger radius than the

resultant crests. Then, when m a k i n g up such

threads there would be heavy bearing between mat-

ing crests and roots with the result that the flanks

would no longer meet. So the le ss er of two ev il s is

chosen and positive clearance between cre sts and

roots is provided. Thi s clearance gradually becomes

less and less as the ch ase rs wear, but there i s no

guarantee that the fit ever will be perfect a s to all

elements.

Mechanism of Leakage

It must be evident, then, that if perfectly dry pipe

and coupling thread s could be so tightly scre wed

together (without galling) that the flanks came into

proper contact, there still would be a small helical

aperture between the crests and roots through which

fluid could and would flow. Eve n a light lubricant

can seal off this h e l i c a l passageway, but only

against a relatively low differential pressure. The

pressure to c a u s e a leak would be only that re-

quired to overcome the capillary attraction inertia

of the fluid already in the hel ical passag eway.

It shoul d be mentioned th at there c an be a "hill-

and-da le" form of leakag e in which the fluid moves

axially instead of helically. In this case the path of

leakag e, a s indicated by the name, i s up and down

a s well a s forward. However, to p er ni i t this, the

flank contact would have to be extremely poor. The

ide a i s mentioned only to conlplete the picture.

Going back to helical leakage , a 1u b r i c a n t of

higher visc osi ty would withstand a greater pres-

sure. Still better, then, would be to mix some sort

of sol id filler with the lubricant, and this i s pre-

cis ely what i s done to make pipe-thread compounds.

Comnlon fillers ar e red l ead, graph ite, and powdered

metals such a s zinc, lead, or c o p p e r . 'hese are

usually mixed with calcium or aluminum soap-base

greases.16 Even su ch mixtures a s white lead and

linseed oil make excellent se ali ng agents; but they

are not true..ilubricants beca use they harden with

age and will not break out satisfactorily.

It can be demonstrated that properly cut pipe and

coupling threads, w i t h i n the API specifications

using existing lubricants, can be screwed together

s o a s to make joints that will be tight agains t wa-

ter, oil, or gas up to the yield strength of the pipe

body for e i t h e r inter nal or external pressure. At

le as t this can be done in a mill or shop; but to do

it in the field, under often adverse working condi-

tions and weather, i s an entir ely different proposi-

tion, hence the great amount of leakage trouble re-

ported when casing or t u b i n g is field-tested for

leakage, or strings are tested in wells.

Preventive Steps

In API RP 5C l: Rec omm end ed Practice for Care

and U se of C as in g, Drill Pip e, and Tubing, instruc-

tions and suggestions are given for properly clean-

ing, lubricating, and r u n n i n g pipe into wells for

maximum assurance against leakage. The instruc-

tions will not be repeat ed here, but i t might be well

to list the various factors and their effects:

a Cleaning. If not properly cl eaned, the joint s will

not make up far enough and leakage can result.

Also sand or grit can cause galling, with likeli-

hood of leak age ( se e Fig. 53).-b. Lubricant.16 Should contain one or more of the

fillers, such a s zinc, powdered copper, gap hi te ,

etc., mentioned previously. Too thin a lubricant

may seal tenlporarily but be forced out in time

under pressure. Too heavy a grease will not

s p r e a d properly, especi ally in cold weather;

leaving unprotected metal with resultant galling.

c. Stabbing. Careless stabbing can damage start-

ing threads and cause galling.

d. Spinning. Rope spinning, before tonging, can

be done at too high speed and result in galling,especi ally becaus e of si de pull involved.

e. Tonging. Insufficient tonging can'result in leak-

age. Over-tonging may cause galling, with pos:

sible leakage.

f. Tension. There i s a common misconceptiori that

h i g h tension i s conducive to leakage of API

threads. Th is i s based on the theory (disc ussed

under Tension) that the threads s li p apart as a

result of the radial con~ponentof the longitudin-

al force, and provide an opening for escape of

fluid. However, i t is now known that there i s 'n o

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34 H . G . TEXTEH

more tendency for a threaded connect ion to leak

at high tens ion loads than at zero load.17 So,

th i s be ing t rue for e i ther ca s ing or tub ing , there

i s no mer i t in let t ing off tens ion af te r s t r ing s

h a v e b e e n l a n d ed , s o f a r a s t h i s s c o r e i s c on -

cerned . O ther f ac tor s should be the c r i t e r i a gov-ern ing l and ing prac t i ce .

Of the var ious factors t ied in wi th leakage, the

lubr ican t used i s o f h ighes t importance . Be cau se of

th i s the API r ecen t ly has comple ted a s tudy , w i th

the Mel lon Ins t i tu te , to de te rmine nha t charac te r i s -

t i cs are most impor tant and what combinat ion of in-

gred ien t s w i l l g ive these charac te r i s t i cs .16

Pa r t ly becau se of the p rac t i ca l d i f f i cu l ti es en-

countered with the present API threads , even with

t h e b e s t l u b r i can t s , s ev e r a l p a t en t ed ca s i n g an d

t ub in g j o i n t s 6 ~ l 8 a v e b e en d e ve lo p ed i n w h i c h

there i s some sor t of po s i t ive m eta l- to -meta l o r gas -

ke t s ea l . These do no t depend upon a lubr ican t ex-

cep t to p reven t ga l l ing , and have been used suc-

cessful ly for many years . An object ion to their use

i s t h e ir g r ea t e r co s t an d t h e f ac t t h a t, b e i n g p a t en t-

ed, they have never b e c o nl e an API s t andard and

ar e therefore not interchange able, one mil l wi th an-

other .

P reven t ion of L e a k a g e

T h e n ece s s a r y s t ep s t o p r ev en t l e ak ag e s h o u l d

be obvious . Of nios t in~por tanceare cleaning of the

thread s , u se of a proper lubr ican t , and careful make-up. In other words , fol low n~ et i cu lo us ly he recom-

mendat ions in Sect . I , API RP 5 C l .

B e c a u s e of t ime or handling f ac tor , s t i l l ano ther

s t e p i s becoming a lmos t mandatory w here high pres -- .

s u r e s a r e i n vo l ved .

E xp er ie nc e h as s h o ~ nhat threaded tubular prod-

ucts wi l l of ten show 1 e a k a g e ( b et we e n p i p e a nd

coupl ing) dur ing f ield tes t ing even though no leak-

a g e w a s e vi d en t d u r i n g m ill t e st in g . T i m e i s t h e

fac tor invo lved , p lus handl ing . The lubr ican t s eems

to dry out and the vibrat ion caused by rai l road and

other hau ling aggrav a tes the condi t ion so tha t sh ip-

ping to South America, for examp le, b i l l show up to

30 , 40 , and even 50 percen t l ea ker s when h igh-pres -

sure t es t ed on loca t ion . l 'he l eak er s a re all a t t h e

mil l connect ion.

l 'he l eaka ge can be and i s stopped by s imply un-

screwing the coupl ings , r e -greas ing bo th th reads ,

and re -app ly ing , T hi s i s now regarded a s a very e f -

f i cac ious method of l eaka ge preven t ion and i s fo l -

lowed by a number of larg e conlpanies , es pe cia l ly

for tubing s t r i n g s i n h i g h - p r e ss u r e w e l l s . I t s u g -

ge s t s , of cou r se , tke adv i sa b i l i ty of sh ipp in g tub ing

w it h t h e co u p l in g s b o x e d s e p a r a t e l y , o r ap p l ied

only handl ing t igh t , s o tha t bo th male and f emale

t h r ead s w i ll b e f r e sh l y g r ea s ed b e f o r e b e i n g r un

into the wel l and permit the coupl ing to "f loat" into

p o s i ti o n n h i l e b e i n g m ad e u p.

Th e foregoing prac t i ce cou ld jus t a s wel l be ap-p l i ed to cas ing , bu t i t i s not be ing done to the bes t

of the au thor ' s knowledge . One r eason i s bec aus e

cas ing for very h igh-pres sure w el l s o f t en i s a pos i -

t ive se al i ng type of integral de s ign , not involving

a coupl ing.

T es t i n g ag a i n s t L eak ag e

U n d e r Sec t . 3A . Burs t ing-Cas ing , i t w as sug-

g es t ed t h a t c a s n g s t r i n g s m ay b e , an d a r e , p r e s -

sure t es t ed in p lace before d r i l l ing ou t the cement

p lug . T hi s i s not on ly a chec k aga in s t bur s t ing bu t

i s , of cour se , a l so a check aga ins t l eakag e through

the t h r e a d e d c o n ne c ti o ns . T h e s a m e p r a c t i c e i s

son le t imes fo l lowed for tub ing and i s ' t o be h igh ly

recommended, e spe c ia l ly where high pre s sur es a re

involved.

A lso , fo r tub ing there a re a t 1e a s t two t es tin g

serv ices in the f i e ld which can t es t very e f f i c i en t ly

t h e s i n g l e , o r s ev e r a l , m ade -u p j o i n t s a s t h e s t r i n g

i s be ing lowered in to the wel l . T hus , p rede termined

p r e s s u r e s c an b e i m p o s ed on e v e r y co n n ec t i o n , a s

made, and may be he ld fo r as long a s des i r ed (usu-

al ly around 5 s e c ) . T h e a u t h o r very highly recom-

mends this pract ice for running tubing in any high-pres sure wel l .

T h i s s e r v i ce co u ld b e ad ap t ed to c a s i n g b u t, s o

far as the wri ter knows, has not yet become com-

mon practice.

6. CRUSHING BY SLIPS AND TONGS

T h i s s u b e c t h a s a l re a d y b e e n t o u c h ed u p on

under Col l a pse and Burs t ing . Whether it i nvo lv es

cas ing or tub ing , the obvious p reven t ive m easure i s

to use s l i p s of suff icie nt length and tongs of proper

des ign for the load involved.

F ig . 36a and 36b show very f l agran t care les snes s

in tonging some len gth s of 5% -in. OD, 14 1b , H-40

cas ing ( in Magnoli a F ie ld , Arka nsas , in 1943). Th e

r ing sec t io ns show how badly the tub es were de-

formed and the tong marks ind ica te how non-uni-

formly the tong d ies had gr ipped the p ipe . Inves t i -

ga t ion ind ica ted tha t the tong head wa s bo th worn

and out-of- round. Th e remedy for trouble su ch a s

t h i s i s o b v io u s .

F 'ig. 37 i l lu s t ra tes a s l ip -cru she d 5 '4- in. , 20- lb,

N-80 ca sin g length from the top of a 9,200-ft north-

we s t Colorado well . Very ev iden t ly the cas ing hea d

s l i p s h e r e t o o s h o r t fo r t h e l o ad .

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OIL-KELL CASING A1

Fi g. 36b-Result of Very Careless Tonging

Fig. 38 is another example of s l ip-crushed cas-

ing, involving 8,350 t of 7-in. OD, 26-lb, N-80 in

a Ventura , California , well . T he welding at the top

i s where a short length of cas ing had been welded

onto the crushed one for lifting the pipe out of the

s l i p s .

Fig . 37 s h o ws th e s l i p s h a d m e r e 1y been too

short . In th is second example the s l ips may have

VD TUBING TROUBLES 35

I Fig.37-Slip-crushed Casing-Slips too Shon

I Fig.38-Slipcrushed Casing-Slips Ou t of Alignment

been long enough, but they were very evidently not

pos it ioned p roperly and f a i l e d to gr ip uni fo rmly

around the o utsi de of th e pipe.

I There i s no excuse for s l ip c rush ing in cas ing

I heads. Modern hea ds a re so well-designed that any

I loads, up t o th e rnarimum t ens i le s t reng th of the

I body of t h e p ip e, c an be w i t h s t o o d p er ma ne nt ly

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36 H . G . TEXTER

Fig. 39-F a ta l Res u l t o f a " Co cked " T u b in g S pider

w i t h o u t a p p r e c i a b l e d e fo r m a ti o n of t h e c a s i n g . T h u s

c a s i n g s t r i n g s m a y b e l a n d e d a n d h o ok e d u p i n fu l ltension w i t h o u t f e a r o n t h i s s c o r e .

F i g . 39 i l l u s t r a t e s a p i e c e o f 2 % - i n .OD,

6.50-lb,

N-80 t ub in g in wh ich a h o l e h a d b e e n p u n c h e d b y

a c o c k e d t u b in g s pi de r -a n ot h er e x a m p l e o f r a n k

c a r e l e s s n e s s .

?A. WEAR-CASING

T o d a y t h e p ri n c i p a l w e a r o n t h e i n s i d e of c a s i n g

i s tha t c aus ed by ro ta t ing d r i l l p ipe in s ide . F 'o rmer -

ly , when we l l s were qu i t e common ly d r i l l ed - in wi th

c a b l e t o o l s, w i r e -l i n e w e a r ( c u tt i ng ) w a s a s o u r c e

o f s ome t roub le . B o th type s of wea r ca n r e s u l t i n

e i t h e r c o l l a p s e o r b u r s t f a i l u r e s. If t h e w e a r i s c o n -

A c a s e w h e r e w e ar b y dr il l pi pe r e s ul t ed i n a

b u r s t f a i l u r e o f a s u r f a c e s t r in g i s s h o w n i n F i g .

40a . T h i s 10% -in. , 49 .5-1b ca s in g was r ecove red

f ro m a L o u i s i a n a w e ll . I t h ad b e e n s e t at 3,035 f t

and 4' 4- in . d r i l l p ipe w as ro t a t ing in s id e , equ ippe d

wi th tungs ten c a rb ide banded too l j o int s . Wear andf a i l u r e o c c u r r ed a t a b o u t 1 1 5 f t f ro m t h e t o p a n d t h e

c a s i n g w a s e a s i l y r e c o v e r e d f o r e x a m in a t io n . I t c a n

b e s e e n how t h in t h e w a l l h a d b e e n w or n b e fo re

fa i lure .

F i g . 4 0 b w a s t a k e n i n s i d e a t a n u n s p l i t p o rt io n

of the c a s n g b u t d i r e c tl y i n l i n e w i t h t h e s p l i t .

Here , appa ren t ly , t he d r i l l p ipe had ro ta t ed fo r a

t i m e w i t ho u t m ov i ng v e r t i c a l ly , a s i s e v i d e n c e d b y

t h e g r o u n d a r e a s c o rr es po n di n g e x a c tl y w it h t h e

wid th and s pa c in g o f the ha rd - su r f ac ing b ands on

t h e t o o l jo i n ts . O t h e r c i r c u m s t a n c e s , s u c h a s th e

c a s i n g s t r i n g n ot b e i n g v e r t i c a l, w e r e i n v o l v e d i nt h i s p a r t i c u la r w e l l ; b u t t h e f a c t re m a i n s t h a t t h e

c a s i n g was worn ve ry bad ly by the d r i l l s t r ing .

F r o m a w e l l i n s o u t h w e s t T e x a s t h e re w a s r ec o v-

ered , in 1945, a length of 5 '4 - in . T & C c a s i n g i n s i d e

c e n t r a t e d a t a j o i n t , a t e n s i o n f a i l u r e c o u l d b e

c a u s e d , b u t t h i s i s a r a th e r r e m ot e p o s s i b i l i t y .

E v e n t h e s t e e l -c l a d i n s u l a t e d l i n e s u s e d i n e l e c -

t r ica l -logging o r gun -pe r fo r a t ing de v i ce s ca n c au s e

q u i t e a p p r e c i a b l e w e a r . R e c e n t l y t h e a u t h o r l o o k e d

over s ome 11 ,500 f t o f 7 - in . OD c a s i n g w h i c h h a d

been r ecove red f rom a very de ep w el l in Wyoming.

Very s u rp r i s ing ly , 38 l e n g t h s s h o w e d w i r e - li n e c u t-

t i n g u p t o a b o u t '4, i n. i n dep th in s p i t e of the f ac t

t h a t n o c a b l e d ri ll in g h a d b ee n d o n e i n si d e of i t .

On ly e l ec t r i ca l - logg ing o r gun -pe r fo r a t ing l ines had

been run over a per iod of about 3 months , and wi th

a to t a l t ime o f be ing in s ide t he ca s in g of no t ove r- u

3 0 h o ur s . T h i s r a t h e r s t a r t l i n g d i s c o v e r y l e a d s o n eto wonder whe the r va r ious uns o lved bu r s t o r co l -

l a p s e f a i l u r e s m i g h t n o t b e a t t r i b u t a b l e t o t h i s t y p e

of wire-line wear.

G o i n g ba c k t o d r il l -p i p e w e a r , i t i s o b v i o u s t h a t

t h i s c o u ld b e a v o i d e d by e q ui p pi ng t he d ri ll in g

s t r i ng wi th p rope r ly p l aced rubbe r p ro tec to r s . How-

e v e r, t h i s i s n o t a l w a y s d o n e, e s p e c i a l l y w h e r e t h e

c a s i n g p r o du c t io n s t r i n g i s q u i t e l o n g o r w h e r e tu b -

i n g i s u s e d f o r t h e d r i l li n g -i n s t r i n g . S m a l l - s iz e d

r u b be r p r o t e c t o r s a r e n o t a l w a y s i m m e d ia t e ly a v a i l -

a b l e o r t h e y a r e n o t b e l i e v e d t o b e n e c e s s a r y.

Fi g . 40a-Casing Worn by Too l Jo in ts

F i g . 40b-Wear Spots from Tungsten Carb ide Banded

T o o l Jo in ts

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OIL-WELL CASING AND TUBING TRO UB LES 3 7

of wh ich the r e was a worn s vo t unde r the th r eads .

runn ing long i tud ina l ly , wh ich ve ry ev iden t ly was

the r e s u l t of r o t a t ion o f thc tub ing s t r in g wh ich had

d r il l e d o u t t h e c e m e n t pl u g . O t h e r c i r c u n ~ s t a n c e s n-

d i c a t e d th a t th e w e ar w a s a r e s u 1 t of t he cas ing

h a v in g b e e n s e t w i t h e n o u g h c o m p r e s s i o n th a t , a t5 , 2 5 3 f t , o r j u s t 1 7 f t b e l o w t h e o u t e r (9 '4-in .)

s t r ing , i t h ad buck led in to a cav i ty -pe rmi t t ing the

d r i l l in g t u b i n g t o r u b ha r d a g a i n s t t h e c o n v e x s i d e

of the buckle .

In s id e wear o f cas ing f rom tub ing s ome t ime s oc -

c u r s i n pu m p in g w e l l s a n d h a s b e e n s t r o n g ly s u s -

pec ted o f hav ing caus ed f a i lu r e s . Undoub ted ly the

1 a c k o f v e r ti c a l i t y of t h e c a s i n g h a s b e e n a con-

t r i b u t in g f a c t o r ( s e e 7B. Wear-Tubing).

A n e x a m ~ l e f u n u s u a l w e a r o n t h e i n s i d e of c a s -

i n g i s i l l u s t r a t e d i n F i g . 41. Here th r ee long i tud ina l

g rooves may be s een wh ich were worn in s ide ? - in .O D c a s i n g i n t h e O k l a h om a C i t y P o o l . T h e y w e r e

worn by the s p r ing - s t ee l anch o r s of a tub ing pack e r

a s they " b reathed" up and down wi th the s ucke r -

r od p um p in g s t r o k e s . T h a t t h i s c a n b e s e r i o u s w a s

Fig . 41-Grooves Worn by Anchors of a Tubing

Packer

Fi g. 42-Peculiar Case of Externally Worn Casing

ev idenced by one o f the g rooves hav ing f ina l ly cu ten t i r e ly th rough the wa l l a t one s po t , bu t on ly a f t e r

many years of pumping l i fe .

I n 1937 , in the o ld Fo x Poo l of s ou the rn Ok la -

h o m a, a p e c u l i a r c a s e o f o u t s i d e wear was r ecov -

e r ed . I t was in a 5,560-f t s t r i ng of 9- in . OD, 3&lb ,-o ld Grade C c a s i n g . l ' h e w e a r f a i l u r e w a s p r e c e d e d

by a l a s t e n g ag e d t h r e a d f a il ur e in th e s e v e n t h

leng th . Af te r r ecove r ing i t and r econnec t ing wi th a

d i e n i p p l e, t h e r e w a s f ou n d t o b e a h o l e i n t h e c a s -

ing a t 52 8 f t . C u t t ing off be low the ho le , t he r e wa s

recove red a cas ing l eng th wh ich had worn through

fr o m t he o u t s i d e . Apparen t ly th i s l eng th , no t be ingh e l d i n t e n s i o n b e c a u s e o f t h e l a s t e n g a g e d t h r e a d

f ai lu re in t he s e v e n t h l eng th , had been wobb led

around Ly the dr i l l p ipe-caus ing i t to gr ind awa y

ag ai ns t a she l f of hard format ion unt i l worn c le ar

th rough ( s ee F ig . 42). f ' o s sib ly th i s wou ld no t have

occu rred if the s t r ing had been kept in tens ion , bu t

i t i s i n t e r e s t i n g p ro of t h a t c a s i n g c a n be w o r n

through from th e ou t s i de .

P r e ve n t io n of C a s i n g w e a r F a i l u r e s

If d r i l l i n g i s d o n e i n s i d e c a s i n g w i t h c a b l e t o o l s ,

no th ing can be done to p r even t wi r e - l ine cu t t ing .

H a v i n g t h e c a s i n g s t r i n g p e r f e c t ly v e r t i c a l i s , ofc o ur se , th e ~ e r f e c t a ns we r t o w a r d wh ich goa l

n or ma l d r i l l i n g p r a c t i c e u s u a l l y i s a i m e d.

If t he d r i l l i ng i s done wi th ro t a ry too l s , t he d r i l l

p ipe s hou ld be p ro tec ted wi th amp le rubbe r s , k e p t

c l o s e to t he t o o l j o i n t s , and r ep laced when worn ap -

p rec iab ly . Hard - f aced too l j o in t s s hou ld be omi t t ed

f ro m t h e p a r t o f t h e d r i l l s t r i n g r o t at i n g i n s i d e cas-

i ng . Th ey a r e on ly o f va lue in r e s i s t ing fo rmat ion

a b r a s i o n , a n d w o u ld g r i n d a h o l e i n c a s i n g r a t h e r

r a p i d l y if t h e y s h o u l d b e a r a g a i n s t i t , as i l l u s t r a t e d

i n F i g . 40a.

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3 8 H . G . TEXTER

L o n g i t u d i n a l i n s i d e w e a r f r om t u b i n g i n p u m p in g

w e l l s c o u l d b e p r e v e n t e d b y t h e u s e o f t u b i n g an -

c h o rs . H o w e v e r, t h i s i s h a r d ly w o r t h- w h il e u n l e s s

i t i s k n ow n t h a t t h e c a s i n g s t r i n g i s q u i t e d e c id e d ly

o f f ve r t i ca l o r has one o r m ore bad dog - l egs .

7B. WEAR-TUBING

T u b i n g w e a r o c c u r s m o s t o f t e n i n pu m p in g w e l l s .

I t d e p e n d s l i t t l e u p o n w h e t h e r t h e h o le i s v e r t ic a l

o r s 1 a n t i n g , b u t i t i s m u ch w o r s e i n d o g- le g g e d

h o l e s r e g a r d l e s s of t h e d e v ia t io n .' '

I t m a y b e e i t h e r e x t e r n a l o r i n t e r n a l . If e x t e r n a l ,

i t i s u s u a l l y t h e c o u p l in g s w h i c h a r e a f fe c te d a n d

t h e c a u s e i s th e r u b bi n g a g a i n s t t h e i n s i d e of t h e

c a s i n g i n p h a s e w i t h t h e r e v e r s i n g s t r o k e s o f t h e

s u c k e r r o d s . O f t e n th e c o u pl in g s a r e w o r n do wn

u n t i l t h e t h r e a d s a r e v i s i b l e , as i n F i g . 43 .

If t h e w e a r i s i n t e r n a l , i t i s , o f c o u r s e , c a u s e d

b y t h e s u c k e r r o d s. A g o od i l l u s t r a ti o n i s i n F i g . 44f ro m a w e l l n e a r B ea u m on t, T e x a s . C o r ro s io n i s

a l s o i n v o l v e d, b u t w e a r i s t h e p ri m ar y c a u s e o f t h e

fa i l u re .

F i g . 45, f ro m a w e l l n e a r M a d i s o n , K a n s a s , i s a

g o o d c r o s s - s e c t i o n a l v i e w o f s u c k e r - r o d w e a r .

I t i s w e ll k n o w n t h a t f r ee ly s u s p e n d e d tu bi ng

e l o n g a t e s d u r in g t h e d o w n s t r o k e a n d s h o r t e n s d u r i ng

t h e u p s t r o k e , w h i c h i s g e n e r a ll y r e f e r r e d to a s t ub -

i ng "b rea th i ng . " B rea t h i ng , t oge t he r w i t h ho l e dog -

l e g g in g , i s r e s p o n s i b l e f o r t u b i n g - a g a in s t - c a si n g

w e a r . I t m a y b e s t o p p e d by u s e o f a t ub i ng ancho r .

An o p i n i o n h a s s o m e t i m e s b e e n e x p r e s s e d t h a t

t u b i n g- a g a i n st - s u c ke r - r o d w e a r i s a l s o c a u s e d by

b r e a t h in g . T h i s i s i l l o g i c a l i n v i e w of t h e f a c t t h a t

Fig. 43-Extreme Case of Tubing Coupling WearI Fi g. 44-Sucker-rod-against-Tubing Wear

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OIL-WELLCASING AND TUBING TROUBLES 39

Fig. 45-Cross-sectional View of Sucker-rod Weart h e v e r t i c a 1 mot io n of tubing , i. e. , b rea th ing , i s

s m a l l c om p ar ed t o t h e v e r t i c al m o t o n of s u c k e r

r o d s. T h e r e f o r e , u s e o f a t u b i n g a n c h o r c a n n o t p r e -

v e n t t u b i n g v s . s u c k e r -r o d w e a r d u e t o a d o g - le g g ed

hole.

F r e q u e n tl y , h o w e v e r , t ub i ng -aga i n s t - s ucker - rod

w e a r i s f o u nd i n w e l l s i n w h i c h t h e r e a r e n o a p p re -

c i a b l e d o g - l e g s. S u c h a w e a r o c c u r s a t t h e b ot to m

por t i on o f t he s t r i ngs , gene ra l l y w i t h i n a f ew hun -

d r e d s o f f e e t a b o v e t h e p um p.'9 T h i s is i n acco rd -

a n c e w i t h a phenom enon b rough t t o l i gh t by Ar t hu r

L u b i n s k i of S t a n o l i n d O i l a n d Gas C o m p a n y Z 0whi che x p l a i n s t h a t , d u r i n g p u m p u p s t r o k e s , t h e b o tt om

p o r t i o n o f t h e t u b i n g b u c k l e s a n d w r a p s a r o u n d t h e

s u c k e r ro d ( s e e F ig . 46). O n t h e o t h e r ha n d, t h e t u b

i n g s t r a i g h t e n s a n d u n w r a p s during pump down-

s t r o k e s . A m o d e 1 s h o w n a t t h e 1953 I n t e r n a t i o n a l

O i l E x p o s i t i o n ( T u l s a ) d e m o n s t r a te d t h i s u n e x p ec t -

e d p h e n o m e n o n . T u b i n g b u c k l i n g a n d s u b s e q u e n t

t u b i n g v s . s u c k e r - r o d w e a r m a y b e s t o p p e d b y u s e

of a t u b i n g a n c h o r o n l y i f t h e t u b i n g is a n c h o r e d a t

t h e m o s t e x t e n d e d p o r t io n w h i c h t h e t u b i n g m a y

r e a c h d u r i n g t h e b r e a th i n g c y c l e. S u i t a b l e a n c h o r s

a r e n o w a p p e a r i n g o n t h e m a rk e t.T h e o r e t i c a l l y , t u b i n g b u c k l in g m a y a l s o b e p r e-

v e n t e d w i t h o u t a n c h o r in g b y s u b j e c t i n g t h e t u b i ng

t o t h e s a m e p r e s s u r e o u t s i d e a s i n si d e. T h i s c o uld

b e a c c o m p l i s h e d w i t h a p a c k e r , w h i ch v e r y o ft e n i s

n o t d e s i r a b l e f o r m an y r e a s o n s .

I n a n y e v e n t , r e g a r d l e s s of w h e h e r t u b in g -

a g a i n s t- s u c k e r -r o d w e a r i s c a u s e d by b u c k l i n g o r

h o l e d o g - l e g g i n g , t h e w e a r m a y b e m i n i m i z e d b y a

d e c r e a s e o f t h e c o e f f ic i e n t o f f ri c t io n . P r e v e n t i n g

t h e s a n d f ro m e n t e r i n g t h e w e l l i s u s e f u l . C o r r o s i o n

i n c r e a s e s f r i c t i o n a n d , t h e r e fo r e , u s e o f i n h i b i t o r s

i s a l s o u s e f u l from t h e m e c h a n i c a l - w e a r s t a n d p o in t .

T h e e f f e c t s of e x t e r n a l w e a r m ay b e l e s s e n e d a p -

p r e c i a b l y b y r o t a t in g t h e t u b in g s t r i n g , if p r a c t i c a l .

T h i s m a y b e d o n e a u t o m a t ic a l l y w i th e a c h p u m p in g

s t r o k e , s l o w l y r o ta t i n g a t t h e r a t e o f , s a y , o n e o r

m o re r e v o l u t i o n s p e r d a y o r p e r w e e k .

H o w e v e r , t h i s r o t at i o n i n t r o d u c e s a n e w p o s s i b i l -i t y , v i z ., t h a t of c u t t i n g t h e t u b i n g i n t w o a n d d ro p -

p i n g t h e l o w e r pa r t. W he n t h e t u b i n g i s n o t r o t a t e d ,

t h e s u c k e r ro d m a y w e a r a h o l e i n s o m e s p o t ( u s u a l -

l y i n t h e b od y) a n d t h e r e w i l l b e w a r n i n g ( b e c a u s e

o f l a c k o f p r o d uc t i o n ) a n d t h e s t r i n g w i l l b e p u l l e d

f o r r e p a i r s . But, w h e n t h e s t r i n g i s r o t at e d t h e w e a r

Fig . 46-Tubing Buckling on Pump Upstrokes

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40 H . G. TEXTER

w i ll b e u nif orm a r o u n d t h e c i rc u m f er e n ce of t h e

p o i n t s o f w e a r a n d e v e n t u a l l y t h e t h i n n e s t s e c t i o n

w i ll p a r t w i t h o u t w a r ni ng . T o a v o i d t h i s , c a l i p e r

l o g s s h o ul d b e r un a t r e g u l ar i n t e r v a l s , s a y e v e ry

8 t o 1 2 n lo n th s, if t u b i n g i s t o b e r o t a t e d c o n ti n u -

o u s l y .Tub ing -aga in s t - s u cker -rod we ar n iay be l e s s e ne d

b y t h e u s e o f ru bb er (n e op r en e ) g u i d e s p l a ce d

a ro u nd t h e s u c k e r r o d s a n d k e p t n e a r t h e j o in t s,

m u ch a s d ri ll -c o ll ar p r o te c t or s a r e k ep t n e w - t h e

t o o l j o i n t s. T h e s e a r e n o t u s e d t o a n y g r e a t e xt e n t.

8 A . E R O S I O N - C A S I N G

(from high-velocity f lu ids)

I t i s ve r y u n l i ke l y t h a t o i l , g a s , o r w a t e r, e v e n

t h o u g h c a rr yi ng a n a b ra s i v e i n s u sp e n si o n , w il l

eve r f low th rough un re s t r i c t e d ca s in g a t h igh enough

v e l o c i t ie s t o e r o d e i t , b e c a u s e t h e a n g l e o f im-

p in ge me nt of t he p a r t i c l e s of s a nd , e tc . i n a

s t r a igh t bore i s t oo s ma l l . However , de f l ec t ion o f

the d i r ec t ion o f flow, a s des c r ib ed fo l lowing , cou ld

cu t th rough the cas in g in a r e l a t ive l y s ho r t time . I t

i s c om m on k n o b l e d g e how r a p i d l y s a n d w i l l c u t o u t

s u r f ac e equ ipmen t and f low l ines .

In recover ing some 95/ , - in . cas ing dur ing abandon-

ment of a 13-year -o ld (1936) Oklahom a Ci ty wel l ,

a le ng th w a s found ~ i t hh r e e e q u a l l y s p a c e d h o l e s ,

F i g . 47, which had the appea ran ce o f be ing cu t wi th

Fig. 47-Odd Example of Casing Erosion Because

of Deflected Flow

a s a n d- b la s t. R e c o r d s s h o w t h a t e a rl y O kl a ho m a

C i t y b e l l s w e r e d r i ll e d i n un d e r p r e s s u r e , u s i n g i n-

j e c te d g a s a s a c i r cu la t ing medium. I t i s a s s um ed

tha t , du r ing the t e s t ing o f the we l l , a s t an d of d r i l l

c o l la r s an d a b it w e re l e ft h a n g i n g a nd a s a nd -

b la s t in g ef f ec t had cu t t hrough the c as ing , as s h o w ni n t h e p ho t og r ap h . T h e h o l e s , s p a c e d j u s t 1 2 0 d e g

apa r t , were a t 90 f t f rom the top . Very ev iden t ly a

t r i cone rock b i t h ad d i v e r t e d th e di re ct io n of im-

p i n g e m e n t o f a b r a s i v e m a t e r i a l s o a s t o c u t th ro u gh

t h e w a l l o f t h e c a s i n g . I n t h i s p a r ti c u l a r c a s e no

o the r ha rm had ens ued .

88. E R O S I O N - T U B I N G

A s w i th c a s in g , t h e wr it er k n o w s of n o c a s e s

whe re tub ing h a s been e r o d e d by h igh -ve loc i ty

f lu id s f lowing pe rpend icu la r ly . However , t he r e a r e

o c c a s i o n a l c a s e s w h e r e t r an s ve r s e f lows have

e r o d e d h o l e s i n t h e t u b i n g s t r i n g .

F ig . 4.8 s h o w s e x t e r n a l e r o s i o n i n a p i e c e o f 2'4-in . OD, 6.50-lb, 5-55 t u bi n g. T h i s o c c u r r e d i n a w e l l

n e a r S i n t on , T e x a s , a f t e r o n l y 11 n ion th s s e rv ice .

T h e s p e c i m e n sh o w n h a d b e en j u s t o p p o s i t e a s h o t

per fora t ion in th e sur rou nding ?- in . OD o i l s t r i n g

and inf lowing flu id , p robably w i th som e format ion

s a n d , h a d c u t t h ro u gh t h e t u b i n g w a l l.

To avo id fu tu r e e ro s ion , t he ope ra t ing company

i s c o n s i d e r i n g s o m e s o r t o f c o v e r i n g f o r t h e t u b i n g

o p p o s i t e t h e p e r f o ra t i o n s . O n e l i k e l y s o l u t i o n t o t h e

p ro bl em i s c o a t n g w i t h l e a d , a s t h i s v e r y s o f t

m e t a l i s n o t e a s i l y a b r a d e d by s a n d p a r t i c l e s .

9 A . B U C K L I N G - C A S I N G

( f rom mechan ica l f o r ces )

If a s t r ing of c as i ng i s p l ac ed in heavy long i tu -

d ina l compres s ion in i t s l ower pa r t , and if t he r e i s

a s i z a b l e c a v i t y a t t h a t po i n t, t h e s t r i n g w i ll m o s t

s u r e l y b u c k l e i n t o t h e c a v i t y ( s e e F i g . 49). l ' h e r e-

s u l t may be mere ly a s l i gh t bend wi th s ome r e s t r i c -

t i o n s t o t h e p a s s a g e o f t o o l s ; o r , if i t o c c u r s a t a

coup l ing o r jo in t , t h e r e migh t be a c r acked th r ead

o r p a r t i a l c o l l a p s e of t h e j o in t. API j o in t s a r e not

d e s i g n e d t o w i t h s ta n d b e n d n g s t r e s s e s a n d w il l

f a i l by t e a r i n g o r c r a c k i n g a t a b o u t t he l a s t e n g a g e d

t h r e a d , a t r e l a t i v e l y l o w l o a d s .

U n d er t h e s u b j e c t o f We ar t h e r e w a s m e n t i o ne d

a 5 '4 -in. ca s ing s t r i ng in a s o u t h w e s t T e x a s w e ll ,

w h i c h h a d b u c k l e d a s d e sc r i b e d , a n d as e v i d e n c e d

b y s i g n s o f t u b i n g w e a r at t he po in t o f buck l ing .

R e co r ds ind ica te 'd tha t f a r t oo much t e n s i o n had

b e e n l e t off a f t e r t h e s t r i n g w a s c e m e n te d . T h u s t h e

p i p e b u c k l e d i n t o a c a v i t y b e l o w t h e i n t e rm e d i a t e

s t r i n g , a n d a b e n d in g s t r e s s c o n c en t r a t e d a t a cou-

p l e d j o i n t c a u s e d o n e o f t h e t h r e a d s t o c r a c k o p e n.

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OIL-WELL CASING AND TUBING TROUBLES 4 1

F i g . 4 8 - E x te r n a l E r o s io n o f T u b i n g

O UTER C A S I N G S TR I N G

I N N ER C A S I N G S T R I N G

C A V I T Y J U S T A B OV E C E M E NT

TOP OF CEMENT OR F R E E Z E P O I N T

F i g . 4 9 -B u c k l i n g o f C a s i n g fr om C o m p r es s i o n

I n c i d e n t a l l y , t h i s i s o n e of t h e a l l t o o fe w c a s e s

w h e r e a c a s i n g f a i l u r e i n a d e e p w e l l h a s b e e n r e -

c o v e r e d a n d c o u l d b e e x a m i n e d i n d e t a i l . F i g . 50

i l l u s t r a t e s j u s t w h a t h a d h a p p e n ed .

P r e v e n t i o n o f M e c h a n i c a l B u c k l i n g

K e e p in g t h e c a s i n g in f ul l te n s i o n from to p t o

b o tt om a t a l l t i m e s i s t h e m o s t o b v i o u s p r e v e n ti o n

o f b u c k l in g . I n t h e p a s t , e s p e c i a l l y i n r e l a t i v e l y

s h o r t st r i n g s , c a s i n g w a s s o m e t i m e s r e l e a s e d o f a l l

t e n s i o n a n d a l lo w e d t o " s it d ow n " o n b o tt om w h i l e

rem ov i ng b low-ou t p rev en t e r s , e t c . , p repa ra t o ry to

n i p p li n g up i n t h e c a s i n g h e a d ( s e e p r e c e d i n g p a r -

a g ra p h ). If t h e r e a r e no c a v i t i e s i n t o w h i c h t h e c a s -

i n g c o u l d b u c k le , t h e r e w i l l b e n o t r ou b le , a s c a s -

i n g j o in t s a r e e q u a l ly s t r o n g i n l o n g i t u d in a l t e n s i o n

o r l o n g i t u d i n a l c o m p r e s s i o n if k e p t s t r a i g h t .

P r o d u c t i o n s t r i n g s a r e s e l d o m e n t i r e l y s u rr o u n d e d

b y c e m e n t r i g h t u p to t h e s e c o n d a r y s t r i n g , and

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42 H. G. TEXTER

Fig. 50-Diagram of Failure Caused by Buckling

q u i t e o f t e n c a v i t i e s h a v e b e e n w a s h e d o u t b e l o w

t h e o u t e r s t r i n g a n d a b o v e t h e c e m e n t o r f r e e z e

po in t . Once a s t r i n g h a s b u c k 1 e d i t m a y n o t b e

p u l l e d e n t i r e l y s t r a i g h t b ef o r e b e in g s e t i n t h e c a s -

i n g h e a d . T h e r e f o r e , c a s i n g should always be kept

in tension.

I n t h e a u t h or 's o p i n i o n , a s e x p l a i n e d i n " C a s i n g

Stra i n af t er Cement ing ," ' h e safe procedure would

be to land the casing in the casing head at exactly

the position in which it was hanging on the hook

when the cement plug hit bottom. In o the r words ,

h a n g t h e c a s i n g i n i t s o r i gi n a l t e n s io n s o t h a t n o

l a t e r t e m p e r a tu r e i n c r e a s e s c a n e v e r f o rc e t h e b ot -

t om s e c t i o n i n t o c o n ~ p r e s s i o n .A l o n g , s l e n d e r c o l -

umn, as is c a s i n g , c a n s u p p o r t v e r y l i t t l e c o m p r e s -

s i o n l o a d w i t h o u t t e n d i n g t o b u c kl e .

T h e a r g u m en t t h a t t h e r e c a n b e t e m p e r a t u r e de-

creases n e e d n o t w o r ry t h e o p e r a t o r b e c a u s e a s im-

p l e c a l c u l a t i o n w i l l s h o w t h a t e v e n a 7 5 - d e g aver-

age c o o l i n g over the entire free portion o f t h e c a s -

i n g w i l l n o t q u i t e u s e u p t h e d e s i g n f a c t o r i n t en -

s i o n o n w h ic h t h e s t r i n g w a s l a i d ou t, e v e n t h ou g h

t h i s f a c t or b e a s l ow as 1.4. Sure ly anyone wi l l ad -

m i t t h a t a 7 5 -d e g d e c r e a s e i n t h e a v e r a g e t em p e ra -

tu r e o f a c a s i n g s t r i n g i s f a r b ey o nd a n y o b s e r v e d

deg r ee of coo l ing .

9A. BUCKLING-CASING

( f rom in ternal 4120

H ig h i n t e r n a l p r e s s u r e i s a n o t h e r f or c e t e n d i n g t o

b u c k l e t u b u l a r s e c t i o n s ( a s w a s m e n t i o n e d u n d e r

Wear-Tubing). How ever , be ca us e a c a s i n g s t r i n g

w h i c h m i g h t b e s u b j e c t t o h i g h i n t e r n a l p r e s s u r e i sn e a r l y a l w a y s p r e t t y w e l l c o nf in e d i n t h e d r i l l e d

h o l e, n o a u t h e n t i c a t e d c a s e s o f f a i l u r e f ro m t h i s

c a u s e h a v e e v e r c o m e t o t h e a u t h o r ' s a t t e n t i o n . I t

i s , n e v e r t h e l es s , a r e a l p o s s i b i l i t y a n d is a n o t h e r

a r g u m e n t f o r k e e p i n g c a s i n g s t r i n g s i n t e n s i o n d u r -

i n g a n d a f t e r c e m e n ti n g .

9B. BUCKLING-TUBING

T h e s a m e f a c t o r s a f f e c ti n g b u c k l i n g o f c a s i n g a p -

p ly to tub ing . However , on ac cou n t of tub ing be ing

qu i t e c lo s e ly con f ined , t he mechan ica l buck l ing f rom

s e t t i n g t h e t u bi ng i n c o m p r e s si o n s e l d o m r e s u l t s i n

f a i l u r e s o f t h e t u b i n g i t s e l f . T h e t r o u b le r e s u l t i n g

u s u a l l y h a s t o d o w i t h t h e p a c k e r s e t t i n g a n d n o t

wi th th e con di t ion of th e tubing .

However , r e f e r ring aga in to th e phenomenon of

buck l ing f rom in te rna l p r e s s u re (d i s cus s ed unde i

7B. W ea r-T ub ing ) a n d q u o t i n g fro m a l e t t e r b y

Ar thur Lu b in s k i :

"A s t r i n g of t u bi n g s e t o n a p a c k e r a n d s u b j e c t e d

t o m o r e p r e s s u r e i n s i d e t h a n o u t s i d e m u s t b u c k l e

in to a h e l ix u n l e s s i t is s u b j e c t e d a t t h e p ac k e r t o

a n a p p r e c i a b l e a m o u n t of t e n s i o n . T h i s b u c k li n g

oc cu r s on ly in the lower po r t ion of the s t r ing . T h e

c o nd i ti o ns a r e ge n e ra l ly n ot s e v e r e e n o u g h t o

c a u s e a p er m an e nt d ef o rm a ti o n a n d t h e t u b i n g

c o m e s b a c k t o i t s s t r a i g h t f orm w h e n t h e p r e s s u r e

i s r e m o v e d . In s om e c a s e s t h e de fo rm at io n is

b e y o nd t h e e l a s t i c l i m i t a n d t h e t u b i n g r e m a i n s

co rks c r ewe d when pu l l ed ou t o f the hole. "

10A. TORSION-CASING

W hil e c e m e n t i n g c a s i n g , i t i s e i t h e r r e c i p r o c a t e d

v e r t i c a l l y o r r o ta t e d. I n e i t h e r c a s e t h e p u r p o s e i s

to s c r a t c h off f il t e r ca ke a nd in s u r e a good bond be-

t w e e n t h e c e m e n t a n d t h e w a l l o f t h e h o l e , as w e l l

as t h e p i p e .If t h e r o t a t in g m e t ho d i s u s e d , t h e r e is a s m a l l-

chan ce of exce s s i ve make-up wi th a r e s u l t a n t t o r -

s i on - ty p e f a i lu re . T o a v o id t h i s i t i s ve r y e a s y t o

o b s e r v e t h e t o p j o in t ( b e l o w t h e r o t a r y t a b l e ) a n d

c e a s e r o t a t i n g if i t s h o u l d s t a r t t o m a k e u p. A l s o

t h e t o r q u e l o a d o n t h e t a b l e s h o u 1 d b e w a tc he d

carefu l ly .

10B. TORSION-TUBING

T h e o n l y t i m e t u bi n g i s s u b j e c t t o a p p r e c i a b l e

t o r s i o n i s w h il e i t i s b ei ng u s e d i n p l ac e of a

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sma l l - d i a me te r d r i l l i ng - in s t r i n g . A n y i n c i d e n t a l

da ma ge t he re f rom i s no t p r o p e r 1 y c h a r g e a b l e t o

tubing t r o u bl e b u t s h o u l d b e c l a s s i f i e d a s a dri l l -

pipe t rouble .

11. M I S C E L L A N E O U S T R O U B L E S

C a u s e d b y F i e l d W el di ng o n C a s i n gA r c w e l d i n g o n a n y o f t h e p r e s e n t g r a d e s o f c a s -

i n g , e x c e p t v e ry l o w -c a rb o n , p r o d u c e s d e l e t e r i o u s

e f fe c t s i n t h e w e l d a r e a a n d s h o u l d b e a v o i d e d i f a t

a l l p o s s i b l e . A t b e s t t h e r e w i ll b e a n e m b r i tt l in g

e ff e ct , a n d a t w o r s t t h e r e w i l l b e a c t u a l c r a c k i n g

u n d e r n e a t h o r a d j a c e n t t o t h e a d d e d w e l d m e t a l .

? 'h e p r i n c i p al o p e r a t i o n s o n c a s i n g a r e t h e w e l d -

i n g o n of l u g s o r m e t a l s t o p s f o r h o ld i n g s c r a t c h e r s

o r c e n t r a l i z e r s i n p l a c e a n d s p o t w e l d i n g or b e a d

w e ld in g o f fl oa t i ng e qu ipme n t , s l l oe s , a nd c oup l i ngs

o n t h e bo tto m l en g th s. l ' h e r e a s o n f or t h e l a tt e r

o p e r a t i o n i s t o p r e v e n t t h e u n s c r e w i n g o f t h e bo t-t om a s s e n ? b l y w h i l e r o t a ry d r i l l i n g i n s i d e . H o w e v e r ,

i t i s t h e w r it e r 's o p i n i o n t h a t t h e f a i l u r e s r e s u l t i n g

f ro m a p p a r e n t u n s c r e w i n g a r e a c t u a l l y l a s t e n g a g e d

t h r e a d f a i l u r e s ( s e e S e c t . 4) ~ h i c h ou ld o nly b e

a ggr a v a t e d by t h e e mb r i t t l i ng e f fe c t o f w e ld ing .

E m b r i tt l e m e nt by w e 1 d i n g i s a r es u lt of t he

q u e n c h i n g e f fe c t o f - t h e m a s s o f r e l a t i v e l y c o l d s t e e l

on t he de pos i t e d w e ld me ta l a nd t he mome nta ri l y

h e a t e d p i p e m e t a l a d j a c e n t t o i t ( s e e 1 A . l ' e n s i o n -

F i g . 8 a a n d 8 b ). A p r e c a u ti o n a r y m e a s u r e , t h e n , i s

t h e u s e o f a w e ld i n g ro d w h ic h i s l e s s c o n d u c iv e t o

unde r - be a d c r a c k ing .Of much gre a te r va lu e , however , would b e f lanie

p r e h e a t i n g b e f o re w e l d i n g a n d s l o w c o o l i n g a ft e r-

w a rd . T h e d i ff ic u lt y i s i n t e a c h i n g w e l d i n g c r e w s

t h e a b s o l u t e n e c e s s i t y o f t h e s e m e a s u re s .

I n ge ne r a l t he h ighe r t he c a r bo n a nd a l l o y c on t e n t

o f a s t e e l , t h e m o r e d if fi c ul t i t i s t o w e l d s a t i s f a c -

tor i ly .

T h i s s u b j e c t i s t o o b ro ad t o b e c o v e re d a d e q u at e -

l y he r e a nd t he r e a de r is r e f e r r ed t o a v e r y e x c e l l e n t

pa pe r t he r e on l i s t e d i n t he r e f e r e nc e s . "

C a u s e d by Sho t - pe r f o r a t i ng Ca s ing

With mode r n pe r f o r a ti ng t oo l s , u s in g e i t he r bu l-

l e t s o r s h a p e d e x p l o s i v e c h a r g e s a n d w it h t h e c a s -i n g p r o p er l y s u r r ou n d e d w i t h c e m e n t , t h e r e i s l i t t l e

t h a t c a n h a p p e n t o a n y o f t h e o r d i n a ry g r a d e s of

c a si n g. T h e t ra je c to r y s p e e d of t h e p e rf o ra t in g

f or ce i s s o h ig h t h a t e v e n w h e re t h e c a s i n g i s b a ck -

e d up by no th ing bu t f lu id , t he r e i s no t t oo much

d a n g e r o f i t s s p l i t t i n g o r sh a t t e r i n g .

H o w e ve r, a s h i g h e r a n d h i g h e r y i e ld a n d t e n s i l e

s t r e n g t h m a t e r i a l s a r e a d o p te d , t h e r e i s a n a tt e n d -

a n t r e d u c t i o n i n d u c t i l i t y , e s p e c i a l l y t r a n s v e r s e

d u c t i l i t y , w i th i n c r e a s i n g d a n g e r o f s p l it t i n g . T h i s

i s i l l u s t r a t e d i n F ig . 51 w h i ch i s t h e r e s u l t of a

s u r f a c e t e s t o n v a r i o u s g r a d e s o f 7- in ., 32- lb c a s -i ng . S a m p l e s w e r e l o w e r e d i n t o s h a l l o w h o l e s f il le d

wi th w ate r and per fo ra ted wi th 5/,-in . d iam eter bul-

l e t s . l ' h e o n l y v a r ia t i o n fr om t e s t to t e s t w a s i n t h e

a n a l y s i s ( a n d t h e r e f o r e t h e m e c h a n i c a l p r o p e r t i e s )

o f t h e t u b e s , a n d t h e i l l u s t r a t i o n s h o w s t h e r e s u l t

i n o n e of t h e h i g h e s t - s t r e n g t h s a m p l e s . It s p l i t b ut

d i d n o t s h a t t e r , a s h a p p en e d i n s o m e o f t h e t e s t s .. .T h e r e i s n o t n iu ch t h a t c a n b e d o n e t o p r e v e n t

s p li t t in g , o th er th a n to e n s u r e t h a t th e c a s i ng i s

c omple t e ly su r r ounde d by c e me n t . I t i s un f o r tuna t e -

l y t r u e t h a t d u c t i l i t y i s i n i n v e r s e p r o p o r t i o n w i t h

t h e o t h e r m e c h a n ic a l p r o p e rt i es . T h i s i s p a rt ic u la r-l y t r u e of t h e t r a n s v e r s e d u c t i l i t y .

C a u s e d b y H a n d l i n g ( C a s i n g a n d T u b i n g) 2 21 2 '

D a m a ge c a u s e d b y h a n d l i n g a n d t h e p r e v e n t i o n

t h e re o f i s u s u a l l y t o o o b v i o u s t o n e e d m u c h d i s -

c u s s io n . C a r e l e s s n e s s i s t h e p ri n c ip a l c a u s e a n d

f o r t h i s t he r e i s no j us t i f ic a t i on . What , f o r e xa mple ,

c o u l d b e m o re i n e x c u s a b l e t h a n h a n d l i n g c a s i n g o r

Fig. 51-Casing Split by Shot Perforating

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44 11. G . TEXTER

tubing \\ i t 1 o u t the tl~readprotectors properly in

place?

C/ccasionally the ca use of certa in handling dam-

age is not in~n~ediatelyvident. hote, for exa~nple,

the peculiar transverse grooves on the pipe in k ig.

52.2

rs e turned out to be abrasion marks fron~ heti e- do ~l n \ir e used in binding the pipe load to the

railroad car. i he pipe i s 7%-in. GD, 39-lb, i\r-80

castnh , ind the grooves run fronl 0.046 in. to 0.095

i n . I n clei)th (the .\l'i alio\\able being 0.062 in. for

tl~ib eiF>l>t).

I'robably this par tic ulc ~rength had been a t one

sid e of t l ~ e op ot the c ;l rl o a d and the wire \.as

stretched taut across i t. I l ~ e olling ancl vibrating

Fig . 52 - Wi r e - cu t G r o o ves on C as i n g

F i g . 53 -S a nd i n C a s i n g T h re a d s bout

T o B e S t ab be d

of the car during transit apparently caused the wire

to rub or saw the notches shown in the photograph.

It is evident that suc h grooves c o u 1 d be abradeden ti re ly through the wall if given tinie.

Another handling damage of obsc ure o r i g i n is

leaka ge beth een couplings and pipe. l ' his may be

partly the re sul t of drying out of the grea se, but i s

certainly aggravated by the jolting and jarring of

rail and ocean transportation. It i s particularly no-

ticeable after long hauls and can amount to 30 to

50 percent leakers at far-away delivery points, such

a s blaracaibo: Venezuela. l ' h e high temperature of

this particular delivery point is u~ dou bte dly nother

factor.

Also, if laye rs of pipe sl id e along one another inrailbay cars, the battering of the coupling-protector

end s of one layer aga ins t the back ends of the cou-

plings of the adjacent layers will definitely produce

some leakers. l 'his was proved very conclusively

in e a r y 1938 when several carloads of carefully

numbered and carefully tes te d 5'4-in., 14-1L ca si ng

were shipped to the East l'exas oil fields. Upon re-

te st in g \kith water in the field, most of the c oupli ngs

which showed battering at their back end s sho wed

leakage. Unfortunately there is no siniple way to

avoid thi s trouble althougll the reniedy i s sinlple.

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J u s t u n s c r e w t h e c o u p l i n g s , r e - g r e a s e a n d r e- ap p ly .

An a lmos t unbe l i ev ab ly f l ag rant exam p le of ca r e -

l e s s n e s s i n h an d l in g i s i l l u st r a t e d i n F i g . 5 3. T h i s

i s a photog raph of 7 - in . OD, 24 - lb ca s in g abou t to

b e ru n on a v e ry w in dy d a y i n 1 9 37 n e ar C h a s e ,

K a n s a s . T h e t h r e a d p r o t e c t o r s w e r e b e i n g u n s c r e w -

e d o n th e r a c k a a n d t h e c a s i n g r u n i n t o t h e w e l l

wi th no c l e an ing w h at soe v e r . Smal l wonder the p ipe

engi neer had been cal le d out '\rush" on a g a l l i n g

c o m p la i n t. I t d e v e l o p ed t h a t t h e r e h a d b e e n a n a r g u-

men t as t o w h e t h e r t h e o p e r a t o r o r t h e c o n t r a c to r

s h o u l d p r e p a r e t h e p i p e f o r r u n ni n g , a n d i t s e e m s

th at "no on e did nothing."

A l e s s - o b v i o u s e x a m p l e of c a r e l e s s n e s s i n ha n -

d l i n g i s t h e p r a c t i c e o f t o n g i n g c a s i n g o r t u b i n g t o o

h ig h a b o v e t h e s p i d e r , e i t h e r w h i l e r u n n in g i n t o t h e

h o l e o r u n s c r e w in g . T h e b e n d i n g m o m e n t i n vo l v e d

t e n d s t o g a l l t h e t h r e a d s a s w e ll a s c r oo k t h e p ip e .B o th t h e m o v in g to n g a n d t h e b a c k u p s h o u l d a l w a y s

b e h e l d a s lo w a n d as c l o s e t o g et h e r a s p r ac t ic a l .

F o r a t h o F O u gh , s tudy o f r unn ing and hand l ing

p r a c t i c e , s e e API RP 5C1.'

12. CORROSION-CASING AND TU BIN G

T h i s s u b j e c t r i gh t fu l l y b e lo n g s u n de r C a s i n g a n d

T u b i n g T r o u b l e s, b u t i t i s s o l a r g e a n d c o m p l ic a t ed

t h a t l i t t l e m or e t h a n a n o u t l i n e w i l l b e g i v e n i n t h i s

p a pe r . F o r d e t a i l s , t h e r e a d e r is r e f e r r ed t o w h o l e

l i b r a r i e s o f b o o k s a n d m a g a z i n e s o n t h e s ub jec t -

m o s t s p e c i f i c a l l y , t h e m o n t h l y m a g a z i n e Corrosion,

p u b l i s h e d b y t h e N a t i o n a l A s s o c i a t i o n o f C o r r o si o n

Eng inee r s .

I n a dd i t i o n to a n o u t li n e , t h e w r i t e r w i l l d i s c u s s

i n d e t a i l j u s t a f e w of t h e m o re s p e c t a c u l a r p h a s e s

o f co r ro s ion , more o r l e s s as exa mp les of how com-

p l i c a t e d t h e s u b j e c t c a n b e co m e . I t s r a m i fi c a ti o n s

a r e s e e m i n g l y e n d l e s s .

C o r r o s i on i s a n i n t e r e s t i n g m a n i f e s t a t i o n o f t h e

r u le t h a t a l l s u b st a n c e s te n d to r e v e r t to t he i r

s t a t e s of l e a s t po te n ti a l en er gy ; j u st as mat te r ,

p a r t i c u la r l y w a t er , t e n d s t o s e e k i t s p o s i t i o n (l e v e l )

o f lowe s t po ten t i a l ene rgy . Th us , i n na tu r e , go ld i s

n e ar ly a l w a y s f o u nd i n t h e m e t a l l ic s t a t e b e c a u s e

t h a t is i t s s t a t e of l e a s t p o t e n t i a l e ne r g y. I ro n , o n

t h e o t h e r s i d e of t h e s c a l e , i s p r a c t i c a ll y a l w a y s

found in th e form of a n ox ide o r o the r chemica l com-

p ou nd b e c a u s e t h i s i s i t s s t a t e of l e a s t p o te n ti al

e n er g y. I n o t h e r w o rd s , i r o n ( s t e e l ) i s a l w a y s t r y in g

t o r e v e r t t o t h e c o n d i t i on i n w h i c h i t is found in t he

ear th .

T h i s s u b j e c t m ay b e d i v id e d i n t o casing corro-

s i o n a n d tubing corrosion. E a c h o f t h e s e may be

s e p a r a t e d i n t o out s i de a n d i ns i de co r ro s ion . Each

c r e a t e d i st i nc t ly s e p a r a t e ~ r o b l e m s , om e c a p a b l e

o f s o lu t ion and s ome no t .

A s t o t y p e s of c o r r o si o n , t h e y m a y b e c l a s s i f i e d

i n t o t h e f o ll o w in g :

1. Oxida t ion co r ro s ion .

2. Hydrogen su l f ide cor ros ion .

3. C ondens a te co r ro s ion .

4 . E lec t ro l y t i c and ga lvan i c co rro s ion .Ox ida t ion C or ro s ion

O x i d a t i o n c o r r o s i o n i s o r di n a ry r u s t i n g , t h e s i m -

p l i f i ed chemica l r eac t ion o f wh ich i s :

A s i n d i c a t ed , i t c a n o c c u r o n l y i n t h e p r e s e n c e of

w a t e r a n d o n l y w h e n o x y g e n ( a i r ) i s p r e se n t .

L a c k o f o x y g e n i s o f t e n t h e re a s o n w h y c a s i n g

s t r i ngs f i l l ed wi th mud do no t co r rode . T he in s id e

of t h e c a s i n g an d t h e o u t s i d e of t h e t ub in g w i ll

s h o w o n l y t h e s l i g h t e s t e v i d e n c e of c o r r o s i o n be-.

c a u s e t h e r e i s no r e n e w a l of t h e c o rr o d in g a g e n t

o n c e i t h a s b e e n c o n s um e d.

Ox ida t ion co r ro s ion p r o c e e d s m o s t r a p i d l y i n

e l e c t r o l y t e s , s u c h as s a l t w a t er , s i n c e l o c a l i z e d

o x i d a t i on i s a c t u a l l y e le c t r o - c he m i c a l i n na t u r e .

T h i s e x p l a i n s , t h e n , t h e v e r y m a r k e d l y c o r r o s i v e

e f f e c t s o f a e r a t e d s a l t w a t e r i n m o s t s u r f a c e e q u i p-

m e n t a n d l i n e s .

O f t e n q u i t e s e v e r e o x i da t io n c o r ro s io n t a k e s

p l a c e w h e r e a i r i s u s e d i n ga s - l i ft w el ls -2 9 s e v e r e ,

i n f a c t , t h a t a i r l i f t in g s h o u l d n e v e r b e u s e d i f g a s

i s a v ai l ab l e . O x id a ti on c o r ro s io n i s a l s o a r e s u l ~ f

o x y g e n c o n t a m i n a t io n w h e r e f o rm a t io n s a r e r e p r e s -

s u r e d w i t h a i r o r w it h g a s c o n t a i n i ng s o m e a i r .

L o c a l i z e d o x i d a t i on i s a v e r y c o mm o n p h en o me -

n on , b u t i s b a d l y n li s un d e rs t oo d . A c t u a l l y i t i s s im-

p ly t h e r e s u l t o f e l e c t r o -c h e m i c a l c e l l a c t i o n s e t up

b y d i s s i m i l a r i t i e s w i t h i n t h e m e ta l . T h e s e m a y be :

S c r a t c h e s o r a b r a s i o n s ( a s fr om t o n gs ).

V a r i a t i o n s i n s u r f a c e r o u g h n e s s .

V a r i a t i o n s i n g r a i n s i z e o r g ra i n o r i e nt a t i o n .

Var i a t ions in s t r a in ( a s f rom bend ing) .

A l s o , c e l l a c t i o n m a y r e s u l t fr om d i s s i m i l a r i t i e s

i n t h e e n v ir o n m en t c a u s e d b y:

Di f f e r ences in compos i t ion o r concen t r a t ion o f theco r rod ing s o lu t ion .

Di f f e r ences in ag i t a t ion o f the s o lu t ion .

Dif ferences in ae ra t ion of t he f lu id .

C o n t a c t w i t h d i s s i m i l a r m a t e r i a l s .

ffRingworm" Corrosion

A good example of ox ida t ion co r ro s ion be cau s e

of d i s s i m i l a r i t i e s i n g r a i n s t r u c t u r e - i s t h e s o - c a l l e d6 6r ingwormw co r ro s ion of tub ing , a s i l l u s t r a t e d in

F i g. 5 4 a a nd 5 4b . I t o c c u r s o n l y i n u p s e t t ub in g

and i s an e l ec t ro -ga lvan ic e f fec t r equ i r ing th e p r es -

e n c e o f a d e p o l a r i z e r f o r c o n s u m m a ti o n . T h e v a r ia -

t i o n s i n c o r ro s i on r e s i s t a n c e a r e a s s o c i a t e d w i th

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Fig. 540-Internal Fig . 54b-External

"Ringworm" Corrosion

the change in metallographic structures which occur

at the he at runout zone of up set tubing (when it was

h e a t e d for upsetting). I*ig. 55 shows t hese t w o

str uct ure s in a secti on of 2'4-in., 6.50-lb, 3-55 tub-ing not subse quen tly heat treated (normalized).

It may be visualized that there has been a (sniall)

current flow fro n~ he intermedia te zon e through the

fluid as an electrolyte, with resultant corroding out

of the transition-zone metal. ?h e obvious and per-

fectly eflective remedy for this particular type of

corrosion is to use tubing which has been fully nor-

malized to remove the variations in grain structure.

E'ig. 56 illus tra tes a very odd resul t of ring no rn~

corrosion. It was found toward the bottor~~f a str ing

of 2'4-in. OD, 4.7-lb, 3-55 non-normalized tubing,

which had been dropped on sever al occ as io ns and

which was being discarded. I h e piece illustrated

was hanging together by a shred when brought out

of the hole and then broke c on~ ple tel y part on the

derrick floor.

l'he sequ ence of e vent s was deduced to be:

a. 13ingworni corrosion att ack ed the in si de of the

\ball at the upset heat runout zone.

b. Erosion from sand-laden production cleaned out

the products of corrosion.

c. Dropping the string resulted in an upsetting ef-

fect and cracked the thinned wall at i ts l~eakest

point.

1 e bes t preventive of oxidation corrosion i s the

use of paint and protective coatings; but, unfortun-

ately, these are applicable only to a linrited extentin tubular material in oil wells. 1 e best applica-

tion i s to the i nsi de of tubing where thernlal-se tting

phenol formaldehyde plastics are quite extensively

used, especially for flowing w e l l s . E'or punlping

kvells the c o a t i n g i s too soon rubbed ofl by the

sucker rods.

k'or casing, M he r e coatings are not y

used, the best preventive of outside corrosion is

either cathodic protection or the use of s I i t a 11l e

mud and cement programs prior to and hen the cas-

ing i s run. Cement is an e x c e l 1 e n t protection

aga ins t corro sion, as i s nlud with pIi of 10 to 12left behind the casing.

l'h e bes t preve ntive of inside corrosion of ca sin g

is the use of inhibito rs.

liydrogen Sulfide Corrosion: Generalized

Chemical Reaction

11,o

l l , S + E e ~ k e S + 2 l i

(Again, a s in the c as e of oxidation corrosion.

water is an absolutely es senti al agent.)

l'he major damage resu lting from hydrogen sul f ~ d eattack is really a secondary reaction in ~vliicl: l ~ e

E eS (iron sulfide) se t s up a galvanic cell with t l ~ e

ste el a s the sacrificial anode. 'Ihis resul ts in deep,

rather ch ara cte ris tic pitting. It rilay aflect both cas -

ing and tubing. bi g. 57 sho ws typical attack on the

outside of tubing.

'1 he hydrogen sulfide originates in t l ~ e or~ r~a tio ns

being penetrated. T hi s then is carried up with n, ~te r

vapor, particularly on the insi de of tubing, ,~ ntl e-

act s with the s t e e 1 wherever it dissolves i n con-

densed hater droplets.

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OIL-WELL CASING AND TUIjII'iC TICOUBLES 47

F i g . 55 ( X 1 0 0 ) - T w o M e t a l l o g r a p h i c S t ru c tu r es a t th e H e a t R u no u t Z o n e o f U p s e t T u b i n g

F i g . 5 6- O d d R e s u l t o f "R in g w o r m w C o r r o s io n 1 Fig. 57-Hy drogen Su l t lde Cor ros ion

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4 8 H . G . TEXTEH

,Io n e of the worst res ult s i s not merely corrosion

but hydrogen embrittlement (often nanled hydrogen

sulfide en~brittlerllent).1 is is caused by adsorption

of t he r e l e a s e d atomic hydrogen (in the reaction

given previously) into the steel, resulting in Llis-

te rs and cracks. l 'h is hydrogen embrittlenlent i sparticularly severe on parts under c y c l i c str ess

such as tubing connections in pumping hells, and

on sucker rods.

Fig. 50a i s on illustration of the re s u l ~ s f hy-

drogen en :lritt len~ent . It occurred very sl oh ly in a

string of 2'4-in. Ob, 6.50- lb, h-80 t u b i n g in a

10,000-ft California well. ?'he s tri ng had been run

about the ~iiiddleof 1947 and suddenly failed, for

no apparent reason, in January of 1951 (3'4 years

later). iyhile recovering the string to d i s c a r d it,

quite a nuniber of failures occurred, like the ones

illustrated, all having a very non-ductile appear-ance. Inasn~uchas the material wa s perfectly normal

in all respects except that of ductility, it was con-

cluded thtlt hydrogen sulfide en lbrit tlen~ent, a s an

end resul t of sulfate-reducing bacteria (see folloh -

ing), tr ~i ~ll tave been the explanation of the trouble.

l' he pl~otonlicrograph, Fig . 58b, i s a lOOx nlagni-

fication of one of the r~iyriadsof cracks found in the

Fig. 580-Hydrogen Sulfide Embrittlernent

samples examined. l'he absence of any decarboni-

zation is indication that the crack must have formed

in a cold tube and not during any of the mill hot

piercing and rolling operations.

Preven t ion

Sulfide attack can Le anieliorated by:

1. Setting a p a c k e r and filling the annulu s with

swee t oil. Th is will prevent corrosion on the

inside of casing.

2. Use of a volat ile inhibitor such a s forn~aldehyde

or ammonia.

3. Coating surfaces with a protective film. There

are liquid organic products which adsorb at the

steel surface forming impervious filrns which

quite positively prevent corrosion. 'l'hese are

known a s polar-type inhibitors.

Bacterial Corrosion

A recently recognized cause of external casing

corrosion has been attributed to sulfate-reducing

bacteria. This attack results in very characteristic

corrosion pit s and i s often localized in a r e a s of

s u r f a c e scratches such a s tong and s l ip marks.

k'ig. 59 i s a typical example. q'hese odd forn!ation-

borne bacteria, which exist without air, a re known

a s U e ~ u l ~ hov i b r i onaerobic sulfate-reducing bac-

teria.

'1'0 prevent attack by sulfate-reducing bacteria

there are several expedients:

a. Cathodic protection.

b. t'revent niult iplication of bact eria by pH con-

trol, a s tiith arnn~onia, r use of la c~ er ic id es .

F i g . 58b (X100) - Photomicrograph of Tubing

in F i g . 58a

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OIL-WELL CASING A,ND UBING TROUBLES 49

Fig. 59-Casing Corrosion as a Result of

Sulfate-reducing Bacteria

C. Coatings.

d. 100-percent fill-up (often not economically jus-

tified).

Stre ss Corrosion Cracking

Allied to, but not identical with, hydrogen sul-

fide corrosion, there is corrosion cracking or stress

corrosion cracking of st eel tubing. T hi s sub ject i s

well-covered in the October 1952 is su e of Corrosion

(of NACE).

Very briefly, this refers to the failure of steel

subjected to stress in an hydrogen sulfide environ-

ment. Ma n y different stee ls are suscept ible, but

some of the more spe ctacular fail ures have been in

9-percent nickel s te el tubing.

Quoting from L. W. Vollmer's paper, on page 332

of that i ss ue of Corrosion:"There i s evidence to indicate that most st ee ls

heat-treated to produce mechanical properties in or

above a critical. . .Rockwell hardness of C-25 to

C-26may be susceptible to the failure process.

"Pl ast ic deformation (a s in cold straightening)

greatly increases susceptibility of steel to failure

process.

"There i s evidence that a metallographic struc-

tural p h a s e , as a (martensitic) network, may be

the s ensi ti zi ng factor of hydrogen sulfide corrosion

cracking.

"Embrittlement. . .b y exposure to hydrogen sul-

fide is not considered a s a primary cau se of fa ilure,

but it may be an important contributory factor.

"It may be pos sib le to reduce sus cep tib ili ty. . .by composition changes in steel.

"There i s no evidence t h u s far that 9-percentnickel steel will stress-corrosion crack in sweet

( H , S free) condensate well service."

In another paper by Bowers, McGuire, and Fi eh e

on page 341 of the same i ss ue of Corrosion, the re

i s the following conclusion:6 6

The susceptibility of steels to failure (by cor-

rosion cracking) may be retarded or prevented by

any change in composition or by any heat treat-

ment that eliminates or sufficiently t em p e r s a

niartensitic netw~rk."

Condensate Corrosiona4

?'his i s caus ed principally by attack by organic

acids (acetic, propionic, valeric) as well as carbon

dioxide in the produced fluids. Th es e substa nce s

are niostly found in the high-pressure condensate

fields of the Gulf Coast , but may al so be present in

low-pressure "sweet" oil wells.

The failure of tubing in high-pressure w ell s is

the most important corrosion problem in gas produc-

tion. In wells havi ng a shut-in tubing pressu re of

about 2,000 psi, the industry's experience i s that

85 to 90 percent of s u c h wells are corrosive and

that if no preventive measures are taken, the aver-

age life of tubing i s approximately 3 years. Failure s

as early as 6 months after completion have been ex-

perienced.

Occasionally condensate wells actually have had

to be abandoned because of leaks in both the cas-

ing and tubing as a resu lt of thi s type of corrosion.

It i s probably the most dangerous of all corrosion

problems be cause i t so often involves very deep,

expensive, high-pressure wells where the failure of

tubing and casing can be truly disastrous.

One of t he sim ple st and best means of detecti ng

internal corrosion i s by determining the iron content

of the produced water. After a ga s well i s complete-ly cleaned of drilling fluids, the iron content of the

produced water i s a direc t measure of the lo ss of

iron from the tubing.

Internally plastic-coated tubing i s a solu-

tion of conde nsat e corrosion, but it i s not penna-

nent. ?'he bes t preventive measures are the use of

neutralizers (ammonium hydroxide) or one of the ab-

sorptive-type inhibitors.

Electrolytic and Galvanic Corrosion

Thes e two types of corrosion ar e defined as:

Electrolytic, by impressed (outside) current.

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6

H. G . TEXTER

G a l v a n i c , b y s e l f - g e n e r a t e d c u r r e n t ( a s w i t h d i s -

s imi la r me ta l s ) . 25

Llectrolytrc corrosion, e i t h e r e x t e r n a l o r i n t e r n a l ,

o f e i t h e r c a s i n g o r t u b i n g c a n b e c a u s e d b y s t r a y

e le c t r i c a l c u r re n t s . I f a s t r a y d i r e c t c u r r e n t f e e d s

i n t o a c a s i n g ( o r tu b in g ) s t r i n g th r ou g h t h e w e l l h e a de q ui p m en t , i t w i l l c a r r y a w a y i ro n fr om w h a te v e r

p o in t i t l e a v e s t h e p i p e a n d t h e r e s u l t a t t h a t p o in t

i s corrosion, t o a l l i n t e n t s a n d p u r p o s e s , a n d t h e

a p p e a r a n c e s e l d o m c a n b e d i s t i n g u i s h e d fr om a n y

o the r type o f c or ros ion . When d i re c t c u r re n t l e a v e s

s t e e l t h r o u g h a n y e l ec t r ol y t e, b e i t s a l t w a t e r o r

mud or jus t da mp fo rma t ion , i t c a r r ie s i ron wi th i t ,

a s i o n s, a t t h e r at e of 20 lb per ampere per year.

W here c a s i n g i s s u r ro u n d e d b o th e x t e r n a l l y a n d

i n t e r n a l l y by a n e l e c t r o l y t e ( s u c h a s s a l t wa t er ), i t

c a nno t be p re de te rmine d whe the r e le c t ro ly t ic c or ro -

s i o n w i l l b e e x t e r n a l n o r w h et h e r i t w il l b e s e v e r e

o r n e g l ig i b le . If t h e c u r re n t i s o ff e r ed l e s s r e s i s t -

a n c e t o l e a v i n g t h e o u t s i d e of t h e p i p e t h a n t h e i n -

s i d e , t h e c o r r o s i o n w i ll b e e x t e r n a l a n d v i c e v e r s a .

If t h e p o i n t o f 1 e a v n g t h e p i p e i s g r e a t i n a r e a,

w i t h t h e u s u a l l y l o w a m p e r a g e i n v o l v e d , t h e c o r r o -

s io n p i t t i n g wi l l be o f too low ma gni tude to e ve r

c a us e t roub le ; bu t if the po in t o f e x i t i s s m a 1 1 in

a re a , the r e su l t in g c onc e n t ra te d p i t t i n g ma y go

th rough the wa l l in a ma t te r o f a f ew m o n t h s . T h e r e

a r e j u s t t o o m a n y v a r i a b l e s i n v o lv e d t o m a k e an y

p r e d i c t i o n s a s t o h ow s t r a y c u r r e n t s w il l a c t .

T h e o b v i o u s c o n t r o l fo r e l e c t r o l y t i c c o r r o s i on i s

t h e u s e of i n s u l a t e d f l a ng e s . C a t h o d i c p ro t e c t i o n i s

a no the r r e me dy .

Galvanic corrosion nla y be c on t ro l le d by a vo id ing

t h e u s e o f d i s s i m i l a r m e t a l s a n d b y c a t h o d i c p ro t e c-

t i o n . S o m e t i m e s t h e u s e o f d i s s i m i l a r m e t a l s i s un -

a vo ida b le , a nd a ve ry pe c u l ia r e xa mple of th i s migh t

be wor th r e c i t ing .

I n 1 9 4 9 t h e a u t h or e x a m i n e d 4 9 l e n g t h s of 7 -i n.

OD c a s i n g , a n lo n g s o m e 2 7 0 l e n g t h s r e c o v e r e d f ro m

a d ee p well in Wyoming, in which the re we re myr-

i a d s of i n t er n a l p i t s , a l l i n a na rr ow l o n g i tu d i n alb a nd . l ' h e w i d th o f t h e p a t h of p i t t i n g w a s 2 o r 3 in .

a nd the f i r s t though t w a s tha t a c o r ro s ive fluid ha d

l a i d i n t h e t u b e s w h i l e o n t h e r ac k a n d c a u s e d t h e

c o r ro s i o n. T h i s o ft e n h a p p e n s i n s i d e d r i l l p i p e w i t h

t h e s a l t y m u d s o f W e s t T e x a s P e r m i a n b e d s . H o w -

e v e r , t h i s d e fi n i te l y w a s n o t t h e c a s e .

In the f ir s t p l a c e , th e mud wa s no t c o r ros ive . If i t

h a d b e e n , t h e n a l l of t h e l e n g t h s , i n s t e a d o f o n 1 y

4 9 , w o u ld h a v e s h o w n p i t t i n g . S o w o ul d s o m e 9 % - i n .

le ng ths r e c ove re d f rom the s a me we l l . E 'u r the rmore ,-t h e l e n g t h s h a d b e e n washed out with fresh water

as f a s t a s t h e y w e r e u n s c re w e d a n d l a i d d ow n.

T h a t t he p i t s w e re n ot a n o p i c a l i l lu s io n w a s

p r o v e d b y c u t t i n g o p e n s e v e r a l l e n g t h s b y s p l i t t i n g

long i tud ina l ly wi th a torc h . T he p i t s we re t rue c or-

r o s i o n p i t s , p r e t t y m u c h r o u n d , a n d o f a s i z e r u n -

n ing f rom p in po in t s to '/, n. or s o in d iame te r . On e

of t h e l a r g e r p i t s n ~ e a s u r e d .0 6 0 i n . i n d e p t h w h i c h ,fo r ? -in. , 29- lb c a s in g a mounts to a bo u t 15 p e r c e n t

o f t h e n o n ~ i n a lwa l l .

T h e s i g n i f ic a n ce o f t h i s p a r t i c u l a r p i t t i n g l i e s i n

t h e f a c t t h a t i t m u s t h a v e o c c u rr e d i n t h e s p a c e of

o n l y t h r e e m o n th s . N e w c a s i n g h a d b e e n r un i n t h e

w e l l, w h i c h w a s t h e n d e e p e n e d s o m e 7 5 0 f t. T h e

o n l y o t h e r o p e r a t i o n s w e r e i n t h e n a t u r e o f r u n n i n g

e l e c t r ic a l - l o g g i n g l i n e s a n d g un - p e rf o r at i n g t o o l s ,

a ft er wh ich t h e well w a s a b a n d o n e d an d s om e

11 , 0 00 f t of the ?- in . c a s in g re c ove re d .

If a 0 .060-in . de e p p i t c ou ld oc c u r in 3 months , i t

c o u ld , t h e o r e t i c a l ly , p e n e t r a t e t h e w a l l i n a b o u t 2 0

m o n t h s. H o w e v e r , p i t s u s u a l l y s l o w d o w n t h e i r r a t e

of pe ne t ra t ion as t h e y g o d e e p e r .

T h e o n l y c o n c e i v a b l e e x p l a n a t i o n of t h i s u n i q u e

t y p e of c o r ro s i on i s t h a t i t w a s c a u s e d b y a n e l e c -

t r o -g a l v an i c e ff e ct b e t w e e n t h e s t e e l - c l a d e l e c t r i c a l -

l o g g in g o r g u n -p e r fo r a ti n g l i n e s a n d t h e s t e e l o f t h e

p ipe , w i th the mud as a n e l e c t r o l y te . I t w a s k n o w n

t h a t t h e s t e e l l i n e s t o u c h e d t h e p i pe a t v a r io u s

p o i n t s , b e c a u s e s o m e 4 0 l e n g t h s of t h e re c o v e r e d

c a s i n g s h o w e d d ef in it e w ir e- li ne c u tt in g , a n d a t

t h e s e p o i n t s t h e r e m u s t h a v e b e e n m e t a l - t o - m e t a lc on ta c t . The n , wi th som e e le c t ro -po te n t ia l d i f f e re nc e

b e t w e e n t h e s t e e l o f t h e p i p e a n d t h a t o f t h e w o v e n-

s t e e l - c l a d l i n e s , i t c a n be p i c t u r e d t h a t a c ur re n t

flowed from t h e p i p e i n t o t h e m ud , i n t o t h e l i n e , a n d

b a c k t o t h e p i p e a t t h e p o i n t o r p o i n t s o f c o n t a c t .

F u r t h e r , i t i s b e l i e v ed t h a t t h e c u rr e nt , b e i n g s m a l l ,

c o u ld o n l y c a r r y a w a y i r o n i o n s fr om t h e p i p e w h e r e

t h e l i ne w a s h an gi ng q u i t e c l o s e t o it , t h u s ac -

c oun t ing fo r the r e ma rka b ly s t r a igh t - l ine t r e nd o f

t h e o b s e r v e d p i t ti n g . T h i s w o u l d a c c o u n t , t o o , f o r

t h e f a c t t h a t o n l y 4 9 of t h e r e c ov e r ed l e n g t h s s h ow -

e d a n y s i g n s of p i t t i n g .

Corrosion-erosion

T h i s i s a t er m a p p l i e d t o a n y fo rm o f c o r ro s i o n

whic h i s a ggra va te d by a ny ra p id ly moving f lu id , as

ga s , whe re in t he flow re m ove s the p rod uc ts o f c o r-

r o s i o n a n d p r e s e n t s f r e s h s u r f a c e s t o t h e c o r ro d i n g

inf luence .

C o r r o si o n - er o s io n s o m e t i m e s o c c u r s o n t h e i n s i d e

o f t u b i n g h a n d l i n g s l i g h t l y c o r r o s i v e , h i g h -v e l o ci t y

c o n d e n s a t e p r o d u ct i o n. I t m a n i f e s t s i t s e l f mostly i n

t h e d o w n - p o in t i n g e n d s o f t h e m a t i n g t u b i n g l e n g t h s.

T h i s i s i l l u st r at e d i n F i g . 60.

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Fig. 60-11 lustration of Corrosion-eros ion

Courtesy NGAA, " C o n d e n s a t e Well C or ros i on"

T h e e x p l a n a t i o n of t h i s p h e no m e n o n w o u ld s e e m

t o b e as f o ll o w s: A s t h e c o n d e n s a t e f lo w s up w a rd

t h ro u g h a n y g i v e n l e n g t h o f t ub i ng , i t i s f o ll o w i n g

a l m o s t t r u e s t r e a m l i n e f low . B u t w h e n i t r e a c h e s t h e

g a p a t a c o u p l i n g , t u r b u l e n t f lo w i s s e t up w h i c h

c o n t i n u e s a f o o t o r s o i n to t h e n e x t l e n gt h . T h e n

c o r r o s i o n p r o d u c t s , w h i c h t e n d t o fo rm a n d w h i c h

o r d i n a r i l y p r e v e n t f u r t h e r c o r ro s i o n , a r e s c o u r e da w a y a n d c l e a n m e t a l i s p r e s e n t e d t o t h e c o r ro d in g

i nf lu en ce . H e n c e t h e g r ea t er c o r ro s io n of t h e s e

d o w n - p o i n ti n g e n d s .

S um m ary

C a s i n g a n d t u b in g c o r r o s i o n m a y b e c h e m i c a l o r

e l e c t r o - c h e m i c a l , o r b ot h .

Corrosion can occw only in the presence of mois-

t ure.

C o r r o s io n i s t h e r e s u l t o f t h e f ol lo w i n g c a u s e s

( n o t l i s t e d i n t h e o r d e r o f t h e i r i m p o r t a n c e ):

a. Oxi da t i on .

b. Hydrogen s u l f i de f rom s o u r p roduc t i on .

c . M i c ro -b io l og i cal r eac t i on f rom ane ro b i c s u l f a t e -

r e d u c i n g b a c t e r i a .

d . O r g a n i c a c i d a n d c a r b o n d io x i d e i n c o n d e n s a t e

w e l l s .

e . E l e c t r o l y s i s a n d e l e c t ro - g a l v a n ic c u r r e n ts .P r e v e n t i o n o f c o r r o si o n i s r e l a t i v e l y c h e a p if t h e

c a u s e s a n d t y p e s a r e t h o r o u g h l y u n d e r s t o o d .

C h e m i c a l a t t a c k m a y b e r e t a r d e d b y i n h i b i t o r s ,

co at in gs , or by a f ilm of co rros ion pr oduc ts .

T h e o n l y t y p e of c o r r o s io n w h ic h p o s i t i v e l y c a n

b e p r e v e n t e d b y t h e p i p e m a n u f a c t u r e r, a t r e a s o n -

a b l e c o s t , i s " r ingworm"corros ion of tubing - th i s by

f u ll y n o r m a 1 z i n g a f t e r u p s e t t i n g a n d b e f o r e

f in i sh ing .

O t h e r c o r r o s i o n c o u l d b e p r e v e n t e d b y t h e u s e o f

h i gh c h r om e - n ic k e l s t e e l s , s u c h as t h e s t a i n l e s s -

s t e e l a l l o y s ; b u t t h e s e a r e n o t e c o n om i c al i n m o s tc a s e s b e c a u s e o f t h e v e r y hi g h c o s t. L o w e r - c os t

a l l o y s s u c h a s 9 -p e r ce n t c h ro m e o r % p e r c e n t n i c k e l

h a v e s e e m e d e n c o u r a g i n g a t f ir st , b u t h a v e n o t at a l l

p r o v e d t o b e e c o n o m i c a l . I n f a c t , i n h y d r o ge n s u l -

fide, t h e n i c k e l a 1 1 0 y s h a v e s e e m e d t o b e w o r se

t h a n u s e l e s s b e c a u s e o f u n be l i e v ab l y r a p i d h yd ro -

g e n e m b r i t t l em e n t o r s t r e s s - c o r r o s i o n c r a c k i n g.

I n t e rn a l p l a s t i c c o a t i n g o f t u b in g by e i t h e r t h e

m anufac t u re r o r f i el d conc e rn s i s wor th -wh il e ; bu t ,

un fo r t una t e l y , i t is o n 1 y t e m p o ra r y a n d t h e r e f o r e

o n l y a p a r t i a l a n s w e r .

In h i gh - su l fu r o i l f i e 1 d s,

s u c h a s S m a ck o ve r,A r k a n s a s , t h e u s e o f g a l v a n iz e d t u b i n g h a s m et

w i t h s o m e s u c c e s s . H o w e v e r, h e r e a g a i n , t h e h ig h

c o s t u s u a l l y o v e r b a l a n c e s t h e g a i n a n d t h e w r i te r

k n o w s of n o g a l v a n i z e d tu b in g b e i n g u s e d t o da y.

V e r y p e r t i n e n t i s t h e p o t e n t i a l r e v e r s a l p r o p e r t y of

z i n c a t a b o u t 150 F. A b o v e t h a t p o i n t i t b e c o m e s

e l e c t r o - n e g a t i v e t o i r o n a n d c a n n o l o n g e r p r o t e c t i t .

T h e d e g r e e of i n t e r n a l c o r r os i o n i n t u b e s c a n b e

m e a s u r e d w i t h m e c h a n i c a l l y r e c o r d i n g f i n g e r s s u c h

as u s e d f or w ell- bo re s u r v e y s . T h i s m e th od of

c h e c k i n g i n t e r n a l s u r f a c e s i s v e r y c o mm o n ly u s e d

f o r t u b i ng , e s p e c i a l l y i n c o n d e n s a t e w e l l s , a n d c a ng i ve a ve ry accu ra t e p i c t u re o f t he deg ree o f co r ro -

s i o n p i t t i n g a n d t h e p o s i t io n t h e re o f . I t s u s e i n s i d e

c a s i n g , h o w e v e r , is n o t so common, but is q u i t e

w o r th - w h il e e v e r y t i m e t h e t u b i ng i s o u t of t h e w e l l ,

t o i n s u r e t h a t i n te r n al c a s i n g c o r ro s io n i s u n de r

cont ro l .

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5 2 H. G. TEXTER

I n c o n c l u s i o n t h e w r i t e r w o ul d l i k e t o p o i n t o u t

a g a i n t h a t t h e c o r r o s i o n p h a s e o f t h i s p a p e r on c a s -

i n g a nd t u b i n g t r o u b l e s i s m o s t i n a d e q u a t e l y c o v e r -

e d . ? 'h e o n l y e x c u s e i s t h e c o n l p le x i ty o f t h e s u b -

j e c t a s c o m p a r e d to t h e p r e c e d i n g t y p e s o f tr o u b l e s .

REFERENCES

' Hills, J . 0: Desig n o f Casing Programs, Drrllrng and

Productron Practrce, 369 (1939).

Illlls, J . 0: Review of Casing-string D e s i g n Prin-

ciples and Practice, Drrllrng and Production Practrce,

91 (1951).

Texter , I i . G: Casing Strain aft er Cementing, 0 1 1 Ga s

J . , April 8 (1948).

Texter, H . G: Various hlethods of High-pressure Te st -

ing Oil-country Tubular M a t e r i a 1 , ASME, Petroleum

Division, Kansas City, Missouri, September 1952.

Saye, J . E. and Richardson, T . W . G: Field T est ing o f

Casing-string Design Factors, Drlllrng and Production

Practrce, 23 (1954).

Shutts, W . C: Recent Developments i n Joint Deslgns o f

Tubula r Goods, Drillrng and Productron Pract ice, 132

(1934).

7 ~ P ~td 5 A: Speclfic atron for Casr ng, Tubrng, and Drill

Prpe, American P e t r o 1 e u m Instit ute, Dallas, March

1954, 19th Edn.

' A P I R P 5C1: Recomnlended Practice for Care and Us e

of Casrng, Drill P ~ p e , nd Tubrng, American Petroleum

Institute, Dallas, December 1951, 4th Edn.

Iiolmq uist, J . L. and Nadai, A: A Theor etical and E x -

perimental A p p r o a c h to the Problem of Collapse of

Deep-well Cas ing , Drillrng and Production Practice,

392 (1939).

l o Edwar ds, S. H. and Miller, C. P: Discu ssi on on the

Ef fe ct of Combined Longitudinal Loading and External

Pres sure on the Strength o f Oil-well Casin g, Drilling

and Productron Practice, 483 (1939).

l iPayne , John M: A Study-group Investigati on of Equip-

ment and Techniques for 20,000-Ft Drilling, Drtllrng

and Productron Practrce, 123 (1949).

l ZRoberts, D. L: S h e a r Prevention in the Wilmington

Field, Drrllrng and Product~on racttce, 146 (1953).

l 3 Te xt er , H. G: Internal Hydrostatic P ress ure Tes ti ng as

a Measure of Performance Val ues of Oil-well Casing

and Tubing, T h l r d World Petroleum Congress, Th e

Hague, Holland , 195 1.

l 4 Wais, John, Jr: Recent Developments i n Casing Stand-

ards and Design, Drillrng and Production Practice, 249

(1947).

I S Kemler,'E. N: Factors Influencing the Leakage Resist-

ance o f Threaded Pipe Jo in ts , Drrllrng and Productron

Practrce, 275 (1946).

1 6 ~ ~ ~ul 5A2: Bullet rn on Hrgh:pressure Thread Com-

pound, American Petroleum Institut e, Dallas, Novem-

ber 1952, 1st Edn.

17 1ho ma s, P. D. and Bartok, A. W: Leak Res ist anc e of

Casing J o i n t s In T en si on , Drillrng and Production

Practrce, 243 (1941).

'' Kel ly, Harold S. and Hebard, G. G: Tub ing Joi nt s for

High-pressure W ell s, Drillrng and Production Prac tlce ,

381 (1949).19

Eastman, H. John: Producing Directionally D r i 1 1 e d

We ll s, World Oil, 230, April (19 54).20

Lubins ki, Arthur: Influence o f Tens ion and Compression

on Stra ght nes s and Buckling of Tubular Goods in Oil

Wells , Proc. Am. Pet. Inst., Sect. ZV (Prod. Bul. 237)

31 (1951).

"Biron, J. N. and Frazier, B. G: Field Welding on 0 1 1 -

field Tubular Goods, AIME, Petroleum Branch, Paper

No. 388-G.

22Smit h, L. R: Abuse i n Handling Tubular Goods, Drrll-

ing and Productron Practrce, 279 (1937).

z3 Cooley, Herbert M : R e c e n t Developments i n Recom-

mended F ield Practice on Care and Us e of Oil-country

Tubular Goods, Drrllrng and Productron Practrce , 229

(1948).

24 Buchan, R. C: Corrosion and Pr eventi ve Methods in the

K a t y Fie ld, Drtlllng and Productron Practrce, 310

(1946).

25 Copson, H. R: Galvanic Corrosion o f Dissimilar Metals

i n Salt-water So lution s, Drillrng and Productlon Prac-

trce, 219 (1940).

BIBLIOGRAPHY

Dunlop, C. A: T r e n d s and Developments in API Stand-

ards, Drzllrng and Productzon Prac trce, 361 (1939) .

Wescott, Blaine B; Dunlop, C. A; and Kemler, E. N : Set-

ting Dep ths for Casin g, Drrlling and Productron Prac-

tice, 125 (1940).

Cllnedinst, W. 0: Collapse Safety Factors for Tapered

Oil St rings , Drrllrng and Production Practrce, 181 (1945).

Iiettenburg, Robert J . and Schmieder, F. R: Oil-well Cas-

ing Failures, D r t 1 1 ng and Production Practice, 185

(1945).

Peret, J . W: Comparison ,>o fC a s 1 n g Landing Methods,

Petroleum Engrneer, B-101, October (1953).

DISCUSSION

C. A. Dunl op (Hum bl e O i l & Refin ing Co. , H ous-

t o n ) ( ~ r i t t e n ) :H a v i n g b e e n a c q u a i n t e d w i t h H. G.

T e x t e r f o r m a n y y e a r s a n d k n o u i n g h o w . p a i ns t a k in g

a n d t h or ou g h h e i s , i t i s n o t s u r p r i si n g t o s e e h ow

c o ~ l i ~ l e t e l ye h a s c o v e r e d t h e s u b j e c t o f o i l - w e l l

c a s i n g a nd t u b i ng t r o u b l e s . L i t t l e , i f a n y t h in g , i s

l e f t t o a d d t o h i s d i s c u s s i o n o f t h i s s u b j e c t ; a n d

w h a t r e m a r k s I n ia ke n ~ u s t , s a c o n s e q u e n ce , b ec o n fi n ed t o c i t i n g s o m e of t h e e x p e r i e n c e s of t h e

H u m b l e C o m p an y , i n s o f a r a s t h e y ni ay b e r e l a t e d t o

f a i l u r e s o f t h e t y p e s d e s c r i b e d b y t h e a u th o r .

C o n c e r n i n g t h e m a t t e r of c a s i n g , w e , l i k e o t h e r

o p e r a t o r s , h a v e h a d o u r t r o u b l e s w it h d r o p p ed c a s -

i n g s t r i n g s a n d w i t h f a il u r e s f ro m o t h e r c a u s e s i n -

c l ud i ng m il l d e fe c ts . I t i s c u st o m ar y w it h s e a n ~ l e s s

c a s i n g t o c r o p t h e e n d s o f e a c h j o i n t of p i p e ; a n d

u n l e s s t h i s c r op i s s u ff i ci e nt , i t i s p o s s i b l e t o h a v e

m i s a l i g n m e n t of t h e t h r e a d e d e n d of t h e j o i n t , o r a

h oo k, s o t ha t w he n m a k i n g up s u c h p ip e i n t h e

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OIL-WELL CASING AND TUBING TRO UBLE S 53

s t r i n g , e x t r e m e c a u t i o n w i l l h a v e t o b e t a k e n t o p re -

v e n t s t r i p p i n g t h e t h r e a d s a n d h a v i n g a s u b s e q u e n t

f a i l u r e i n t e n s i o n o r c a s i n g - j o in t l e a k s . T h i s c o n di -

t io n c a n b e d e t e c te d b y a n o t a b 1e a n d a b n o r m a l

wo b b l in g of t h e u p pe r e n d o f t h e j o in t of c a s in g as

i t i s m a de u p i n t o t h e s t r in g , a n d is c o r r e c t e d b yf u r th e r c r o p p in g th e e n d s b e f o re t h r e a d in g a n d a

m o re c a r e f u l m i l l i n sp e c t io n o f t h e f i n i sh e d p ip e .

I n d uc t io n h a r d e n i n g of t h e i n s i d e of c a s i n g a t t h e

e n d s o f e a c h l e n g th of p ip e t o o b t a i n 1 0 0 - p e r c e n t

j o i n t e f f i ci e n cy w i t h o ut u p s e t t i n g h a s r e s u l t e d i n t h e

f a i l u r e i n t e n s i o n of c a s i n g . T h i s m e t ho d of s e l e c -

t i v e h e a t tr e a tm e n t c a u s e d h i gh r e s i d u a l s t r e s s e s i n

t h e en d c on n e c ti o n s s o t ha t w he n u nd er t e n s i o n

lo a d , m in u te ' c r a c k s o c c u r r e d wh ic h wo u ld p r o g r e s s

w i t h t i m e t o u lt i m a t e f a i l u r e a n d c a u s e a d r o p p e d

s t r i n g . F ig . 1 i l l u s t r a t e s t h i s c o n d it i on , t h e f a i l u r e

h a v i n g o c c u r r e d a s a c i r c u m f e r e n ti a l b r e a k in t h ebody of the p ipe approx imate ly 1 in . back o f the

t h r e a d s i n t h e i n d u ct i on - t re a t ed s e c t i o n . T h e u s e of

i n d u c t i o n h a r d e ni n g of t h e e n d s of c a s i n g t o o b t a i n

h i g h j o i n t s t r e n g t h h a s s i n c e b e e n a ba n d on e d , a n d

e n d s of c a s i n g a r e n ow b e i n g h e a t t r e a t e d w i t h a

q u e n c h a n d a d r a w to o b t a in h ig h jo in t s t r e n g th .

T h e p r e s e n c e o f a s e a m i n t h e c o u p l i n g i s a m il l

d e f e c t t h a t h a s r e s u l t e d i n q u i t e a f e w t e n s i o n f a i l -

u r e s. T h i s t y p e o f d e f e c t i s i l l u s t r a te d i n F i g . 2.

A l t h ou g h a s e a m i n t h e p i p e b o dy m a y n o t c a u s e

f a il u re i n t e n si o n , w h e n t h i s d e f e c t i s p r e s e n t i n a

c o u p l in g , e v e n th o u g h th e d e p th of t h e se a m is q u i t e

s h a ll o w , i t w i l l i nv a r ia b l y r e s u 1 t i n s p l i t t i n g t h e

c o u p li n g a n d a l lo w t h e s t r i n g t o p a r t a t t h e c o nn e c-

t i o n b e c a u s e of t h e h i gh h o o p s t r e s s e s t o w h i c h

c o u p li n g s a r e s u b j e c t e d w h e n t h e p i p e i s h un g i n

t e n s i o n . B e c a u s e s o m e s e a m s a r e di ff ic u lt , if n o t

i m p o s s i b l e t o d e t e c t v i s u a l l y w i t h o ut s o m e v i s u a l

a i d , m a g n e t i c - p a r t i c l e i n s p e c t i o n m e t h o d s are con-

s i d e r e d to b e n e c e s s a r y if t h i s t y p e of f a i l u r e i s t o

b e p r e v e n t e d .

T h e e f f e c t of a p u l l -o u t o r t e n s io n f a i l u r e a t a n

API c a s i n g c o n n e c t i o n is f u l l y c o v e r e d i n t h i s p a-

p e r. Ho we v e r , i t m a y b e of i n t e r e s t t o f ur th e r i l l u s -

t r a t e b y a n a c tu a l p h o tog r a ph ( F ig . 3) t h e n e c k i n g

d o wn of t h e p ip e o r m a l e m e m b er of t h e c o n n e c t io ni n t h e v i c i n i t y o f t h e l a s t 7 o r 8 e n g a g e d t h r e a d s .- -I t w i l l b e n o t e d t h a t t h e 4 u n e n g a g e d t h r e a d s h a v e

b e e n e l o n g a t e d t o a l m o s t t w i c e t h e o r i g i n a l p i t c h ,

a n d t h e 10 o r 11 t h r e a d s a t t h e e n d o f t h e p i p e h a v e

b e e n d i s t o r t e d i n t h e d i r e c t i o n of t h e l o a d a t t h e

t im e th e j o in t ju m pe d o u t. T h i s w a s a c a s in g t e n -

s i o n f a i l u r e t h a t l a t e r w a s f o un d t o b e t h e r e s u l t o f

t h e p i p e n o t h a v i n g t h e r e q u i r e d p h y s i c a l p r o p e r t i e s

o f t h e g r a d e u s e d .

duct ion-hardened Casing

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54 H. G . TEXTER

Fig . 2 (Dunlop)-Seams in Couplings

Reference is made by the author to failures in I effect on the grain structure. Although the actual

tension caused by a localized hardened area in the

pipe. Fig. 4 illustrates such a fai lure which origi-nated at the area marked "AA" and which had a

Brinell hardness of 514 making it extremely brittle.

Th e remainder of the pipe had a Brine11 hardness of

only 229. The photomicrographs clearly show the

- -cause of the localized hardening was never deter-

mined, the condition could have been caused by thepipe being heated with a welding torch and then

quenched or rapidly cooled with water.

In r e g a r d to the fac to r of safety on the joint

strength of API c a s i n g that i s to be used, th is

- - I S T O R T E D T H R E A D S I NI D I R E C T I O N O F L O A D -

- - - --- - - - - .. . . - -

+ I T C H OF -+

T H R E A D E L O N G A T E D

Fig. 3 (Dunlop)-Casing Pull-out or Tension Fa ilu re

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M A G N I t I C A I I U N 10 0

Sl H U C l UR E I N HARDENED AREA

BRINELL HARDNESS 514

MAGNIFICATION 100 XSTRUCTURE IN UNHARDENED AREA

BRI.NELL HARDNESS 229

Fig.4 (Dunlop)-Casing Fai lur e at Point of Localized Hardening

should take into consideration the leak-resistance

properties of the joint. A factor of s a f e t y of 1.8

means that the actual load placed on the joint will

not be greater than 55.5 percent of the pull-out or

ultimate joint strength. Tests of the leak resistance

of cas ing joints in t ension reported by T. McLean

Jaspe r in his discussion of the p a pe r by Thomas

and Bartok, given in the author's li st of references ,

indicates that the average stress at first l e a k a s

compared to the st re ss a t pull-apart of the joint can

be 81.4 percent. Also, the minimum st re ss value

relation can be 55.2 percent of the pull-apart value.

This latter value would support the use of a factor

of safety of 1.8 on the basis of the leak-resistance

property of the API cas ing joint.

A s st at ed by the author, the majority of oil-well

tubing being used today i s of the external-upset de-

sign. With the use of N-80 and higher-grade material,

there have been ca ses of defect ive upset s reported,

some of which have resulted in very costly failures.

Almost every operator knows from sad experience

tha t a dropped string of tubing ins ide of a relatively

small-diameter str ing of cas ing d oes not afford very

much clearance in which to work success'fully with

the fishing t o o 1s available. Consequently, when

faced with a situation such a s this, the operator has

his investment clearly i n mind and therefore i s in

no frame of mind to think kindly of anyone respon-

sible in any way for such a failure. Defects in the

upset which can cause a failure in tension may be

in the form of cracks or cold shuts which are best

detected by suitable magnetic-particle inspection.

The def ect s can be the resul t of using improper up-

setting technique or faulty steel, and some are il-

lustrated in Fig. 5.

Plug scores on the inside of tubing have been the

source of many burst failures at relatively low in-

ternal pressures. Although lug scores are some-

t h i n g that one must live with, I suppose, severe

lug scores indicate poor housekeeping in not keep-

ing the plug-mill operations clean. An examination

of m a n y plug-score failures indicates that, in the

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56 H. G . TEXTER

CRACK IN UPSET TUBIN(

MAGNIFICATION 100 X

Fig. 5 (Dunlop)-Defects in Pip e Upsets

s t r a ig h ten ing ope ra t ion a t t he mil l , t oo much p res -

s u r e w it h t h e r o l l s h a s b e e n a p p l i e d a n d t h e p i p e

h a s b e e n c r a c k e d wi th t h e c r a c k s t a r t i n g a t a p l u g

s c o r e and ex tend ing pa r t way in to the wa l l o f t he

tub ing as s h o w n i n Fig. 6. T h e c r a c k s m a y n o t b e of

a d e p th t o c a u s e f a i l u r e a t t h e r e l a t i v e ly l ow m i 1 1

h y d r o s t a t i c t e s t p r e s s u r e , b u t d o r e s u l t i n f a i l u r e a t

h i g he r p r e s s u r e s a p p l i e d i n t h e fi el d.

T u b i n g l e a k a g e , p a r t i c u l a r ly i n g a s - c o n d e n s a t e

w e l l s , h a s b e e n a n u i s a n c e p r o b l em f o r ma ny o p e r-a t o r s a n d h a s c a u s e d l a r g e e x p e n d i t u r e s of m o ne y

fo r pu l l ing 1 e a k i n g t u b in g s t r i n g s a n d i n w o rk in g

o v e r w e l l s b e c a u s e of t h i s t r o ub l e . A s a r e s u l t of

th i s d i f fi cu lty , s p ec ia l p r emium-p ri ced tub ing ha s

b e e n u s e d i n p l a c e o f A P I t ub in g . P r o p e r c a r e a n d

a t t en t ion to the make-up o f API tub ing wi l l el im-

ina te mos t o f the d i f f i cu l ty expe r i enced wi th the API

c o n n e c t i o n s f ro m t h e s t a n d p o i n t of l e a k s . I n s p e c t i o n

o f th r eads be fo re runn ing , i i s e o f a prope r lub r i can t

a p p l i e d i n t h e f i el d t o a l l c o n n e c t i o n s , a n d a n a d e -

qua t e number of th r ead s make-up , wh ich i s bes t ob -

t a i n e d b y t h e u s e o f p o w e r tu b i n g t o n g s , w i l l r e s u l t

in i m p r o v e d pe r fo rmance o f the API jo int . I n the

ca s e of th readed -and -coup led p ipe , a l low ing the

coupl ing to f loat or make up f ree ly OI ? bo th p ipe ends

wi l l r e s u l t i n a be t t e r -made co nnec t ion f rom a l eak -

r e s i s t a n c e s t a n d p o i nt . I t w il l b e of i n t e r e s t p e r h a p s

to men t ion tha t r a the r ex haus t ive t e s t s o f 2 'h -in. OD

API round - th r ead tub ing connec t ions , bu t wi th the

OD of the f ema le o r box member inc r eas ed by 0 :25

in ., h ave s hown th a t t h i s t ype o f connec t ion wi lls u c c e s s f u l l y h o l d a n it r og e n g a s p r e s s u r e o f 1 5 , 0 0 0

p s i w i t h o u t l e a k i n g a n d w i t h t h e p r e s s u r e b e i n g

v a r i e d b e t w e e n 1 3 , 00 0 p s i a n d 15,000 p s i b y t h e

c h a n g i n g t e m p e ra t u re o f t h e g a s w h i c h i s k e p t a t

c o n s t a n t v ol um e . T h i s d o e s n o t m e a n t o s a y t h a t

the AP I tub ing connec t ion wi th s om e mod i fi ca tion

o f the coup l ing OD i s r ecommended fo r 15 ,000-ps i

s e r v i c e , b u t d o e s i n d i c a t e t h e API connect ion modi-

f ie d t o b e s u i t a b l e a nd i n e x p e n s i v e f o r r e l a t i v e l y

h i g h - p r e ss u r e s e r v i c e .

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OIL-WELL CASING AND TUBING TR OU BLE S 57

M A G N I F I C A T I O N 100 X

D E C A R B U R I Z A T I O N AT

P L U G S C O R E A N D C R A C k

M A G N I F I C A T I O N 100 X

C R A C K A T P L U G S C O R E C A U S E D

B Y R O T AR Y S T R A I G H T E N I N G

F i g . 6 (Dunlop)-Plug Scores a n d Subsequent Cracking

Roy A. Bobo (Phi ll ips Petroleum Co., Houston)

(written): T he one item of equipment th at plays a

predominant role in the drilling and con~pletio n fwells alike i s the tubular material. Th e casi ng pro-

gram has to be se lecte d before the well i s drilled.

It governs si ze of the bi ts tha t can be used in drill-

ing. It figures prominently in the type of completion

that can be made. Tubing itsel f has a more promi-

nent influence on the completion of wells . With the

advent of deeper higher-pressure wells, casing and

tubing become critical items, both in regard to safe-

ty and to total cost in the drilling and completion

processes.

M. Texter i s among those individuals who are

respon sible for the remarkable advancement that ha s

been made in tubular goods. We are fortunate tha t

he has set his 30 years of tubular-goods experience

in writing. My own company has experi enced many

of the fai lures li st ed by Mr. Texter. Th is informa-

tion will be valuable reference material for devel-

opment of better tubular products.

In order to be constructive, two comments will be

offered. Th e first pertains to the second paragraph

on page 19, wherein the author prefers to apply bi-

axial loading correction factors to a string of cas-

ing on the assumption that tensile loading varies

from zero at the bottom to a maximum at the top. Afraction of' a total strin g of casing, equivalent tothe ratio of the mud density to the st ee l density, i s

in compression. Inasmuch as this bottom portion

(12.7 percent for water; 26 percent for heavy muds)

i s in compression, the generally acce pted biaxial-

stress correction factors are incorrect and result in

placing too much steel in the wrong p 1a c e s in a

str ing of casing. A s an e x am p 1e of the effect of

buoyant force on tensile loading, it i s interesting to

note that the bottom 1,570 ft of the casing string in

the author's example on page 18 was a c t u a11y in

compression. Neglecting compressive e f f e c t s of

pressure, biaxial correction for the effect of tensionon casing should begin a t this point. Safety factors

are always in order but they should be based on the

correct principle.

On page 20, the statement i s made: "The us e of

rubber protectors, adequate in s i z e and properly

placed on the drill pipe, will effectively p r e v e n t

wear of the casing." In th is regard, h4r. Texter is

asked whether, i n his experience, he has acquired

data to show the comparative cutting action of hard-

ened steel with that of rubber rotating against cas-

ing in an abrasive circulating medium.

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58 H. G. TEXTER

Blr. Texter i s deserving of the highes t cornmenda-

tion for this contribution.

Mr. Texter : Effectively, what Rlr. Bobo i s saying

in his third paragraph i s that the author did not take

buoyancy into account when cal cula ting the effect of

biaxial str es se s. This , however, i s justified by com-mon usage. In no i n s t a n c e , to the best of the

writer's knowledge, does anyone ever take into ac-

count the buoyancy of the liquid in which a casing

string is being s e t #hen calculating safety factors.

hlr. Uobo i s right in sayin g that "safety factors

are always in order but they should be based on the

correct principles." However, rather than correc t

just one phase of design-factor calculations, the

whole subject s I1o u 1d be reviebbed and everyone

should revise his method so as to take into accouiit

all pertinent factors. The writer was not discussing

the s e l e c t o n of design factors, but w a s merely

pointing out a case of trouble resulting from an in-

sufficient design factor, based on the con~monlyac-cepted method of arriving at these facto rs.

The author i s very glad to have hlr. Bobo's next

to last paragraph added to his paper, intimating that

rubber rotating against casing in an abrasive circu-

lating medium can cut into the steel \\ith, it is pre-

sumed, resultant c a s i n g failure. Thi s i s a worth-

while addition to the long list of troubles discussed

in the paper.

CO2.5

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