8-Casing Setting Depth Design

8/10/2019 8-Casing Setting Depth Design

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CASING SETTING DEPTHDESIGN


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Casings are set in a drilled hole for one

or more of the following reasons: to protect contamination of underground

water formations,

to prevent drilling problems such as holecollapse, differential sticking of pipes,

to prevent fracturing of subsurface formations,

to produce oil or gas from producing pay zoneor zones.


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Three types of casings are used for these

purposes:

Surface casings,

Intermediate casings (and liners),

Production casings (and liners).


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Surface casing setting depth:

As a drilling engineer you will not be in a position

to select a setting depth for surface casing.

This depth is provided by the authorities of local

government by considering depth of water table.

Surface casings should be set deep enough so thatdrilling fluid can not contaminate underground

fresh water sources by mud filtration.


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Production casing setting depth:

As a drilling engineer youwill not select setting depthfor a production casingstring.

Production divisions of oilcompanies provide thisdata for you.

They consider productionaspects of well such as payzone depth, well completiontype, etc.


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You will bedesigning setting depthsfor intermediate casings considering

safety of drilling only,

Properly designed intermediate

casings are the ones which help

drilling engineer reach to specified

targetsafely


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Containing 20 bbl of gas kick means

satisfing condition in the left figure

Pinside

< Pfrac

@ shoe Pinside

> Pfrac

@ shoe

Formations

should never be

fractured by

internal

pressures

Safe


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Avoiding differential sticking

I nternal pressure

should never exceed

1000 psi formationpressure across

permeable zones in

order to avoid

dif ferential sticking

(Pmud

-Pformation

)1,000 psi


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So, how do I select a setting depth

for the next casing set ?

You will be given

a depth at which

surface casing shoe

will reside C.

You need to determine

maximum depth to

which you can drill while

satisfying previous two

criteria D.

This depth

will be the setting depth

for your next set of

casings.


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Procedure When you take 20 bbl CH4

kick in the hole at D,

pressure inside the hole

at c must be less than

formation fracture

strength at C

Pinside @ C

= Pformation @ D

- 0.052kiL

ki

- 0.052 MW (D-C-Lki

)

Pformation @ D

= 0.052 (ePf @ D

) D

ePf @ Dis pressure gradient (ppg) at D


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Satisfy the first design criteria byselecting a formation, which will

satisfy following condition:Pfr acture @ C

Pinside @ C

Note that, the formations getstronger as you drill deeper.

Formation fracture gradients(ePff) can be determinedempirically as a function ofdepth (see the example).

Pfr acture @ C

= 0.052 (ePf f) C

ePff @ C is fracture gradient (ppg)at C


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Pfr acture @ C Pinside @ C

0.052(ePff @ C

) C 0.052 (ePf @ D

) D-0.052kiL

ki-0.052 MW (D-C-L

ki)

( ePf @ D

- MW ) [C (ePff @ C

- MW) - Lki

(MW -ki

)] /D

ePf @ D

-MW is known as kick tolerance(KT)

For a proper casing setting depth, the value of KT is recommended to be


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0.052 ePff C 0.052 ePf D 0.052 L 0.052 MW D C L( )

ePff C ePf D L MW D C L( )

ePff C ePf D L MW D MW C MW L

eP

ff

C MW C MW L L eP

f

D MW D

C ePff MW L MW L L D ePf MW

C

DePff MW L

DMW L L ePf MW

KT C

D

ePff MW L

D

MW L L


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Example

The pore pressure for a projected well are determined as in the

following table (see Dec 2000-newsletter). The surface casing is

set at 5,100 ft. what is the maximum depth to which the second

string may be set and maintain a kick tolerance of

Kick Volume = 20 bbl

Bit Size = 12 inch

ODdp= 5 inch

CH4kick

DCs = 8 x 3, 360

MW = pore pressure + 150 psi


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Table of pore pressures

I assume, you already know how to estimate formation pore

pressures prior to drilling (see previous newsletter if you dont).

Depth, ft Pore

Pressure,

psi0 0

5,100 2 519.4

7,800 3 853.2

8,200 5 628.5

12,000 8 236.8

12,100 5 977.4


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Calculate ePf = Pore Pessure/(0.052 x Depth)

Depth, ft Pore

Pressure, psi

ePf

ppg

0 0 8.6

5,100 2 519.4 9.5

7,800 3 853.2 9.5

8,200 5 628.5 13.2

12,000 8 236.8 13.2

12,100 5 977.4 9.5

for depth=0, ePf= 8.6 ppg


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Calculate over-burden stress (s) based on Ben Eatons data:

Depth, ft Pore Pr.

psi

ePf

ppg

s

0 0 8.6

5,100 2 519.4 9.5 0.9097

7,800 3 853.2 9.5 0.9332

8,200 5 628.5 13.2 0.9363

12,000 8 236.8 13.2 0.9611

12,100 5 977.4 9.5 0.9617

s = 0.84753 + 0.01494 D/1,000 - 0.0006 (D/1,000)2 + 1.199 x 10-5

(D/1,000)3


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Poissons ratio

is the negative ratio of transverse to axialstrain. When a materialis compressed in

one direction, it usually tends to expand in

the other two directions perpendicular tothe direction of compression. This

phenomenon is called the Poisson effect.

Poisson's ratio (nu) is a measure of this

effect. The Poisson ratio is the fraction (or

percent) of expansion divided by the

fraction (or percent) of compression, for

small values of these changes.
http://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Materialshttp://en.wikipedia.org/wiki/Nu_(letter)http://en.wikipedia.org/wiki/Nu_(letter)http://en.wikipedia.org/wiki/Materialshttp://en.wikipedia.org/wiki/Strain_(materials_science)


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Poissons Ratio (v) based on Ben Eatons data:

Depth, ft Pore Pr. psi ePf

ppg

s v

0 0 8.6

5,100 2 519.4 9.5 0.9097 0.4088

7,800 3 853.2 9.5 0.9332 0.4360

8,200 5 628.5 13.2 0.9363 0.4384

12,000 8 236.8 13.2 0.9611 0.4546

12,100 5 977.4 9.5 0.9617 0.4545

v = 0.23743 + 0.05945 D/1,000 - 0.00668 (D/1,000)2 + 0.00035

(D/1,000)3

- 0.671 x 10-5 (D/1,000)4


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Finally, the formation fracture gradient, ePffcan be predicted

empirically

eP

vv

s eP eP

ff

f f

10 052 0 052

0 052

. .

.

Depth,

ft

Pore Pr. psi ePf

ppg

s v ePff

ppg0 0 8.6

5,100 2 519.4 9.5 0.9097 0.4088 15.10

7,800 3 853.2 9.5 0.9332 0.4360 16.03

8,200 5 628.5 13.2 0.9363 0.4384 16.95

12,000 8 236.8 13.2 0.9611 0.4546 17.60

12,100 5 977.4 9.5 0.9617 0.4545 17.01


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Remember, we are trying to find a value for D at which KT =

KT [C(ePff @ C- MW) - Lki(MW- ki)]/D

C = 5,100 ft and ePff @ C = 15.10 ppg

Lki= 20 bbl/CapacityDC-H

CapacityDC-H= 0.0836 bbl/ft

Lki= 20 bbl / (0.0836 bbl/ft) = 239 ft

since 239 ft < ldc(360 ft), Lki= 239 ft


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Trial & error solution, assume D = 10,000. From 5,100 to 10,000 you

will be drilling open hole. So you will design a MW such that the mud

pressure must not fall below the maximum anticipated formation

pressure in the open hole section.

Determining the required MW


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So, Mud Weight isselected as MW = 13.55ppg

finaly, assuming methanekick, ki @ D= ?

real gas law

PV = z n RT

PV = z (m/MW) RT

P MW = z (m/V) RT

P MW = z () RT

ki@D= (P@DMW)/( z RT@D)

molecular weight of CH4 (MW) = 16

P@D= 0.052 (ePf @ D) D

= 0.052 (13.2) 10,000

= 6,864 psi

T@D= 65 + (3of/1,000 ft) D

= 65 + (3of/1,000 ft) 10,000

= 95 of or (460+95 = 555 oR)

R = 80.186

z = a + (1-a) / eb+ c pprd


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a = 1.39(Tpr- 0.92)0.5- 0.36 T pr- 0.101

b = (0.62-0.23 Tpr) Ppr+ { [0.066/(Tpr- 0.86)] - 0.037 }Ppr

2+ (0.32 Ppr6) / [10 9 (Tpr

- 1)]

c = (0.132 - 0.32 log Tpr

)

d = antilog (0.3106-0.49 Tpr+ 0.1824 Tpr2)

Tpr= T/343,

Ppr= P/667.8


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T= 555

P= 6,864

Tpr= 1.62

Ppr= 10.28

A= 0.48

B= 7.84C= 0.07

D= 0.99

Z= 1.13

ki@10,000= (6,864x16)/(1.13x80.186x555 )

ki@10,000= 2.184 ppg


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substituting,

C=5,100 ft Lki= 239 ft

ePff @ C

=15.1ppg ki@10,000= 2.184 ppg

MW = 13.55 ppg D = 10,000 ft

into [C (ePff @ C

- MW) - Lki(MW -

ki)] /D

[5,100(15.1-13.55)-239(13.55-2.184)]/10,000 = 0.519

0.519 > for your next iteration choose a greater depth (i .e. 10,500 ft).

F inal ly, iterate unti l KT = .

8-Casing Setting Depth Design

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