222669368 L8 Drill String Design in Directional Wells

8/12/2019 222669368 L8 Drill String Design in Directional Wells

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Drill String Design in Directional Wells

Arun S Chandel

Assistant [email protected]

09997200339

1


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Neutral

Point The neutral point in a drill string can be defined as the point

where the string changes from tension to compression

This point is a function of bit weight and buoyancy

, ,the whole drillstring is suspended off bottom, in this case theentire string is in tension with neutral point right at the bit.Secondly, imagine the whole drillstring is set on bottom with noload being taken by the surface equipment, in this case the stringis in compression and the neutral point is at surface.

s mpor an o now e oca on o s rans on po n orneutral point for several reasons. If the neutral point is at the jars,for example, then the drill string and jars could both be damaged.If the neutral oint is allowed to move u into the drill i e

buckling could occur.


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assembly for regular vertical and directional drilling operations whenpossible. There may be a problem in high angle and horizontal drillingin this respect because of the difficulty in maintaining bit weight.Dama e at the neutral oint ma be stron l de endent u ondrillstring rotation, and consideration should be given to critical rotary

speeds and their associated harmonics.

Theformulatocalculatethelengthfromthebittotheneutral

pointinaverticalholeifonlydrillcollarsarebeingusedis:

Ln = {BitWeight}/{WxBF}

(W h e r e W = Co l l a r o r p i p e w e ig h t i n l b s / f t & B F = B u o y a n cy Fa ct o r )


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e. . Determine the neutral oint in:8x213/16DCsifWOB=30klbsin11ppg mud

L = 30 000 150 x 0.832 = 240SoNPis240ftupincollars

Butoftenwellhavetheneutralpointabovethe

collars somewhereintheheviwate,lets

havealook

at

this.


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Lnphw= {BW (WcxLcxBF)}/{WhwxBF}

Where:

L = Distance from bottom of HWDP to NP

BW = Bit Weight

Wc

= Weight per foot of collars

L = Len th of collars

Whw = Weight per foot of Hevi-Wate

BF = Buoyancy Factor

Check first though to see if the NP is within the collars


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. . DCs&600ftof5HWDPifWOB=40k.lbsin13ppg mud

Is NP within collars?Lnp = Bit Weight / [W x BF] or:

, x . =

So NP i sabove collars..

But where?

L = 40 000 150 x 300 x 0.801 / 50 x 0.801

Or: 3,955 / 40.05 = 98.75 ft

So - NP i s ~ 9 9 f t into the HWDP


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NeutralPointCalculations

in

(DrillCollars+HWDPs)


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Directional Well, Neutral Point in the

r o ars

When the neutral point is in the drill collar section and thecollars are all of the same diameter, the formula for neutralpoint is:

np

WOB

L =DC

ere: = borehole inclination

WDC = weight per foot of the drill collars


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Directional Well Neutral Point in the HWDP

When the neutral point is in the HWDP section and the drillcollars are all of the same diameter, the formula for neutralpoint is:

{ ( ) cos }DC DCnphw

WOB W L BF

L

=hw

ere:= borehole inclination

Whw = weight per foot of the HWDP


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The last formula can be expanded in the case of a taperedw t r co ars o more t an one ameter. or examp e,

if there were two sizes drill collars but the neutral point was inthe hevi-wate the formula would become:

1 1 2 2{ cos ( }DC DC DC DCWOB BF W L W LL +

=

coshw

W BF

= borehole inclination

WDC1 & WDC2 = weight per foot of first and second size of drill


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PROBLEM3

Determine the neutral point in a 300 inclined well:300 of 6.5 x 2-1/4 DCs + 200 of 7-1/4x 2 DCs

,

WOB = 45k-lbs in 12 ppg mud

Lnp1 = Bit Weight / [W x BF] or:, . . .So NP i s a b o v e t h i s c o l la r ..

np2

45,000-(300x99.5x0.82xcos30)/[129.3 x 0.82 x cos30]

.So NP i s a b o v e t h i s c o l la r ..


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PROBLEM3

But where?

Lnphw

= {45,000 [0.82x cos30x (99.5x 300+ 129.3x 200 ]}/ [50 x 0.82x cos30]

Or: 523 / 41.0 = 153.15 ftSo - NP i s ~ 1 5 3 .1 5 f t i n t o t h e HW D P


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TENSIONDESIGN


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1. Static Load

The design of the drillstring for static tension loadsrequires sufficient strength in the topmost joint of

each size, weight, grade and classification of drillpipe to support the submerged weight of all the drill,

stabilizers, and bit.

e an s a zer we g s are e er neg ec eor are included with the drill collar weight.

This load may be calculated as shown in thefollowing equation:


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1.StaticLoad


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Tensile

Yield

Stren thinpoundscanbecalculatedforClassIdrillpipe

(newdrill

pipe)

using

the

following

formula:

TensileYieldStrength(lbs),Ym =

Min.YieldStrength(lb/in2

)x /4(OD2

ID2

)


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formula above it is likely that some permanent stretch will

occur and difficulty may be experienced in keeping the

pipe straight.

90% of the tabulated tension value is recommended.


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2.Overpull

If the drill string were to get stuck in the well bore,the operator would want to know how much additionalens on, or pu , can e app e o e s r ng e ore

exceeding the yield point of the drill pipe. This isknown as overpull since it is pull force over the weight

.

Maximum overpull is the difference between the

overpull (MOP) is normally applied)


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MarginofOverpull(MOP)

The difference between the calculated load FTENand the maximum allowable tension load

represents the Margin of Over Pull (MOP):

called the Safety Factor (SF).


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FinalDesignEquation


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Example5:DrillStringDesignbasedonMOP


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v wd a t a .

Can the final well depth be reached with thisassembly?Finally make a table showing all the drillstrin com onents with their air & buo edweight.

1. The Yield Strength of grade E drill pipe=

225,771 lb and weight/ft = 18.37 lb/ft.

2. The Yield Stren th of rade X-95 drill i e=329,542 lb and weight/ft = 18.88 lb/ft.


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Solution


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222669368 L8 Drill String Design in Directional Wells

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