18. Casing Design Example

8/13/2019 18. Casing Design Example

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PETE 411Well Drilling

Lesson 18

Casing Design Example


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Read: Applied Drilling Engineering, Ch.7

HW #9 - Velocity ProfilesDue 10-18-02

PETE 411 Lessons can be found at:

http://pumpjack.tamu.edu/~juvkam-wold/Multimedia Programs can be found at:

Network Neighborhood / juvkam-wold2 / Multimedia


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Casing Design Example

Design a 9 5/8-in ., 8,000-ft combination

casing string for a well where the mud wt.will be 12.5 ppg and the formation porepressure is expected to be 6,000 psi .

Only the grades and weights shown areavailable (N-80, all weights) . Use APIdesign factors.

Design for worst possible conditions.


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Casing Design - Solution

Before solving this problem is it necessary to

understand what we mean by Design Factorsand worst possible conditions.

API Design FactorsDesign factors are essentially safety factorsthat allow us to design safe, reliable casing

strings. Each operator may have his own setof design factors, based on his experience,and the condition of the pipe.


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Casing DesignIn PETE 411, well use the design factors

recommended by the API unless otherwisespecified .

These are the API design Factors :

Tension and Joint Strength: NT = 1.8Collapse (from external pressure): Nc= 1.125Burst (from internal pressure): Ni = 1.1


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Casing Design

What this means is that, for example, if weneed to design a string where the maximumtensile force is expected to be 100,000 lbf ,

we select pipe that can handle 100,000 * 1.8= 180,000 lbf in tension.

Note that the Halliburton Cementing Tableslist actual pipe strengths, without safetyfactors built in.


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Casing Design

Unless otherwise specified in a particular

problem, we shall also assume the following:

Worst Possible Conditions1. For Collapse design, assume that thecasing is empty on the inside (p = 0 psig)

2. For Burst design, assume no backupfluid on the outside of the casing (p = 0 psig)


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Casing Design

Worst Possible Conditions, contd

3. For Tension design,assume no buoyancy effect

4. For Collapse design,assume no buoyancy effect

The casing string must be designed to stand up to theexpected conditions in burst, collapse and tension .Above conditions are quite conservative. They are alsosimplified for easier understanding of the basic concepts.


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Burst Requirements (based on the expected porepressure)

The whole casing string must be capable of withstanding this internal pressure without failing inburst.

psi600,6P1.1* psi000,6

Factor Design* pressure poreP

B

B

=

=

=

D

e p t h

Pressure


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Collapse Requirements

For collapse design, we start at the bottom of the string and work our way up.

Our design criteria will be based onhydrostatic pressure resulting from the 12.5

ppg mud that will be in the hole when thecasing string is run, prior to cementing.


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Casing Design

Collapse Requirements, contd

severelessaretsrequiremencollapsetheholetheupFurther

.bottomtheatd'reqpsi850,5P125.1*000,8*5.12*052.0

factor design*depth*weightmud*052.0P

c

c

=

=

=

D e p t h

Pressure


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Casing Design

Reqd: Burst: 6,600 psi Collapse: 5,850 psi


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Casing Design

Note that two of the weights of N-80 casing

meet the burst requirements, but only the53.5 #/ft pipe can handle the collapserequirement at the bottom of the hole ( 5,850

psi ).

The 53.5 #/ft pipe could probably run all the

way to the surface (would still have to checktension), but there may be a lower costalternative.


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Casing Design

To what depth might we

be able to run N-80 , 47#/ft? The maximumannular pressure that thispipe may be exposed to,is:

psi231,4125.1760,4

factor design pipeof pressureCollapsePc ===

D e p t h

Pressure


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Casing Design

First Iteration

At what depth do we see this pressure (4,231psig) in a column of 12.5 #/gal mud?

ft509,65.12*052.0

231,45.12*052.0

Ph

h*5.12*052.0P

c1

1c

===

=


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Casing DesignThis is the depth to which the pipe

could be run if there wereno axial stress in the pipe

But at 6,509 we have (8,000 - 6,509) =1,491 of 53.5 #/ft pipe below us.

The weight of this pipe will reduce thecollapse resistance of the 47.0 #/ft pipe!

8,0006,509


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Casing Design

Weight, W1 = 53.5 #/ft * 1,491 ft

= 79,769 lbf This weight results in an axialstress in the 47 #/ft pipe

psi877,5in13.572

lbf 769,79

areaend

weight

Sof 21 ===


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Casing Design

The API tables show that the abovestress will reduce the collapse resistancefrom 4,760 to somewhere between

4,680 psi (with 5,000 psi stress)

and 4,600 psi (with 10,000 psi stress)


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Casing Design

Interpolation between these values showsthat the collapse resistance at 5,877 psiaxial stress is:

psi148,4

125.1

666,4P

psi666,4)600,4680,4(*)000,5000,10(

)000,5877,5(680,4P

cc1

1c

==

==

With the design factor,

( )2112

11c1P P P S S

S S P

=


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Casing Design

This (4,148 psig) is the pressure at adepth

Which differs considerably from theinitial depth of 6,509 ft , so a seconditeration is required.

ft382,65.12*052.0

148,4h 2 ==


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Casing Design

Second IterationNow consider running the 47 #/ftpipe to the new depth of 6,382 ft .

psi378,6in572.13lbf 563,86

S

lbf 563,865.53*)382,6000,8(W

22

2

==

==


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Casing Design

Interpolating again,

This is the pressure at a depth of

( ) psi pcc 140,4600,4680,4*50005000378,6680,4125.1 12 ==

ft369,65.12*052.0

140,4h 3 ==

( )

= 21

12

11c1 D.F.

1P P P S S

S S P


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Casing Design

This is within 13 ft of the assumed value. If more accuracy is desired (generally notneeded), proceed with the:

Third Iteration

psi429,6572.13259,87S

lbf 259,875.53*)369,6000,8(W

'369,6h

3

3

3

==

==

=

Pcc3

= ?


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Casing Design

Third Iteration, contd

2

3

140,4

)600,4680,4(*000,5

000,5429,6680,4

125.1

1

cc

cc

P psi

P thus

==

=


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Casing Design

Third Iteration, contd

This is the answer we are looking for, i.e.,we can run 47 #/ft N-80 pipe to a depth of 6,369 ft , and 53.5 #/ft pipe between 6,369and 8,000 ft .

Perhaps this string will run all the way to thesurface (check tension), or perhaps an evenmore economical string would include some43.5 #/ft pipe?


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N-8053.5 #/ft

N-8047.0 #/ft

N-8043.5 #/ft ?

Depth = 5,057 ?5,066?5,210?

Depth = 6,3696,3696,382

6,509

8,000

Burst?


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N-8053.5 #/ft

N-8047.0 #/ft

N-8053.5 #/ft ?

Depth = 6,3696,369

6,3826,509

8,000

Tension?


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Tension Check

The weight on the top joint of casing

would be

With a design factor of 1.8 for tension, apipe strength of

weightactual602,386

)/#5.53*631,1()/#0.47*369,6(

lbs

ft ft ft ft

=

+

requiredislbf 080,695602,386*8.1 =


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Tension Check

The Halliburton cementing tables give ayield strength of 1,086,000 lbf for the pipebody and a joint strength of 905,000 lbf for LT & C.

surfacetoOK isft/#0.47


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Casing Design Review

We have 4 different weights of casingavailable to us in this case:1. Two of the four weights are unacceptable

to us everywhere in the string because

they do not satisfy the burstrequirements.

2. Only the N-80 , 53.5 #/ft pipe is capable of withstanding the collapse requirementsat the bottom of the string


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3. Since the 53.5 #/ft pipe is the mostexpensive, we want to use as little of itas possible, so we want to use asmuch 47.0 #/ft pipe as possible.

4. Dont forget to check to make sure thetension requirements are met; both for pipe body, and for threads andcouplings (T&C).


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The collapse resistance of N-80 , 47 #/ft will

determine to what depth it can be run. Twofactors will reduce this depth: Design Factor Axial Stress (tension)

Halliburton collapse resistance: 4,760 psi Apply design factor: psi231,4

125.1760,4

=


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To determine the effect of axial stress

requires an iterative process:

1. Determine the depth capability withoutaxial stress

2. Determine axial stress at this point

ft509,65.12*052.0

231,4depth ==


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3. Determine corresponding collapse resistance

4. Determine depth where this pressure exists5. Compare with previous depth estimate

6. Repeat steps 2-6 using the new depthestimate

7. When depths agree, accept answer (typically 2-4 iterations) (agreement towithin 30 ft will be satisfactory)


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Linear Interpolation

= )SS(mPP)ii()iii( 121212

12

S S P P m =

)()(P)()( 11212

11 S S S S P P

S S m P iii

==


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Linear Interpolation

( )1212

11 PPSS

SSPP

+=

With design factor:

( )

=

2112

11cc PPSS

SSP

.F.D1

P


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18. Casing Design Example

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