1

PETE 411

Well Drilling

Lesson 18

Casing Design Example

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Casing Design Example

Example Problem

API Design Factors

“Worst Possible Conditions”

Effect of Axial Tension on Collapse Strength

Iteration and Interpolation

Design for Burst, Collapse and Tension

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Read:Applied Drilling Engineering, Ch.7

HW #9 - Velocity Profiles

Due 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 pore pressure is expected to be 6,000 psi.

Only the grades and weights shown are available (N-80, all weights). Use API design 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 Factors” and “worst possible conditions”.

API Design Factors

Design factors are essentially “safety factors” that allow us to design safe, reliable casing strings. Each operator may have his own set of design factors, based on his experience, and the condition of the pipe.

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Casing Design

In PETE 411, we’ll use the design factors recommended by the API unless otherwise specified.

These are the API design Factors:

Tension and Joint Strength: NT = 1.8

Collapse (from external pressure): Nc= 1.125

Burst (from internal pressure): Ni = 1.1

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Casing Design

What this means is that, for example, if we need to design a string where the maximum tensile 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 Tables list actual pipe strengths, without safety factors built in.

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Casing Design

Unless otherwise specified in a particular problem, we shall also assume the following:

Worst Possible Conditions

1. For Collapse design, assume that the casing is empty on the inside (p = 0 psig)

2. For Burst design, assume no “backup” fluid on the outside of the casing (p = 0 psig)

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Casing Design

Worst Possible Conditions, cont’d

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 the expected conditions in burst, collapse and tension.Above conditions are quite conservative. They are also simplified for easier understanding of the basic concepts.

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Casing Design - Solution

Burst Requirements (based on the expected pore pressure)

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

psi600,6P

1.1*psi000,6

FactorDesign*pressureporeP

B

B

Dep

th

Pressure

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Casing Design - Solution

Collapse Requirements

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

Our design criteria will be based on hydrostatic pressure resulting from the 12.5 ppg mud that will be in the hole when the casing string is run, prior to cementing.

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Casing Design

Collapse Requirements, cont’d

severe less are

tsrequiremen collapse the hole the up Further

.bottom the at d'reqpsi 850,5P

125.1*000,8*5.12*052.0

factor design*depth*weight mud*052.0P

c

c

Dep

th

Pressure

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Casing Design

Req’d: 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 the 53.5 #/ft pipe can handle the collapse requirement 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 check tension), but there may be a lower cost alternative.

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Casing Design

To what depth might we be able to run N-80, 47 #/ft? The maximum annular pressure that this pipe may be exposed to, is:

psi 231,4125.1

760,4

factordesign

pipe of pressure CollapsePc

Dep

th

Pressure

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Casing Design

First Iteration

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

ft 509,65.12*052.0

231,4

5.12*052.0

Ph

h*5.12*052.0P

c1

1c

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Casing Design

This is the depth to which the pipe could be run if there were no 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 the collapse resistance of the 47.0 #/ft pipe!

8,000’

6,509’

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Casing Design

Weight, W1 = 53.5 #/ft * 1,491 ft = 79,769 lbf

This weight results in an axial stress in the 47 #/ft pipe

psi 877,5in 13.572

lbf 769,79

area end

weightS of

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19

Casing Design

The API tables show that the above stress will reduce the collapse resistance from 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 shows that the collapse resistance at 5,877 psi axial stress is:

psi 148,4125.1

666,4P

psi 666,4)600,4680,4(*)000,5000,10(

)000,5877,5(680,4P

cc1

1c

With the design factor,

2112

11c1P PP

SS

SSP

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Casing Design

This (4,148 psig) is the pressure at a depth

Which differs considerably from the initial depth of 6,509 ft, so a second iteration is required.

ft 382,65.12*052.0

148,4h2

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23

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Casing Design

Second Iteration

Now consider running the 47 #/ft pipe to the new depth of 6,382 ft.

psi 378,6in 572.13

lbf 563,86S

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

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2

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Casing Design

Interpolating again,

This is the pressure at a depth of

psipcc 140,4600,4680,4*5000

5000378,6680,4

125.1

12

ft 369,65.12*052.0

140,4h3

2112

11c1 D.F.

1P PP

SS

SSP

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Casing Design

This is within 13 ft of the assumed value. If more accuracy is desired (generally not needed), proceed with the:

Third Iteration

psi 429,6572.13

259,87S

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

'369,6h

3

3

3

Pcc3 = ?

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Casing Design

Third Iteration, cont’d

2

3

140,4

)600,4680,4(*000,5

000,5429,6680,4

125.1

1

cc

cc

Ppsi

Pthus

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Casing Design

Third Iteration, cont’d

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 6,369 and 8,000 ft.

Perhaps this string will run all the way to the surface (check tension), or perhaps an even more economical string would include some 43.5 #/ft pipe?

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Casing Design

At some depth the 43.5 #/ft pipe would be able to handle the collapse requirements, but we have already determined that it will not meet burst requirements.

!NO

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N-8053.5 #/ft

N-8047.0 #/ft

N-8043.5 #/ft?

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

Depth = 6,3696,3696,3826,509

8,000

Burst?

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N-8053.5 #/ft

N-8047.0 #/ft

N-8053.5 #/ft?

Depth = 6,3696,3696,3826,509

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, a pipe strength of

weightactual 602,386

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

lbs

ftftftft

required is lbf 080,695602,386*8.1

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Tension Check

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

surface to OK is ft/# 0.47

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Casing Design Review

We have 4 different weights of casing available 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 burst requirements.

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

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Casing Design Review

3. Since the 53.5 #/ft pipe is the most expensive, we want to use as little of it as possible, so we want to use as much 47.0 #/ft pipe as possible.

4. Don’t forget to check to make sure the tension requirements are met; both for pipe body, and for threads and couplings (T&C).

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Casing Design Review

The collapse resistance of N-80, 47 #/ft will determine to what depth it can be run. Two factors will reduce this depth:

• Design Factor

• Axial Stress (tension)

“Halliburton” collapse resistance: 4,760 psi

• Apply design factor: psi 231,4125.1

760,4

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Casing Design Review

To determine the effect of axial stress requires an iterative process:

1. Determine the depth capability without axial stress

2. Determine axial stress at this point

ft 509,65.12*052.0

231,4depth

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Casing Design Review

3. Determine corresponding collapse resistance

4. Determine depth where this pressure exists

5. Compare with previous depth estimate

6. Repeat steps 2-6 using the new depth estimate

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

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Linear Interpolation

(iii) CmSP

(ii) CmSP

(i) CmSP

cmxy

22

11

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Linear Interpolation

)SS(mPP )ii()iii( 121212

12

SS

PPm

)()(P )()( 112

1211 SS

SS

PPSSmPiii

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Linear Interpolation

1212

11 PP

SS

SSPP

With design factor:

2112

11cc PP

SS

SSP

.F.D

1P

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