10 Casing Design

7/30/2019 10 Casing Design

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Casing Design

Why Run Casing?

Types of Casing Strings

Classification of Casing Wellheads

Burst, Collapse and Tension

Example

Effect of Axial Tension on Collapse Strength

Example


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Casing

Cement

What is casing?

Why run casing?

1. To prevent the hole from caving in

2. Onshore - to prevent contamination of fresh water sands

3. To prevent water migration to producing formation

Casing Design


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Why run casing?

4. To confine production to the wellbore

5. To control pressures during drilling

6. To provide an acceptable environment for subsurfaceequipment in producing wells

7. To enhance the probability of drilling to total depth(TD)


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Example Hole and String Sizes (in)

Structural casing

Conductor string

Surface pipe

Intermediate String

Production Liner

Hole Size

30

20

133/8

9 5/8

7

Pipe Size

36

26

171/2

121/4

e.g., you need 14 ppg to control a lower zone, but an upper zonewill fracture at 12 lb/gal.

What do you do?


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Diameter Example

16-60 30

16-48 20

8 5/8-20 13 3/8

1. Drive pipe or structural pile

{Gulf Coast and offshore only}150-300 below mudline.

2. Conductor string. 100 -

1,600 (BML)

3. Surface pipe. 2,000 - 4,000

(BML)

Types of Strings of Casing


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Types of Strings of Casing

4. Intermediate String

5. Production String (Csg.)

6. Liner(s)

7. Tubing String(s)

Diameter Example

4. Intermediate String

5. Production String (Csg.)

6. Liner(s)

7. Tubing String(s)

7 5/8-13 3/8 9 5/8

4 1/2-9 5/8 7


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Structural casing

Conductor string

Surface pipe

Intermediate String

Production Liner

Hole Size

30

20

13 3/8

9 5/8

7

Pipe Size

36

26

17 1/2

12 1/4

8 3/4


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Structural casing

Conductor string

Surface pipe

Intermediate String

Production Liner

250

1,000

4,000

Mudline


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Classification of Casing

1. Outside diameter of pipe (e.g. 9 5/8)

2. Grade of material (e.g. N-80)

3. Nominal weight (Avg. wt/ft incl. Wt. Coupling)

(e.g. 47 lb/ft)

4. Type to threads and couplings (e.g. API LCSG)5. Length of each joint (RANGE) (e.g. Range 3)


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se


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Length of Casing Joints

RANGE 1 16-25 ft

RANGE 2 25-34 ft

RANGE 3 > 34 ft.


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Casing Threads and Couplings

API round threads - short { CSG }

API round thread - long { LCSG }

Buttress { BCSG }

Extreme line { XCSG } Other

See Halliburton Book...


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API Design Factors (typical)

Collapse 1.125

Tension 1.8

Burst 1.1

Required

10,000 psi

100,000 lbf

10,000 psi

Design

11,250 psi

180,000 lbf

11,000 psi


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Normal Pore Pressure Abnormal Pore Pressure

0.433 - 0.465 psi/ft gp > normal

Abnormal


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Burst: Assume full reservoir pressure all along the wellbore.

Collapse: Hydrostatic pressure increases with depth

Tension: Tensile stress due to weight of string is highest at top

Casing Design

STRESS

Tension

Burst

Collapse

Collapse

Tension

Depth

Burst


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Casing Design

Collapse (from external pressure)

Yield Strength Collapse

Plastic Collapse

Transition Collapse

Elastic Collapse

Collapse pressure is affected by axial stress


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Casing Design - Collapse


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Casing Design - Tension


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Casing Design - Burst

p pInternalPressure

Internal Yield Pressure for pipe

Internal Yield Pressure for couplings

Internal pressure leak resistance


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Casing Design - Burst

Example 1

Design a 7 Csg. String to 10,000 ft.

Pore pressure gradient = 0.5 psi/ft

Design factor, Ni=1.1

Design for burst only.


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Burst Example

1. Calculate probable reservoir pressure.

2. Calculate required pipe internal yield pressure rating


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Burst Example

3. Select the appropriate csg. grade and wt. from theHalliburton Cementing tables:

Burst Pressure required = 5,500 psi

7, J-55, 26 lb/ft has BURST Rating of 4,980 psi

7, N-80, 23 lb/ft has BURST Rating of 6,340 psi7, N-80, 26 lb/ft has BURST Rating of 7,249 psi

Use N-80 Csg., 23 lb/ft


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23 lb/ft

26 lb/ft

N-80


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Collapse Pressure

The following factors are important:

The collapse pressure resistance of apipe depends on the axial stress

There are different types of collapsefailure


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Collapse Pressure

There are four different types of collapsepressure, each with its own equation for

calculating the collapse resistance:

Yield strength collapse

Plastic collapse

Transition collapse

Elastic collapse


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Casing Design

YPA = yield strength of axial stress

equivalent grade, psiYP = minimum yield strength of pipe, psi

SA = Axial stress, psi (tension is positive)

Collapse pressure - with axial stress

1.

P

A

P

A

PPA Y

S

Y

SYY 5.075.01

2/12


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Casing Design2. Calculate D/t to determine proper equation

to use for calculating the collapse pressure

Yield StrengthCollapse :

Plastic Collapse:

2

1

2

t

D

t

D

YP pYP

CB

t

D

AYP pp


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Casing Design

Transition

Collapse:

ElasticCollapse:

G

tD

FYP pT

2

6

E

1t

D

t

D

10X95.46P


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Casing Design - Collapse

If Axial Tension is Zero:

Yield Strength Plastic Transition Elastic

J-55 14.81 25.01 37.31

N-80 13.38 22.47 31.02P-110 12.44 20.41 26.22


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Example 2

Determine the collapse strength of 5 1/2O.D., 14.00 #/ft J-55 casing under zero axialload.

1. Calculate

the D/t ratio:


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Example 2

2. Check the mode of collapse

Table on p.35 (above) shows that,

for J-55 pipe,

with 14.81 < D/t < 25.01

the mode of failure is plastic collapse.

54.22t

D


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Example 2

The plastic collapse is calculated from:

psi117,3Pp Halliburton Tablesrounds off to 3,120 psi


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Example 3

Determine the collapse strength for a 5 1/2 O.D.,14.00 #/ft, J-55 casing under axial load of 100,000lbs

The axial tension will reduce the collapse pressure asfollows:

P

p

A

2

p

APA Y

Y

S5.0

Y

S75.01Y


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Example 3

Here the axial load decreased the J-55 ratingto an equivalent J-38.2 rating

P

p

A

p

APA Y

Y

S

Y

SY

5.075.01

2

The axial tension will reduce the collapse pressure rating to:


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Example 3

psi550,2Pp

compared to 3,117 psi with no axial stress!


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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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Casing Design Example

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

Before solving this problem is it necessary tounderstand what we mean by DesignFactors and worst possible conditions.

API Design Factors

Design 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 Design

Well use the design factors recommended by theAPI 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 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 Tables listactual pipe strengths, without safety factorsbuilt in.


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Casing Design

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

Worst Possible Conditions1. For Collapse design, assume that the casing is

empty on the inside (p = 0 psig)

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


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Casing DesignWorst 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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Casing Design

Burst Requirements (based on the expected pore pressure)

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

Dep

th

Pressure


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Casing Design

Collapse Requirements

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

Our design criteria will be based on hydrostaticpressure resulting from the 12.5 ppg mud that

will be in the hole when the casing string isrun, prior to cementing.


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Casing Design

Collapse Requirements, contd

severelessare

tsrequiremencollapsetheholetheupFurther

bottomtheatdreqpsiP

factordesigndepthweightmudP

c

c

.'850,5

125.1*000,8*5.12*052.0***052.0

Depth

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 casingmeet the burst requirements, but only the53.5 #/ft pipe can handle the collapse

requirement at the bottom of the hole (5,850psi).

The 53.5 #/ft pipe could probably run all theway 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 ableto run N-80, 47 #/ft? The

maximum annular pressurethat this pipe may be exposedto, is:

Depth

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 ppg mud?


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Casing Design

This is the depth to which the pipecould 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 collapseresistance 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 inthe 47 #/ft pipe


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Casing Design

The API tables show that the above stresswill 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 DesignInterpolation between these values shows that the

collapse resistance at 5,877 psi axial stress is:

With the design factor,

2112

11c1P PPSS

SS

P


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Casing Design

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

Which differs considerably from the initialdepth of 6,509 ft, so a second iteration is

required.


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Casing Design

Second Iteration

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


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Casing DesignInterpolating again,

This is the pressure at a depth of

21

12

11c1

D.F.

1P PP

SS

SSP


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Casing Design

This is within 13 ft of the assumed value. If moreaccuracy is desired (generally not needed), proceedwith the:

Third Iteration

Pcc3 = ?


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Casing Design

Third Iteration, contd


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Casing Design

Third Iteration, contd

This is the answer we are looking for, i.e., we canrun 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 moreeconomical string would include some 43.5 #/ftpipe?


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Casing Design

At some depth the 43.5 #/ft pipe would beable to handle the collapse requirements,but we have already determined that it willnot meet burst requirements.


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N-8053.5 #/ft

N-8047.0 #/ft

N-8043.5 #/ft?

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

Depth = 6,369

6,3696,3826,509

8,000

Burst?

Casing Design


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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 pipestrength of


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Tension Check

The Halliburton cementing tables give a yieldstrength of 1,086,000 lbf for the pipe body

and a joint strength of 905,000 lbf for LT & C.


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Casing DesignReviewWe 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 becausethey do not satisfy the burstrequirements.

2. Only the N-80, 53.5 #/ft pipe is capable ofwithstanding the collapserequirementsat the bottom of the string


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Casing DesignReview3. Since the 53.5 #/ft pipe is the most

expensive, we want to use as little of itas possible, so we want to use as

much 47.0 #/ft pipe as possible.

4. Dont forget to check to make sure the

tension requirements are met; both forpipe body, and for threads andcouplings (T&C).


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Casing DesignReviewThe 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:


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Casing DesignReviewTo determine the effect of axial stress

requires an iterative process:

1. Determine the depth capability withoutaxial stress

2. Determine axial stress at this point


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Casing DesignReview3. Determine corresponding collapse resistance

4. Determine depth where this pressure exists

5. Compare with previous depth estimate6. Repeat steps 2-6 using the new depth estimate

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


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Linear Interpolation


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With design factor:

21

12

1

1ccPP

SS

SSP

.F.D

1P


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Question

A combination production casing string is toconsider of 15.5 #/ft and 17 #/ft, J-55casing to a total depth of 6300 and is to

be run in 12 ppg mud. Determine the depthat which the casing weight per foot shouldchange because of collapse-pressureconsiderations with a design factor of1.125, assuming that the fluid level couldfall down as low as 6300 in subsequentdrilling operations.

10 Casing Design

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