1

PETE 661 Drilling Engineering

Lesson 5

Casing Design

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Assignments:

READ: ADE Ch. 7

HW # 4: API Pressure Drop

Due Monday, Sept. 29

3

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

4

Casing Design

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

What is casing? Casing

Cement

5

Casing Design

4. To confine production to the wellbore

5. To control pressures during drilling

6. To provide an acceptable environment for

subsurface equipment in producing wells

7. To enhance the probability of drilling to total

depth (TD)

e.g., you need 14 ppg mud to control a lower zone,

but an upper zone will fracture at 12 lb/gal.

What do you do?

6

Types of Strings of Casing

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)

Diameter Example

16”-60” 30”

16”-48” 20”

8 5/8”-20” 13 3/8”

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Types of Strings of Casing

4. Intermediate String

5. Production String (Csg.)

6. Liner(s)

7. Tubing String(s)

7 5/8”-13 3/8” 9 5/8”

Diameter Example

4 1/2”-9 5/8” 7”

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Example Hole and String Sizes (in)

Structural casing

Conductor string

Surface pipe

IntermediateString

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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Example Hole and String Sizes (in)

Structural casing

Conductor string

Surface pipe

IntermediateString

Production Liner

250’

1,000’

4,000’

Mudline

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Classification of CSG.

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

2. Wall thickness (e.g. 1/2”)

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

4. Type to threads and couplings (e.g. API LCSG)

5. Length of each joint (RANGE) (e.g. Range 3)

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

(e.g. 47 lb/ft)

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s

e

12

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...

14

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

15

Normal Pore Pressure Abnormal Pore Pressure

0.433 - 0.465 psi/ft gp > normal

Abnormal

16 Design from bottom

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X-mas Tree Wing Valve

Choke Box

Master

Valves

Wellhead

• Hang Csg. Strings

• Provide Seals

• Control Production

from Well

Press. Gauge

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Wellhead

19

Wellhead

20

Casing Design

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

STRESS

Tension

Burst

Collapse

Collapse

Tension

Depth

Burst

21

Casing Design - Collapse

Collapse pressure is affected by axial stress

22

Casing Design - Tension

23

Casing Design - Burst

(from internal pressure)

Internal Yield Pressure for pipe

Internal Yield Pressure for couplings

Internal pressure leak resistance

p p Internal

Pressure

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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.

psi 000,5 ft000,10*ft

psi5.0pres

2. Calculate required pipe internal yield

pressure rating

psi 500,51.1 *000,5N *pp iresi

Ni = API Design Factor for BURST = 1.1

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Example

3. Select the appropriate csg. grade and wt.

from the Halliburton 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 psi

7”, N-80, 26 lb/ft has BURST Rating of 7,249 psi

Use N-80 Csg., 23 lb/ft

27

28

23 lb/ft

26 lb/ft

N-80

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Collapse Pressure

The following factors are important:

The collapse pressure resistance of a pipe

depends on the axial stress

The API Design Factor

30

Casing Design

Collapse pressure - with axial stress

1.

P

A

2/12

P

APPA

Y

S5.0

Y

S75.01YY

YPA = yield strength of axial stress

equivalent grade, psi

YP = minimum yield strength of pipe, psi

SA = Axial stress, psi (tension is positive)

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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 100,000 lbf

The axial tension will reduce the collapse pressure

as follows:

P

p

A

2

p

APA Y

Y

S5.0

Y

S75.01Y

psi

Area

FS A

A 820,24

012.55.54

000,100

22

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Example 3 cont’d

The axial tension will reduce the collapse

pressure rating to:

psi 216,38

000,55000,55

820,245.0

000,55

820,2475.01Y

2

PA

Here the axial load decreased the J-55

rating to an equivalent “J-38.2” rating

P

p

A

p

APA Y

Y

S

Y

SY

5.075.01

2

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Example 3 - cont’d

We shall be using API Tables to correct for the

effect of axial tension on collapse strength of

casing.

The Halliburton Cementing Tables list the

collapse resistance of 5 ½ -in, 14.00 lb/ft J-55

casing at 3,120 psi.

The axial tension in this case would derate the

collapse strength to about 2,550 psi.

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35

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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 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 661, 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

46

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.

48

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

49

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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52

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,

21

12

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

55

56

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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

22

2

58

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

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

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?

62

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-80 53.5 #/ft

N-80 47.0 #/ft

N-80 43.5 #/ft?

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

Depth = 6,369 6,369 6,382 6,509

8,000

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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