1 casing design

7/30/2019 1 Casing Design

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Dri ll ing Eng ineering

Lesson 5

Casing Design


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Casing Design

Why Run Casing?

Types of Casing Strings

Classification of CasingWellheads

Burst, Collapse and Tension

Example Effect of Axial Tension on Collapse Strength

Example


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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


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Casing Design

4. To confine production to the wellbore

5. To control pressures during drilling

6. To provide an acceptable environment forsubsurface 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?


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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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Functions of Casing

IndividuallyConductor pipe

Provides a mud return path

Prevents erosion of ground below rig

Same as Drive pipe

Supports the weight of subsequent casing

strings Isolates very weak formations


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Surface casing

Provides a means of nippling up BOP

Provides a casing seat strong enough tosafely close in a well after a kick.

Provides protection of fresh water sands

Provides wellbore stabilization


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Intermediate or protective casing

Usually set in the first abnormally

pressured zone Provides isolation of potentially

troublesome zones

Provides integrity to withstand the highmud weights necessary to reach TD or

next csg seat


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Production casing

Provides zonal isolation (prevents

migration of water to producing zones,isolates different production zones)

Confines production to wellbore

Provides the environment to installsubsurface completion equipment


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Liners

Drilling liners

Same as Intermediate or protective casing

Production liners

Same as production casing

Tieback liners Tie back drilling or production liner to the

surface. Converts liner to full string of casing


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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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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


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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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Burst

Design for maximum pressure on the

inside of the casing. API design

recommendations call for the worstcase scenario, which is the casing is

empty, and no external pressure. The

pressure to design for is the estimatedformation pressure at TD for production

casing, or estimated formation pressure

at the next casing depth.


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Collapse

API design recommendations call for

worst case, where there is no pressure

inside the casing, and we design for themaximum mud weight at the casing

depth. We also allow for the reduction

of the collapse rating from the weight ofthe casing hanging below the depth of

interest.


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Tension

API recommendations call for worst

case, where there is no buoyancy

effect. Design is based on the weight ofthe entire casing string.


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Normal Pore Pressure Abnormal Pore Pressure

0.433 - 0.465 psi/ft gp

> normal

Abnormal


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P G


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X-mas TreeWing Valve

Choke Box

Master

Valves

Wellhead

Hang Csg. Strings

Provide Seals

Control Productionfrom Well

Press. Gauge


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Wellhead


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Wellhead


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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


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Casing Design - Collapse

Collapse pressure is affected by axial stress


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Casing Design - Tension


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Casing Design - Burst

(from internal pressure)

Internal Yield Pressure for pipe

Internal Yield Pressure for couplings

Internal pressure leak resistance

p pInternalPressure


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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 forburst only.


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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 of4,980 psi

7, N-80, 23 lb/ft has BURST Rating of6,340 psi

7, N-80, 26 lb/ft has BURST Rating of7,249 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 a pipe

depends on the axial stress

The API Design Factor


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Casing Design

Collapse pressure - with axial stress

1.

P

A

2/12

P

A

PPA

Y

S5.0

Y

S75.01YY

YPA= yield strength of axial stress

equivalent grade, psiYP= 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 of100,000 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 AA 820,24

012.55.54

000,100

22

2


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Example 3 contd

The axial tension will reduce the collapse

pressure rating to:

psi216,38

000,55000,55

820,24

5.0000,55

820,24

75.01Y

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 - contd

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 -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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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 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 tounderstand what we mean by Design Factors

and worst possible conditions.

API Design FactorsDesign factors are essentially safety factors

that 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 the

API unless otherwise specified.

These are the API design Factors:

Tension and Joint Strength: NT = 1.8

Collapse (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 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 lbfin tension.

Note that the Halliburton Cementing Tableslist actual pipe strengths, without safety

factors built in.


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Casing Design

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

Worst Possible Conditions1. ForCollapse design, assume that the

casing is empty on the inside (p = 0 psig)

2. ForBurstdesign, assume no backup

fluid on the outside of the casing (p = 0 psig)


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Casing Design

Worst Possible Conditions, contd

3. ForTension design,

assume no buoyancy effect

4. ForCollapse 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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Burst Requirements(based on the expected porepressure)

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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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 thecasing string is run, prior to cementing.


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Casing Design

Collapse Requirements, contd

severelessare

tsrequiremencollapsetheholetheupFurther

.bottomtheatd'reqpsi850,5P

125.1*000,8*5.12*052.0

factordesign*depth*weightmud*052.0P

c

c

Depth

Pressure

C i D i


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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 ofN-80 casingmeet the burst requirements, but only the

53.5 #/ft pipe can handle the collapse

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

The 53.5 #/ft pipe could probably run all the

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

alternative.


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C i D i


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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?

ft509,65.12*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 pipecould be run if there were

no axial stress in the pipe

But at 6,509 we have (8,000 - 6,509) =

1,491 of53.5 #/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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C i D i


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Casing Design

Interpolation between these values showsthat the collapse resistance at 5,877 psi

axial stress is:

psi148,4125.1

666,4P

psi666,4)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 theinitial depth of6,509 ft, so a second

iteration is required.

ft382,65.12*052.0

148,4h2


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Casing Design

Second Iteration

Now consider running the 47 #/ft

pipe to the new depth of6,382 ft.

psi378,6in572.13

lbf563,86S

lbf563,865.53*)382,6000,8(W

22

2

Casing Design


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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

ft369,65.12*052.0

140,4h3

21

12

11c1

D.F.

1P PP

SS

SSP

Casing Design


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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

psi429,6572.13

259,87S

lbf259,875.53*)369,6000,8(W

'369,6h

3

3

3

Pcc3 = ?

C


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Casing Design

Third Iteration, contd

2

3

140,4

)600,4680,4(*000,5

000,5429,6680,4125.11

cc

cc

Ppsi

Pthus

C i D i


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Casing Design

Third Iteration, contdThis 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 the

surface (check tension), or perhaps an evenmore 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 willnot 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 = 6,3696,369

6,3826,509

8,000


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Tension Check


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Tension Check

The Halliburton cementing tables give a

yield strength of1,086,000 lbffor the pipe

body and a joint strength of905,000 lbffor

LT & C.

surfacetoOKisft/#0.47

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