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PETE 411

Well Drilling

Lesson 17

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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Read Applied Drilling Engineering, Ch.7

HW #9 Due 10-18-02

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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 - Why run casing, cont’d

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 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

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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

Hole Size

30”

20”

13 3/8

9 5/8

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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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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

17Design from bottom

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X-mas TreeWing Valve

Choke Box

MasterValves

Wellhead

• Hang Csg. Strings• Provide Seals• Control Production

from 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 (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 (from internal pressure)

Internal Yield Pressure for pipe Internal Yield Pressure for couplings Internal pressure leak resistance

p pInternal 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

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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

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30

23 lb/ft26 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

There are different types of collapse failure

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Collapse Pressure

There are four different types of collapse pressure, 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

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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Casing Design - Collapse

2. Calculate D/t to determine proper equation to use for calculating the collapse pressure

Yield Strength Collapse :

Plastic Collapse:

2pYP

tD

1tD

Y2P

CB

tDA

YP pp

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Transition

Collapse:

Elastic Collapse:

G

tDF

YP pT

2

6

E

1tD

tD

10X95.46P

Casing Design - Collapse, cont’d

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If Axial Tension is Zero:

Yield Strength Plastic Transition Elastic

)t/D(

J-55 14.81 25.01 37.31

N-80 13.38 22.47 31.02

P-110 12.44 20.41 26.22

Casing Design - Collapse

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Example 2

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

1. Calculate

the D/t ratio:

book nHalliburto From

54.22012.5500.5

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500.5

t

D

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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:

206,10541.054.22

991.2000,55

CBt/D

AYP pp

psi117,3Pp Halliburton Tables rounds 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 100,000 lbs

The axial tension will reduce the collapse pressure as follows:

Pp

A

2

p

APA Y

Y

S5.0

Y

S75.01Y

psi

Area

FS A

A 820,24012.55.5

4

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

Pp

A

p

APA Y

Y

S

Y

SY

5.075.01

2

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Example 3 - cont’d

551,243.70010x557.454.22

945.2216,38

CBt/D

AYP

2

PAp

psi 550,2Pp

…compared to 3,117 psi with no axial stress!

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