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PETE 411Well 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 pipeIntermediateStringProduction 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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σσσσεεεε

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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 Pressure0.433 - 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

TensionDepth

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)

4 Internal Yield Pressure for pipe4 Internal Yield Pressure for couplings4 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/ftDesign 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 psi7”, 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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30

23 lb/ft26 lb/ft

N-80

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Collapse Pressure

The following factors are important:

4 The collapse pressure resistance of a pipe depends on the axial stress

4 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:

4 Yield strength collapse4 Plastic collapse4 Transition collapse4 Elastic collapse

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Casing Design

Collapse pressure - with axial stress

1.

−

−=

P

A

2/12

P

APPA Y

S5.0YS75.01YY

YPA = yield strength of axial stress equivalent grade, psi

YP = minimum yield strength of pipe, psiSA = Axial stress, psi (tension is positive)

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Casing Design - Collapse

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

Yield Strength Collapse :

Plastic Collapse:

−

= 2pYP

tD

1t

D

Y2P

CB

tDAYP pp −

−

=

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Transition Collapse:

Elastic Collapse:

−

= G

tDFYP pT

2

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E

1t

Dt

D

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. Calculatethe D/t ratio: ( )

book nHalliburto From

54.22012.5500.5

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500.5tD

↑

=−

=

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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.

54.22=tD

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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 3Determine 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

YS5.0

YS75.01Y

−

−=

( )psi

AreaFS A

A 820,24012.55.5

4

000,10022

=−

== π

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Example 3 cont’d

The axial tension will reduce the collapse pressure rating to:

psi 216,38

000,55000,55820,245.0

000,55820,2475.01Y

2

PA

=

−

−=

Here the axial load decreased the J-55 rating to an equivalent “J-38.2” rating

Pp

A

p

APA Y

YS

YSY

−

−= 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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