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Casing Design

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Casing Design

Why Run Casing? Types of Casing Strings Classification of Casing Burst, Collapse and TensionEffect of Axial Tension on Collapse Strength

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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 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?

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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’

3. Surface pipe. 2,000’ - 4,000’

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 casing

Surface casing

Intermediate casing

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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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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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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Burst, Collapse, and Tension

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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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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

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.

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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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CasingVarious Types of CasingConductor pipe (A):

Size 18-5/8” & over. Outer protection of the well to prevent the surface formation from caving into the well

Surface casing (B): To avoid contamination to surrounding surface water or to protect from a well collapsing caused by free running water.

Intermediate casing (C): Used in the area where abnormally high pressure structure encountered. It is for extra-strengthened protection of a well.

Liner casing (E): Same usage as intermediate casing but not run from the surface.

It hangs from the preceding with 100-150 meter overlaps.

Production casing (D): Placed at the production zone. Since the lower part is exposed to fluid and gas, it may be exposed to corrosion. Proper material selection is needed. This casing is used as a second containmentbarrier (the first one is the production tubing) hence connection leak tightness is important.

Pressure gradients

Fracturation

Gradient

Pore pressure Gradient

A

B

C

D

E

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Example of Typical String Design

Reservoir zone

Conductor 20”

Surface casing 13-3/8”

Intermediate casing 9-5/8”

Liner casing 7”

Tubing 3-1/2” Production casing 5-1/2”

(Tie Back string)

Packer