Casing Design
By
Dr. Eissa Shokir
Casing Design
• Why Run Casing?
• Types of Casing Strings
• Classification of Casing
• Burst, Collapse and Tension
•Effect of Axial Tension on Collapse Strength
Casing Design
What is casing?
Casing
Why run casing?
Cement
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
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?
Types of Strings of Casing
Diameter Example
1. Drive pipe or structural pile
{Gulf Coast and offshore only}
150’-300’ below mudline.
2. Conductor string. 100’ - 1,600’
16”-60”
30”
16”-48”
20”
8 5/8”-20”
13 3/8”
(BML)
3. Surface pipe. 2,000’ - 4,000’
(BML)
Types of Strings of Casing
Diameter Example
4. Intermediate String
5. Production String (Csg.)
6. Liner(s)
7. Tubing String(s)
7 5/8”-13 3/8”
4 1/2”-9 5/8”
9 5/8”
7”
Example Hole and String Sizes (in)
Hole Size
36”
26”
Structural casing
Conductor string
Pipe Size
30”
20”
17 1/2
Surface pipe
13 3/8
12 1/4
IntermediateString
9 5/8
8 3/4
Production Liner
7
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)
σ
ε
Casing Threads and Couplings
API round threads - short
{ CSG }
API round thread - long
{ LCSG }
Buttress
{ BCSG }
Extreme line
Other …
{ XCSG }
See Halliburton Book...
Burst, Collapse, and Tension
API Design Factors (typical)
Required
Design
10,000 psi
Collapse 1.125
11,250 psi
100,000 lbf
Tension
1.8
180,000 lbf
10,000 psi
Burst
1.1
11,000 psi
Abnormal
Normal Pore Pressure
0.433 - 0.465 psi/ft
Abnormal Pore Pressure
gp > normal
Casing Design
Tension
Tension
Depth
Burst
Collapse
Collapse
STRESS
Burst:
Burst
Collapse:
Tension:
Assume full reservoir pressure all along the wellbore.
Hydrostatic pressure increases with depth
Tensile stress due to weight of string is highest at top
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)
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.
Casing Design - Solution
PB = pore pressure * Design Factor
= 6,000 psi *1.1
PB = 6,600 psi
Depth
Burst Requirements (based on the expected pore pressure)
Pressure
The whole casing string must be capable of withstanding
this internal pressure without failing in burst.
Casing Design - Solution
Collapse Requirements
For collapse design, we start at the bottom of the
string and work our way up.
Tension Check
The weight on the top joint of casing would be
( 6 ,369 ft * 47 . 0 # / ft ) + (1, 631 ft * 53 . 5 # / ft )
= 386 , 602 lbs actual weight
With a design factor of 1.8 for tension, a pipe
strength of
1.8 * 386,602 = 695,080 lbf is required