PETE 411
Well Drilling
Lesson 17
Casing Design
1
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
2
Read Applied Drilling Engineering, Ch.7
HW #9
Due 10-18-02
3
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
4
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?
5
Types of Strings of Casing
1. Drive pipe or structural pile
Diameter
Example
16”-60”
30”
16”-48”
20”
8 5/8”-20”
13 3/8”
{Gulf Coast and offshore only}
150’-300’ below mudline.
2. Conductor string. 100’ - 1,600’
(BML)
3. Surface pipe. 2,000’ - 4,000’
(BML)
6
Types of Strings of Casing
Diameter
4. Intermediate String
5. Production String (Csg.)
7 5/8”-13 3/8”
4 1/2”-9 5/8”
Example
9 5/8”
7”
6. Liner(s)
7. Tubing String(s)
7
Example Hole and String Sizes (in)
Hole Size
36”
26”
Pipe Size
Structural casing
Conductor string
17 1/2
Surface pipe
12 1/4
IntermediateString
8 3/4
Production Liner
30”
20”
13 3/8
9 5/8
7
8
Example Hole and String Sizes (in)
Hole Size
36”
26”
Pipe Size
Structural casing
Conductor string
17 1/2
Surface pipe
12 1/4
IntermediateString
8 3/4
Production Liner
30”
20”
13 3/8
9 5/8
7
9
Example Hole and String Sizes (in)
Structural casing
Mudline
Conductor string
250’
1,000’
4,000’
Surface pipe
IntermediateString
Production Liner
10
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)
11
s
e
12
Length of Casing Joints
RANGE
1
16-25 ft
RANGE
2
25-34 ft
RANGE
3
> 34 ft.
13
Casing Threads and Couplings
API round threads - short
{ CSG }
API round thread - long
{ LCSG }
Buttress
{ BCSG }
Extreme line
{ XCSG }
Other …
See Halliburton Book...
14
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
15
Abnormal
Normal Pore Pressure
0.433 - 0.465 psi/ft
Abnormal Pore Pressure
gp > normal
16
Design from bottom
17
Press. Gauge
Wing Valve
X-mas Tree
Choke Box
Master
Valves
Wellhead
• Hang Csg. Strings
• Provide Seals
• Control Production
from Well
18
Wellhead
19
Wellhead
20
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
21
Casing Design
Collapse (from external pressure)




Yield Strength Collapse
Plastic Collapse
Transition Collapse
Elastic Collapse
Collapse pressure is affected by axial stress
22
Casing Design - Collapse
23
Casing Design - Tension
24
Casing Design - Burst
(from internal pressure)
 Internal Yield Pressure for pipe
 Internal Yield Pressure for couplings
 Internal pressure leak resistance
p
Internal
Pressure
p
25
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.
26
Burst Example
1. Calculate probable reservoir pressure.
pres
psi
 0.5
* 10,000 ft  5,000 psi
ft
2. Calculate required pipe internal yield
pressure rating
pi  pres * Ni  5,000 * 1.1  5,500 psi
27
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
28
29
23 lb/ft
26 lb/ft
N-80
30
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
31
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
32
Casing Design
Collapse pressure - with axial stress
1.
YPA
1/ 2
2



 SA 
 S A 

 YP 1  0.75     0.5  
 YP  
 YP 


YPA = yield strength of axial stress
equivalent grade, psi
YP = minimum yield strength of pipe, psi
SA = Axial stress, psi (tension is positive)
33
Casing Design - Collapse
2. Calculate D/t to determine proper equation
to use for calculating the collapse pressure
Yield Strength
Collapse :
Plastic Collapse:
PY P
 D 

  1

t 



 2 Yp
  D 2 
 


  t 



 A

Pp  Yp 
 B  C
 D 



 t 



34
Casing Design - Collapse, cont’d
Transition
Collapse:
Elastic
Collapse:


 F

PT  Yp 
 G
 D 

  t 



PE 
46.95 X10 6

 D   D 
     1
 t   t 

2
35
Casing Design - Collapse
If Axial Tension is Zero:
Yield Strength
Plastic
Transition
Elastic
J-55
14.81
25.01
37.31
N-80
13.38
22.47
31.02
P-110
12.44
20.41
26.22
(D / t ) 
36
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:
D
5.500

 22.54
1
t
5.500  5.012
2

From Halliburto n book
37
Example 2
2. Check the mode of collapse
D
 22.54
t
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.
38
Example 2
The plastic collapse is calculated from:
 A

Pp  Yp 
 B  C
D/ t

 2.991

 55,000 
 0.0541  1,206
 22.54

Pp  3,117 psi
Halliburton Tables
rounds off to 3,120 psi
39
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:
YPA

 SA


1  0.75 
Y

 p

2

 SA
  0.5 

Y

 p


 YP



FA
100,000
SA 

 24,820 psi
Area  5.52  5.012 2
4


40
Example 3 cont’d
YPA
2






S
S
  1  0.75 A   0.5 A  YP
Y 
 Y 

p 

 p 

The axial tension will reduce the collapse
pressure rating to:
YPA
2


24,820 
24,820 


  1  0.75 
  0.5 
55,000

 55,000 
 55,000 


 38,216 psi
Here the axial load decreased the J-55
rating to an equivalent “J-38.2” rating
41
Example 3 - cont’d
 A

 Pp  YPA 
 B  C
D/ t

 2.945
2 
 38,216 
 4.557 x10   700.43  2,551
 22.54

Pp  2,550 psi
…compared to 3,117 psi with no axial stress!
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