Casing Design Example The table below is a data set from a real land well. As a drilling engineer you are required to calculate the burst and collapse loads that would be used to select an appropriate weight and grade of casing for the Surface, Intermediate and Production strings in this land well:
As s u m p t i o n s :
Pro duction test data:
Gas density above 10000ft
: 0. 1 p si/ f t
Well test completion fluid density
:
8 . 6 0 pp g .
Design factor (Burst)
: 1. 1
Test packer depth
:
11000 ft TVD RKB
Design factor (Collapse)
: 1. 0
Test p erforation depth
:
11250 ft TVD RKB
Pressure at top of perforation
:
1 4 . 0 pp g .
Well test shut-in fluid gradient
:
1 0. 15 p si/ f t
Gas lift may be required
Surface Casing (20" @ 3000 ft) From the Drilling Program it can be seen that the following data is to be used for the design: Casing size
:
20"
Setting depth
:
3000
Pore pressure above 3000
:
8.6
ppg.
Mud Weight in hich the casing is to be run
:
9.0
ppg.
Depth of next 17½" hole
:
6000
Max. pore press re at bottom of 17½" hole
:
9.5
ppg.
Frac. pressure gradient at the 20" shoe
:
13
ppg.
0.1
psi/
Expected gas gra dient
Design Factors: Burst
:
1.1
Collapse
:
1.0
Burst Design - Drilling Internal Load: Assuming that an influx of gas has occurred and the well is full of gas to surface.
Pore pressure at bottom of 17 ½" hole
= =
Pressure at surface = Pressure at bottom of 17 ½" = hole - pressure due to column of gas = Pressure at 20" casing shoe
LOT pressure at 20" casing shoe
9.5 x 6000 2964
psi
2964 - (0.1 x 6000) 2364
psi
=
2961 - (0.1 x 3000)
=
2664
= =
psi
13 x 0.052 x 3000 2028
psi
The formation at the casing shoe will breakdown at 2028 psi and therefore it will breakdown if the pressure of 2664 psi is applied to it. The maximum pressure inside the surface casing at the shoe will therefore be 2028 psi. The maximum pressure at surface will be equal to the pressure at the shoe minus a column of gas to surface: =
2028 - (0.1 x 3000)
=
1728
psi
External Load: Assuming that the pore pressure is acting at the casing shoe and zero pressure at surface. Pore pressure at the casing shoe
External pressure at surface Line up against equals Summary of burst loads
=
8.6 x 0.052 x 3000
=
1342
psi
=
0
psi
Collapse Design - Drilling Internal Load: Assuming that the casing is totally evacuated due to losses of drilling fluid.
Internal pressure at surface
=
0
psi
Internal pressure at shoe
=
0
psi
External Load: Assuming that the pore pressure is acting at the casing shoe and zero pressure at surface.
Summary of collapse loads
Intermediate Casing (13!" @ 6000 ft) From the Drilling Program it can be seen that the following data is to be used for the design: Casing size
:
13⅜"
Setting depth
:
6000
Minimum pore ressure above 6000
:
8.6
ppg.
Maximum pore ressure above 6000
:
9.5
ppg.
Mud Weight in hich the casing is to be run
:
11.0
ppg.
Depth of next 12¼" hole
:
10000
Max. pore press re at bottom of 12¼" hole
:
11.0
ppg.
Frac. pressure gradient at the 13 ⅜" shoe
:
16
ppg.
0.1
psi/
Expected gas gra dient
Design Factors: Burst
:
1.1
Collapse
:
1.0
Burst Design - Drilling Internal Load: Assuming that an influx of gas has occurred and the well is full of gas to surface.
Pore pressure at bottom of 12 ¼" hole
= =
Pressure at surface = Pressure at bottom of 12 ¼" = hole - pressure due to column of gas = Pressure at 13⅜" casing shoe
LOT pressure at 13 ⅜" casing shoe
11 x 0.052 x 10000 5720
psi
5720 - (0.1 x 10000) 4720
psi
=
5720 - (0.1 x 4000)
=
5320
= =
psi
16 x 0.052 x 6000 4992
psi
The formation at the casing shoe will therefore breakdown when the well is closed in after the gas has flowed to surface. The maximum pressure inside the casing at the shoe will be 4992 psi. The maximum pressure at surface will be equal to the pressure at the shoe minus a column of gas to surface: =
4992 - (0.1 x 6000)
=
4392
psi
External Load: Assuming that the minimum pore pressure is acting at the casing shoe and zero pressure at surface. Pore pressure at the casing shoe
External pressure at surface
Summary of burst load
=
8.6 x 0.052 x 6000
=
2684
psi
=
0
psi
Collapse Design - Drilling Internal Load: Assuming that the casing is totally evacuated due to losses of drilling fluid.
Internal pressure at surface
=
0
psi
Internal pressure at shoe
=
0
psi
External Load: Assuming that the maximum pore pressure is acting at the casing shoe and zero pressure at surface. Pore pressure at the casing shoe
External pressure at surface
Summary of collapse loads
=
9.5 x 0.052 x 6000
=
2964
psi
=
0
psi
Production Casing (9 "" @ 10000 ft) From the Drilling Program it can be seen that the following data is to be used for the design: Casing size
:
9⅝"
Setting depth
:
10000
Top of 7" liner
:
9500
Test perforation depth
:
11250
Pressure at top of perforation
:
14.0
ppg.
Minimum pore ressure above 10000
:
9.5
ppg.
Maximum pore ressure above 10000
:
11.0
ppg.
Mud Weight in hich the casing is to be run
:
14.0
ppg.
Density of completion/packer fluid
:
8.6
ppg.
Packer depth
:
11000
Expected gas gradient
0.15
psi/
Design Factors: Burst
:
1.1
Collapse
:
1.0
Burst Design - Production Internal Load: Assuming that a leak occurs in the tubing at surface and that the Closed In Tubing Head Pressure (CITHP) is acting on the inside of the top of the casing. This pressure will then act on the column of packer fluid. The 9 ⅝" casing is only exposed to these pressure down to the Top Of Liner (TOL). The 7" liner protects the remainder of the casing.
Max. pore pressure at the top of the production zone
CITHP (at surface) - Pressure at top of prefs pressure due to column of gas (0.15 psi/ )
=
14 x 0.052 x 11250
=
8190
psi
=
8190 - 0.15 x 11250
=
6503
psi
External Load: Assuming that the minimum pore pressure is acting at the liner depth and zero pressure at surface. Pore pressure at the top of liner
External pressure at surface
Summary of burst loads
=
9.5 x 0.052 x 9500
=
4693
psi
=
0
psi
Collapse Design - Drilling Internal Load: Assuming that the casing is totally evacuated due to gaslifting operations.
Internal pressure at surface
=
0
psi
Internal pressure at top of liner (TOL)
=
0
psi
External Load: Assuming that the maximum pore pressure is acting on the outside of the casing at the TOL. Pore pressure at the TOL
