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Production Casing (9 5/8” @ 10000 ft)
From the Drilling Program it can be seen that the following data is to be used for the design:
Casing Size : 9 5/8"
Setting Depth : 10000 ft
Top of 7” Liner : 9500 ft
Test Perforation Depth : 11250 ft
Pressure at Top of Perforation : 14.0 ppg
Minimum Pore Pressure above 10000 ft : 9.5 ppg
Maximum Pore Pressure above 10000 ft : 11.0 ppg
Mud weight in which 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/ft
Design Factors : (Burst) 1.1
(Collapse) 1.0
Solution:
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 5/8” 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
= Pressure at Top of Perforation x 0.052 x Test Perforation Depth
= 14 x 0.052 x 11250 = 8190 psi
CITHP (at surface) = Pressure at Top of Perfs - pressure due to column of gas (0.15 psi/ft)
= Max. Pore Pressure at the top of the production zone - Expected gas gradient x Test Perforation Depth
= 8190 - 0.15 x 11250 = 6503 psi
Pressure at Top of Liner = CITHP plus hydrostatic column of packer fluid
= CITHP + (Density of Completion/Packer fluid x 0.052 x Top of 7” Liner)
= 6503 + (8.6 x 0.052 x 9500) = 10751 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
= Minimum Pore Pressure above 10000 ft x 0.052 x Top of 7” Liner
= 9.5 x 0.052 x 9500 = 4693 psi
External pressure at surface = 0 psi
Summary of Burst Loads
|
Depth |
External load |
Internal load |
Net load |
Design Load (Net Load x 1.1) |
|
Surface |
0 |
6503 |
6503 |
7153 |
|
TOL (9500 ft) |
4693 |
10751 |
6058 |
6664 |