CASING DESIGN EXERCISE

Table below show a real land well. You are require to calculate the burst and collapse loads that would be used to select an appropriate weight and grade of casing for the (1) Surface, (2) Intermediate and (3) Production string.

ASSUMPTIONS:Gas density below 10000ft : 0.1 psi/ftGas density above 10000ft : 0.15 psi/ftDesign Factor (Burst) : 1.1Design Factor (Collapse) : 1

SolutionsSurface Casing (20” @ 3000 ft)

Burst Design

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 = 9.5 x 0.052 x 6000 = 2964 psi

Pressure at surface = Pressure at Bottom of 17 ½” hole – pressure due to colom of gas= 2964 – (0.1 x 6000) = 2364 psi

Pressure at 20” casing shoe = 2964 – (0.1 x 3000) = 2664 psi

LOT Pressure at 20” casing shoe = 13 x 0.005 x 3000 = 2028 psi

Above 2028 psi, the formation will breakdown. This is the maximum pressure inside the surface casing at the shoe.

Maximum pressure at surface = pressure at shoe – colom of gas to surface= 2028 – (0.1 x 3000) = 1728 psi

External Load: Assuming that the minimum pore pressure is acting at the casing shoe and zero pressure at surface.

Minimum pore pressure at the casing shoe = 8.6 x 0.052 x 3000 = 1342 psiExternal pressure at surface = 0 psi

Summary of Burst Load

Collapse Design

Internal Load: assuming that the casing is totally evacuated due to losses of drilling fluid.

Internal pressure at surface = 0 psiInternal pressure at shoe = 0 psi

External Load: Assuming that the maximum pore pressure is acting at the casing shoe and zero pressure at surface.

Maximum pore pressure at the casing shoe = 8.6 x 0.052 x 3000 = 1342 psi

External pressure at surface = 0 psi

Summary of Collapse Load

Intermediate Casing (13 3/8” @6000 ft)

Internal Load: Assuming that an influx of gas has occurred and the well is full of gas to surface.

Burst Design

LOT Pressure at 13 3/8” casing shoe = 16 x 0.005 x 6000 = 4992 psi

Above 4992 psi, the formation will breakdown. This is the maximumpressure inside the surface casing at the shoe.

Maximum pressure at surface = pressure at shoe – colom 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.

Minimum pore pressure at the casing shoe = 8.6 x 0.052 x 6000 = 2684 psiExternal pressure at surface = 0 psi

Summary of Burst Load

Collapse Design

Internal Load: assuming that the casing is totally evacuated due to losses of drilling fluid.

Internal pressure at surface = 0 psiInternal pressure at shoe = 0 psi

External Load: Assuming that the maximum pore pressure is acting at the casing shoe and zero pressure at surface.

Maximum pore pressure at the casing shoe = 9.5 x 0.052 x 6000 = 2964 psi

External pressure at surface = 0 psi

Summary of Collapse Load

Production Casing (9 5/8” @10000 ft)

Internal Load: Assuming that an influx of gas has occurred and the well is full of gas to surface. This pressure act on the colum of packer fluid. The 9 5/8” casing is only exposed to the pressure down to the Top of Liner (TOL). The 7” liner protects the reminder of the casing.

Burst Design

Max. Pore Pressure at the top of the production zone= 14 x 0.052 x 11250 = 8190 psi

Pressure at surface = Pressure at Top of Perfs – presure due to colom of gas= 8190 – (0.15 x 11250) = 6503 psi

Pressure at Top of Liners = Pressure at surface + hydrostatic colom of packer fluid= 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.

Minimum pore pressure at the top liner = 9.5 x 0.052 x 9500 = 4693 psiExternal pressure at surface = 0 psi

Summary of Burst Load

Collapse Design

Internal Load: assuming that the casing is totally evacuated due to losses of drilling fluid.

Internal pressure at surface = 0 psiInternal pressure at shoe = 0 psi

External Load: Assuming that the maximum pore pressure is acting on the outside of the casing at the LOT.

Maximum pore pressure at the LOT= 11 x 0.052 x 9500 = 5434 psi

External pressure at surface = 0 psi

Summary of Collapse Load