Casing pressure decrease when bringing well on choke

When bringing the well on choke with a subsea stack, the casing pressure (annulus pressure) must be allowed to decrease by the amount of choke line pressure loss (friction pressure):

• Example: Shut-in casing (annulus) pressure (SICP) = 800 psi

• Choke line pressure loss (CLPL) = 300 psi

• Reduced casing pressure, psi = 800 psi ? 300 psi

• Reduced casing pressure = 500 psi

Pressure chart for bringing well on choke

Pressure/stroke relationship is not linear. When bringing the well on choke, to maintain a constant bottomhole pressure, the following chart should be used:

Strokes side:

• Example: kill rate speed = 50 spm

• Pressure side:

• Example: Shut-in casing pressure (SICP) = 800 psi Choke line pressure loss (CLPL) = 300 psi

Divide choke line pressure loss (CLPL) by 4, because there are 4 steps on the chart:

Maximum allowable mud weight, ppg, subsea stack as derived from leak-off test data

• Example: Leak-off test pressure = 800psi

• TVD from rotary bushing to casing shoe = 4000ft

• Mud weight in use = 9.2ppg

• Maximum allowable mud weight, ppg = 800 0.052 4000 + 9.2

• Maximum allowable mud weight = 13.0ppg

Maximum allowable shut-in casing (annulus) pressure

• Example: Maximum allowable mud weight = 13.3ppg

• Mud weight in use = 11.5ppg

• TVD from rotary kelly bushing

• to casing shoe = 4000ft

• MASICP = (13.3ppg 11.5ppg) 0.052 4000ft

• MASICP = 374

Casing burst pressure subsea stack