Gas migration after shut-in: controlling pressure without breaking the shoe
After a 12 bbl kick, the well is shut in on the annular BOP. Initial SIDPP is 500 psi and SICP is 700 psi. The choke is closed and the driller is waiting for the kill sheet. Over 20 minutes, both pressures rise by about 60 psi; SICP reaches 760 psi. MAASP is 1,150 psi. The drillstring is full of 12.6 ppg mud, TVD is 10,000 ft, and the last shoe test was 14.0 ppg equivalent at 7,500 ft. No pump is running. The driller proposes to open the choke and bleed 5 bbl to bring SICP back to 700 psi. The company man is concerned about losing the kick and about breaking the shoe.
1. Why are both pressures rising with the pump off?
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The rise is caused by gas migrating upward in the closed well. Gas is lighter than mud and moves up even with no circulation. As it rises, it cannot expand because the well is shut in, so it displaces mud and adds pressure to the closed system. That is why both SIDPP and SICP increase. The increase is not necessarily a new influx, and it is not caused by the pump. The rate of rise depends on how fast the gas migrates and on the well volume. The crew must not assume the well is safe because the pump is off. If the pressure is allowed to rise unchecked, the weakest point, usually the casing shoe, can be exceeded. The correct response is to control the migration, not to ignore it or to bleed randomly.
2. Evaluate the driller's plan to bleed 5 bbl to restore SICP to 700 psi.
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The plan is unsafe as stated. Bleeding 5 bbl simply to return SICP to its original value can reduce bottomhole pressure and allow more influx, especially if the volume is not calculated from the pressure change and the mud gradient. It also ignores the reason for the rise: gas is migrating and expanding. The correct approach is a controlled volumetric bleed or, preferably, starting circulation with kill mud as soon as the kill sheet and choke are ready. If waiting, the crew should bleed small, measured volumes from the choke while monitoring casing pressure and keeping bottomhole pressure constant. The volume bled must be tied to the pressure increase and recorded. Opening the choke to a target pressure without a procedure can turn a controlled shut-in into a second kick.
3. What pressure limit should be monitored, and what is the correct way to control pressure before circulation starts?
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The main limit is MAASP, 1,150 psi, but the shoe at 7,500 ft TVD must also be checked against the 14.0 ppg equivalent leak-off test. The current SICP of 760 psi is below MAASP, but it is rising. Before circulation, the crew should use the volumetric control procedure: allow the gas to migrate, bleed measured volumes of mud from the choke, and keep casing pressure within the approved limit while maintaining bottomhole pressure. They should record pressure and volume at each step and watch for gas at the choke. If pressure approaches the limit, circulation should start sooner. The driller should not open the choke wide or bleed to a previous pressure. The company man should confirm the procedure and the maximum allowable casing pressure before any bleed.