Gas Migration While Waiting on Kill Mud
At 08:10 the well is shut in after a kick taken while drilling at 11,200 ft MD/TVD with 12.4 ppg mud. The 9-5/8 in shoe is at 8,000 ft TVD with a leak-off test of 15.5 ppg. At shut-in the gauges read SIDPP 300 psi and SICP 700 psi with a 28 bbl pit gain, and the choke is fully closed. The mud engineer needs about 50 minutes to finish the kill mud. At 08:20 the SICP reads 740 psi while the SIDPP is still 300 psi and the pit level has not changed. The trend continues at roughly 40 psi per 10 minutes on the casing. Both the remote choke and a manual choke are available, the drillpipe carries an open TIW valve above closed pipe rams, and the trip tank is lined up to the choke outlet.
1. Interpret the pressure trend. How do you distinguish migration from further inflow and from a leak?
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The signature is a constant SIDPP of 300 psi with a rising SICP (40 psi per 10 min) and a static pit level.
- Gas migration: gas percolates up the annulus faster than the well can be bled. It cannot expand because the well is closed, so it carries its pressure upward and the casing pressure rises. The drillpipe gauge sees only the clean mud column and does not change. Pit level is static because nothing enters or leaves.
- Further influx: the pit level would be rising and both the SIDPP and SICP would climb, because new formation fluid is entering and adding to the trapped volume.
- Leak in the drillpipe, standpipe or a gauge line: the SIDPP would fall, not hold steady.
Migration is the hazard here because it raises bottom hole pressure, which can fracture the shoe or be driven off into a weaker formation. It must be managed, not simply watched. The shut-in SIDPP of 300 psi corresponds to a kill mud weight of 12.4 + 300 / (0.052 x 11,200) = 12.4 + 0.52 = 12.92 ppg, which is comfortably below the 15.5 ppg shoe limit.
2. Calculate the MAASP and the time available before it is reached. State your decision at 08:20 and the reasoning behind it.
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MAASP
MAASP = (15.5 - 12.4) x 0.052 x 8,000 = 3.1 x 0.052 x 8,000 = 1,290 psi
At 40 psi per 10 minutes from 700 psi: (1,290 - 700) / 40 = 14.75 increments, about 148 minutes on the current trend. That is longer than the 50 minutes needed for the kill mud, but migration accelerates as the gas expands, so the trend has to be re-checked continuously.
Decision
Start a controlled bleed now rather than waiting. The rising casing pressure means bottom hole pressure is being pushed above the pore pressure for no benefit, loading the shoe and risking losses into a weaker zone. Holding the pressure near 700 psi keeps bottom hole pressure at its shut-in value, keeps the shoe safe and still leaves a small overbalance. Keep mixing kill mud in parallel and start the kill as soon as it is ready. If for any reason the kill is delayed, the bleed procedure must continue until the well is circulating.
3. Describe the bleed you would perform and how you decide how much to bleed for each step.
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Procedure
- Line up the choke to the trip tank and record the SICP and SIDPP.
- Open the choke slowly and bleed mud until the SICP has dropped by the chosen step - 50 psi is a practical increment.
- Close the choke and let the pressure build again, then repeat. Do not bleed continuously, and do not bleed faster than the pressure builds.
- Record the volume bled, the pressure before and after, and the time for every step.
Volume check
For a 5 in pipe in an 8-1/2 in hole, annular capacity is about 0.046 bbl/ft and the 12.4 ppg mud gradient is 12.4 x 0.052 = 0.645 psi/ft. So 0.645 / 0.046 = about 14 psi per barrel of annulus. A 50 psi step should take roughly 3.5 bbl. If the volume bled is much smaller than that, the choke is plugging or the gas is not behaving as expected - stop and investigate rather than continuing to open the choke. Never bleed so much that the SIDPP disappears; that removes the overbalance.