Gas migration during a five-hour weight-up
The 8½ in hole is at 15,500 ft MD / 14,000 ft TVD when a 20 bbl kick is shut in. Mud weight is 14.5 ppg and pressures stabilise at SIDPP 480 psi and SICP 720 psi. The 9⅝ in shoe is at 11,000 ft TVD with an LOT of 16.2 ppg equivalent. Kill mud weight is calculated as 15.16 ppg, and the mud engineer starts weighting up 900 bbl of active mud, which will take about 5 hours. Forty minutes later, with the well still shut in and nothing else changed, the SIDPP has risen to 780 psi and the SICP to 1,020 psi. The driller proposes bleeding 300 psi of mud off the choke to put the SIDPP back to 480 psi, and then weighting the mud to 15.5 ppg to cover the extra pressure.
1. Explain what has caused the pressure rise and evaluate the driller's proposal point by point.
Read the worked answer
Both pressures rose by exactly 300 psi. That is gas migrating up the closed annulus, not reservoir pressure increasing and not a second influx. Because the well is rigid and shut in, the migrating gas cannot expand, so it carries its pressure upward and adds it to both gauges equally. The kill mud weight needed has not changed: 15.16 ppg is still correct.
Evaluating the proposal:
- Bleeding 300 psi to restore the SIDPP to 480 psi is wrong as written. A straight 300 psi bleed off the choke drops bottom-hole pressure by 300 psi and will almost certainly pull in another influx.
- Weighting up to 15.5 ppg is wrong. It is based on a migrated pressure, not on pore pressure. Kill weight = 14.5 + 480 ÷ (0.052 × 14,000) = 15.16 ppg, and 15.5 ppg adds unnecessary pressure at a shoe that is already under load.
- Bleeding is not wrong in principle, but it must be done as lubricate-and-bleed: bleed a measured volume of mud, let the gas expand, and stop as soon as the SIDPP returns to 480 psi. Bleed only what is needed to stay inside the shoe limit.
2. Calculate the MAASP, decide whether the shoe is already at risk, and state what you would do right now.
Read the worked answer
MAASP with 14.5 ppg = (16.2 − 14.5) × 0.052 × 11,000 = 972 psi.
The SICP is 1,020 psi, so the shoe is already about 48 psi above its leak-off limit. That 48 psi is small and probably not yet damaging, but it will grow with every minute of migration, so this is now a time-critical event.
Right now:
- Start the kill now with the 14.5 ppg mud you have, at the slow circulating rate, rather than waiting 5 hours. Circulating removes the gas and the problem with it; weighting up first only lets the shoe pressure climb.
- If the mud engineer insists on waiting, then lubricate and bleed in small increments - bleed to bring the SICP back to the 972 psi MAASP, maintaining the SIDPP at or above 480 psi, and repeat as migration continues.
- Record every bleed volume and pressure; the migrated gas volume is being replaced by mud, and the record is what tells you whether the technique is working.
- Watch the shoe: any drop in the casing pressure that you did not bleed for is a loss, and that changes the whole picture.
3. Explain how you would run the kill given that only 14.5 ppg mud is available, and what you would tell the driller about the kill sheet.
Read the worked answer
Run a wait-and-weight or a circulating weight-up, whichever the rig can support:
- Preferred: start circulating at the slow rate on the 14.5 ppg mud using the ICP schedule, and weight the active system to 15.16 ppg while the influx is circulated out. The choke holds bottom-hole pressure while the mud weight catches up; the kill sheet is updated for each increment in weight and the FCP is recalculated as the heavy mud reaches the bit.
- If the weight-up cannot be done while circulating: hold the well shut in, keep lubricating and bleeding to hold the shoe at 972 psi, and start the kill the moment 15.16 ppg mud is available.
What the driller needs to hear about the kill sheet:
- The kill weight is 15.16 ppg, not 15.5 ppg. The extra 0.34 ppg has no basis and eats the shoe margin.
- The ICP and FCP must be recalculated from the original 480 psi SIDPP, not from the migrated 780 psi. Using the migrated pressure overweights the mud and over-pressures the shoe.
- The MAASP falls as the mud weight goes up - with 15.16 ppg it is (16.2 − 15.16) × 0.052 × 11,000 ≈ 595 psi, so the choke schedule must be rebuilt before the kill starts, not during it.