Shallow gas under the diverter with a restricted vent line
At 06:20 the 17½ in surface hole is at 1,900 ft MD/TVD, drilled with 9.8 ppg spud mud below 26 in conductor set at 300 ft. No BOP is installed; the well is protected by a diverter with 12 in vent lines to port and starboard. The last function test was 7 days ago and the port line was logged as partially restricted by cuttings. The driller takes a 40 bbl pit gain in 4 minutes, the flow line is running full, the gas detector reads 55% at the shakers, and the rig floor is 40 m from the rotary table. Pump rate is 900 gpm at 350 psi and the pits are being transferred to keep up with the hole. The nearest support vessel is 6 hours away and the weather is closing in.
1. List the actions you take in the first few minutes, in order, and state what you must not do.
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- Stop drilling, pick the string up off bottom and keep it moving so it does not stick, but do not trip.
- Activate the diverter and confirm flow is going overboard downwind through the starboard line, which the last test recorded as clear. Do not put flow through both lines until you know the port line is clear.
- Keep the pumps running at the highest rate the system will take - the flow is killed dynamically, by friction and mud weight, not by shutting in.
- Sound the alarm, muster, account for personnel, kill all hot work and ignition sources, and move non-essential people off the rig floor.
- Notify the OIM, the operator and the standby vessel; put the vessel on notice now, not in an hour.
- Monitor pit volume, flow rate, gas readings and the diverter line for erosion or a leak, and record everything with times.
Must not do: close the BOP, close the diverter in a shut-in configuration, stop pumping to "see what happens", or bleed the well. Any back pressure at 300–1,900 ft will fracture the formation, gas will channel to surface outside the conductor, and you exchange a manageable flow for a crater and possible loss of the rig.
2. Explain why shutting in is not an option here and what a dynamic kill actually requires in this hole section.
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At 1,900 ft the formation strength is close to overburden - of the order of 1 ppg above the pore pressure at best. Shutting in traps the reservoir pressure at surface and transmits it straight into the shoe and the conductor shoe at 300 ft, which will break down. The gas then has a path to surface outside the casing, outside the conductor, or through the seabed, and the rig floor can be undermined or engulfed. That is why shallow gas is diverted, never shut in.
A dynamic kill means killing the flow with circulating pressure and mud weight while the well is open. The available levers are:
- Rate: more gpm raises the annular friction and therefore the bottom-hole pressure; doubling the rate is the fastest lever the driller owns.
- Mud weight: limited by the fracture strength at 300–1,900 ft, so the increase is small - a few tenths of a ppg at most.
- Time: the flow must be killed before the diverter line, the flow line or the rig floor is damaged. If the pit gain keeps climbing steadily despite maximum rate and a modest weight increase, the well is not being killed and you are only buying time.
3. Returns show no sign of decreasing after 20 minutes, the starboard vent line is showing erosion at a flange, and the pit gain is now 180 bbl. What is your decision and what is the trigger for it?
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The trigger for abandoning the location is loss of the ability to divert safely, and an eroding vent line is exactly that. A 12 in line cutting out at a flange will fail, and gas at the rig floor at 55% plus is not survivable.
Decision: stop pumping, shut down the rotary and the mud pumps, muster all personnel at the designated point, and move the crew off the rig floor to the upwind safe area or evacuate to the standby vessel if the OIM judges the risk of cratering or gas ignition is unacceptable. Keep the diverter open - a failed diverter line is still better than a shut-in well. Do not attempt to repair the line while flow is passing through it.
Justification: the well is not being killed dynamically and the diversion path is degrading, so continuing to pump adds gas volume to a rig that can no longer safely dispose of it. Once the crew is safe, the operator can reassess from the vessel, and re-entry only happens after the flow stops and the area is gas-free.
Record the pit gain, rate, pressures and timings throughout; the post-incident review will hinge on whether the decision was taken at the right trigger, not on whether the well was eventually killed.