Losses Followed by a Slow Gain
A well is being drilled at 3,100 m TVD with 1.45 SG mud. Pore pressure in the open hole is estimated at 1.38 SG equivalent, so the margin is narrow. While drilling at 600 gpm the driller sees the pit drop steadily: 320 bbl is lost over about 25 minutes. The pumps are slowed to 300 gpm and the losses slow but do not stop. The toolpusher orders 40 bbl of lost circulation material spotted and says the plan is to 'keep the mud light and keep going'. Two hours later, during a pump-off connection, the derrickhand reports the pit is now rising slowly - 12 bbl over 15 minutes - and the well continues to flow with the pumps off.
1. Why does a well that was losing fluid start gaining it, and what does that change about the situation?
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The two events are directly linked. When 320 bbl of mud is lost into a formation, the mud level in the annulus falls. The hydrostatic pressure at the bottom of the hole is a function of mud density and the true vertical height of the column; lose height, and the bottom-hole pressure drops even though the mud weight reading on the suction pit has not changed.
With only 0.07 SG of original overbalance, the loss of a few hundred barrels is easily enough to take bottom-hole pressure below pore pressure. Once that happens the well flows, and the flow replaces some of the lost volume, which is why the pit begins to rise.
This changes the problem from a lost-circulation problem to a well control problem. The priority is no longer to cure the losses; it is to stop the influx, establish shut-in pressure, and only then decide how to restore the barrier while handling the loss zone. Continuing to drill on a lightened column while the well flows is not a plan.
2. What should the crew do at the connection, and what makes the decision harder than a routine shut-in?
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The crew should stop, confirm flow with the pumps off, and shut the well in. Pressures should be recorded and monitored, the pit gain logged, and the company representative and toolpusher informed with the actual figures.
The difficulty is that shutting in raises pressure at the same weak zone that has just taken 320 bbl. A conventional hard shut-in may simply push more fluid back into the loss zone, and the shut-in pressures may never stabilise in the usual way because the well is leaking off as fast as it is flowing. That is a real conflict between two barriers, and it is a decision to be taken deliberately and documented, not improvised.
What is not acceptable is to keep the pumps off and do nothing while the pit rises. The crew should secure the well as instructed by the responsible person, keep monitoring the pit and pressures continuously, and make sure the loss zone depth and the volumes lost are clearly communicated.