Trip Tank Fill Shortfall on a Night Tour Handover
At 06:20 the night driller is pulling out of hole on a development well at 2,850 m MD. The trip tank is lined up and the driller has compared actual volume with the theoretical displacement sheet every five stands. Five stands have been pulled; the sheet says about 8 bbl should have been taken into the trip tank, but the tank shows only 6.5 bbl. The derrickhand reports the last stand came out with a wet string, and the mud logger calls to say total gas has risen from 0.4% to 2.1% over the last three minutes. The driller is due to go off tour in 20 minutes. The relief driller is already on the drill floor, the toolpusher is in the doghouse, and the driller's instinct is to rack back the stand and hand over a clean floor rather than start something at shift change.
1. How should the driller interpret the trip tank figure, the wet string and the gas reading together?
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The three observations point the same way, which is why they cannot be dismissed as coincidence.
- Fill shortfall of 1.5 bbl. When pipe is pulled, the hole must take exactly the steel volume of the pipe removed. A shortfall means the hole is not taking its proper fill, so something has entered the wellbore to occupy that space.
- Wet string. Pipe coming out wet above the normal mud line is a secondary indicator that formation fluid is in the annulus and being carried up with the pipe.
- Rising total gas. Gas increasing four-fold in three minutes supports an influx now moving upward.
No single one of these is conclusive, and 1.5 bbl is a small number. That is exactly the trap: the correct comparison is against the expected volume, not against some threshold of 'big enough to matter'. A shortfall is a shortfall, and the combination of indicators makes an influx the most likely explanation.
2. What should the driller do before leaving the floor, and why is a handover at this point unsafe?
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The driller should stop pulling, set the slips, and carry out a proper flow check with the pumps off. If the well is flowing, the correct response is to shut in using the agreed procedure, record shut-in drill pipe and casing pressures and the pit gain, and inform the toolpusher and company representative.
A handover is unsafe for two reasons. First, well control actions are time-dependent: gas expanding in the annulus reduces hydrostatic pressure at the bottom, so a kick left alone gets harder to handle, not easier. Second, the incoming driller would inherit a situation with no defined start point - no recorded pressures, no confirmed flow check, and a verbal description of a 'small' anomaly. Handing over a well with an uninvestigated fill shortfall transfers both the hazard and the ambiguity.
The correct behaviour is to escalate, not to tidy up. Shift change is a scheduling matter; a possible influx is not.
3. The relief driller says a 1.5 bbl discrepancy is within gauge error and wants to finish the trip. How should that argument be tested?
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The argument can be tested rather than simply overruled.
Ask what the trip tank gauge error actually is, and whether the same error has been present all trip. If the driller has been matching the sheet within a few tenths of a barrel for the previous twenty stands and now sees 1.5 bbl missing, the discrepancy is new behaviour, not a constant offset - a constant gauge error would cancel out in the comparison. Ask also whether the volume was read with the tank static and the valves correctly lined up, and whether any transfer was in progress.
If the discrepancy cannot be explained by a known, quantified instrument or lineup error, it must be treated as real. The cost of a ten-minute flow check is trivial; the cost of pulling further stands into a live well with a reducing hydrostatic head is not. Doubt is resolved by measurement, not by seniority or by wanting the trip finished.