Shallow Gas in Surface Hole With a Diverter
A 26 in surface hole is being drilled at 420 m with 1.05 SG spud mud. No BOP stack is installed at this stage; a diverter system is rigged up, with lines to both sides of the rig, and it was function tested before drilling out. The well is being drilled at 40 m/hr when the gas alarm sounds. The shaker hand reports mud returning with visible gas bubbles and the pit gains 30 bbl in four minutes. The driller can see the flow line discharging strongly. A crew of twelve is on the drill floor and the rig floor, one contractor is in the mud pit room, and the toolpusher is on the telephone to the office. The driller reaches for the annular preventer control.
1. Why is closing the annular preventer the wrong action here, and what should the driller do instead?
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At 420 m the formation cannot be shut in. The rock at that depth has very low fracture strength, so any pressure applied at surface would be transmitted to a shoe that cannot contain it. If the well is shut in, the gas will either fracture the formation downhole and charge the surface, or find the weakest path - often around the outside of the conductor - and broach to surface uncontrolled.
That is the purpose of the diverter: it does not contain pressure, it redirects the flow safely away from the rig floor and personnel while the gas is allowed to expand and disperse. The driller should open the diverter, confirm the correct lines are open and the others closed, keep the flow path clear, stop rotating, and get non-essential personnel clear of the rig floor and away from the discharge points.
A diverter is a last line of defence, not a well control device. It buys time and protects people; it does not solve the inflow.
2. What must happen on the rig floor and in the pit room, and why does the pit room matter so much in a shallow gas event?
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On the rig floor: sound the alarm and account for personnel, stop rotating, position the string as instructed, direct all ignition sources off, and keep everyone clear of the diverter discharge lines and the flow line. The toolpusher should be pulled off the telephone and given the facts.
The pit room matters because it is where gas can accumulate unburned, and where the person working alone is least likely to notice a developing situation. Gas released at the shakers or through the pit system can collect in an enclosed space and find an ignition source. The mud pit room must be evacuated, ventilation considered, and the person accounted for - not left to finish a sample while the well flows.
Shallow gas gives minutes, not hours. The response works only if it was planned, drilled and everyone knows their part without being told.