Kick while drilling: kill mud and gauge interpretation
At 06:20, while drilling an 8½-in hole at 9,500 ft TVD with 12.0 ppg water-based mud, the pump pressure drops 180 psi and ROP briefly increases. Gas-cut mud appears on the shakers. With pumps off, the flow line continues to flow and the pit gains 18 bbl. The crew closes the annular BOP. Stabilised pressures are SIDPP 520 psi and SICP 780 psi. The slow-circulating-rate pressure is 450 psi at 30 spm. The casing shoe is at 7,200 ft TVD, and the leak-off test gave a 14.2 ppg equivalent. The BOP stack is rated 5,000 psi, the choke manifold is lined up to the trip tank, and the active system is 1,200 bbl. The driller has not yet started circulation. The company man wants the kill mud weight and a check of the pressure readings before any choke action.
1. Which observations confirm a kick, and what should the crew do before the company man's kill calculation?
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The strongest confirmation is the combination of a pump-pressure drop, an 18 bbl pit gain and continued flow with the pumps off. A drilling break or gas-cut mud alone is not proof: gas-cut mud can come from connection gas or contamination, and a fast ROP can be a formation change. Continued flow after pumps stop is the key positive indicator because it shows formation fluid is entering the well. Before calculating, the crew should stop rotation, pick up to clear the tool joints, stop the pumps, flow-check, close the BOP, and record SIDPP, SICP, pit gain and time. They should confirm the drillstring is full, the gauges are reading correctly and no float or back-pressure valve is trapping pressure. Only then should the pressures be used. Starting circulation or opening the choke before stable readings are recorded can lose the kick or hide the true formation pressure.
2. Calculate the kill mud weight and explain why SIDPP is used rather than SICP.
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Kill mud weight = current mud weight + SIDPP ÷ (0.052 × TVD).
KMW = 12.0 + 520 ÷ (0.052 × 9,500) KMW = 12.0 + 520 ÷ 494 KMW = 12.0 + 1.05 = 13.05 ppg.
SIDPP is used because the drillstring contains a known, single mud column with no gas or cuttings. The pressure at the bit therefore reflects formation pressure directly: formation pressure = drillstring hydrostatic + SIDPP. SICP is measured on the annulus, where gas, cuttings or trapped pressure can change the hydrostatic. Using SICP would overestimate or underestimate the kill mud weight. The calculation assumes SIDPP is stabilised, the string is full of the stated mud weight and no trapped pressure is present. If those checks fail, the number must be corrected before pumping.
3. SICP is 260 psi above SIDPP. What could cause this, and does it change the kill mud weight?
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A higher SICP commonly means the annular hydrostatic is lower than the drillstring hydrostatic, for example because gas or cuttings are in the annulus. It can also come from trapped pressure below the BOP, a partially plugged choke line, gauge error, or a closed choke with pressure trapped on the annulus side. The difference does not automatically change the kill mud weight because SIDPP still represents formation pressure, provided the drillstring is full of known mud and the reading is stable. The crew should first verify both gauges, confirm no trapped pressure and check that the drillstring is not plugged or floating. If trapped pressure is suspected, it must be handled by procedure, not by simply bleeding the annulus. The company man should use 13.05 ppg for the kill and treat SICP as the pressure to control while bringing the well under control.