Lessons Learned: Combined Influx of Water & Gas
Reduction of deepwater boosted riser ECD using MPD system allows influx.
An influx occurred during the drilling of a 12¼” hole section of a deepwater exploration well. As the influx happened utilizing an MPD system as the primary barrier envelope, the riser boost pump was turned off while pumping a high vis sweep down the drill string.
With the loss of this marginal annulus friction (estimated to be approximately 20 psi) from stopping the riser boost pump, the BHP was determined to have been reduced below formation pressure allowing a combined influx of water and gas to enter the well bore. The influx was identified and successfully circulated from the well.
Primary lesson learned from this event is to recognize the contribution of the “boosted” riser’s friction to the barrier envelope’s ECD (equivalent circulating density) and compensate as may be necessary, with added surface back pressure when stopping the boost pump acting on the riser.
What happened?
Drilled 12¼” section to 3887 mMDRT, section TD, in MPD mode maintaining13.49-13.51 ppge during drilling (178psi SBP) and 490 psi SBP during connection/pumpoff. Stopped boost pump. Held SBP at 175psi with ECD @ 13.5 ppge. Pumped 100 bbls of 12.7 ppg Hi-Vis sweep. Started boost pump back on riser – circulated out Hi-Vis sweep. ECD dropped by 0.03 ppg (20psi) to 13.48 ppge during the sweep being pumped due to riser boost being off. Observed increase in Active: 113 bbls in 27 mins. Commenced increasing SBP in increments of 10 psi trying to suppress the gain.
After 5 mins, increased SBP more aggressively up to 400 psi. Influx suppressed and observed loss rate 460 bbls/hr.
Reduced string FR from 850 gpm to 600 gpm. Gas noted to be 5.3%. Diverted returns to MGS. Gas climbed to a peak of 12.4%. Observed mud density to be 12.3 ppg at Coriolis flowmeter.
Circulated out influx through MPD/MGS.
What went wrong?
Drop in ECD by 20 psi due to booster pump turned off and not compensated by SBP.
Corrective actions and recommendations
Any drop in ECD should compensated by SBP in MPD operations, maintaining CBHP.
Content credit/source: IOGP (International Association of Oil & Gas Producers), Well Control Incident Sharing 24-11, “Reduction of deepwater boosted riser ECD using MPD system allows influx.”
