Lessons Learned: MPD Operations Incident

Loss of surface back pressure during MPD operations.

An incident occurred while drilling an 8 ½” hole with MPD. It was planned to shut in the well to investigate plugged pressure relief valves. While ramping down pumps, a fault on the MPD choke electrical control system caused the chokes to open allowing surface back pressure to reduce. The well was shut in using the annular BOP. The investigation identified loose fuses in the choke control electrical system which triggered the Variable Frequency Drive (VFD) interlock. The MPD chokes should have been set to “fail as is” however the investigation concluded the chokes were set to “fail open.” The fault caused the chokes to open and loss of surface back pressure. The well was underbalanced for 6-7 minutes but no influx was recorded as no permeable formations were exposed.

What happened?

Plan:

• Drill 8 ½” hole in MPD mode with 1,69 SG OBM and a keeping a Constant Bottom Hole Pressure of 1,96 SG.

Operation with course of events:

• Noticed pressure relief valves 1 and 3 were plugged, after performing dynamic flow-check, established plan to close in well by going into a dummy connection and then trapping pressure on the well using MPD system.
• During ramp down, pressure slightly overshot target connection pressure and operator adjusted set-point. VFD interlock safeguard was triggered, keeping choke #2 in place. Attempted to reset and regain control but continued to trigger interlock. Was losing pressure on well and shut in on annular (Driller first action B). MPD personnel troubleshot the MPD chokes while well remained shut in on red flow path.
• The Surface Back pressure dropped down to 56 bar during the event. This is equal to BHP dropping from 1,96 SG to 1,83 SG. The pore pressure at the current depth is prognosed to 1,89 SG. The well was in underbalance for approximately 6-7 minutes before the back pressure was pumped back up to +/-108 bar with rig pumps down the Drills string.
• No gain was registered during this event. Circulated Bottoms up through red flow path. No gas was observed when bottoms up came to surface.

What went wrong?

Reason for events:

• Found two 16 amp fuses in the 230V distribution box which was completely loose. Fastened fuse holders and all comms and power restored.

Corrective actions and recommendations

Lessons Learned:

• Crew reacted quickly and correctly to the event so that the time the well was left in underbalanced was minimized.
• Post incident investigation showed that MPD chokes were “fail safe open” and not what was requested “fail safe as is”.

Recommended actions:

• Go through all data and investigate the issues that have been seen so far to ensure the mitigations are sufficient.
• Share the learning with similar operations.

Impact:

• Lost time, Barriers where still intact but due to a temporary situation where the MPD choke where left in a state unable to move, the well was in underbalance for 6-7 min. No influx was recorded, and no Hydrocarbon volumes were seen when circulating bottoms up.

Critical Issues:

• MPD Equipment malfunction.

Content/image source/credit: IOGP (International Association of Gas & Oil Producers, “Loss of surface back pressure during MPD operations,” Well Control Lesson Sharing #26-1.