Well Control Event in Exploration Drilling using MPD

A well control event occurred while tripping pipe in exploration drilling using MPD. The failure of the single surface back pressure sensor allowed the automated choke to fully open (fail-open), reducing the bottom hole pressure below the pore pressure and causing the loss of primary (hydrostatic) barrier pressure envelope.

While POOH in MPD mode with constant bottom hole pressure, MPD sensors failed which led the automated choke to open to 100% (due to wrong surface backpressure reading). This reduced the bottom hole pressure below pore pressure and caused an 800l influx. The driller shut-in the well on UAP.

The MPD system was fixed and constant bottom hole pressure was reinstated. The BHA was RIH to bottom with MPD and operations were resumed.

What Went Wrong?:

MPD system failure leading to incidental choke opening (decrease surface back pressure) and thus to loss of primary envelope.

Corrective Actions and Recommendations:

1. Surface Back Pressure (SBP) measurement is driving MPD control system. Wrong SBP value will induce unwanted choke adjustment (overbalance - losses; underbalance - loss of primary well envelope).
2. Based on this statement, the following recommendations apply on any MPD operations:
   • Redundancy is required on SBP measurement system so that SBP reading does not rely on only one sensor. The redundant system shall include a “multi-sensor voting” (min 3 SBP sensors required) allowing the failed sensor to be identified, alerted, and eliminated from the SBP control source.
   • MPD operators have to check that redundancy is properly set and activated.
   • In addition to redundancy process, SBP sensors and related equipment need to be checked on a regular basis and any failed sensor needs to be replaced promptly.
3. Even with automated systems, rigorous well monitoring is of primary importance. In the two examples here above, the commitment of the teams to proper well monitoring allowed them to quickly detect the anomaly and prevent the situation from escalating to a more serious scenario.

safety alert number: 312
IOGP Well Control Incident Lesson Sharing http://safetyzone.iogp.org/

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