IOGP Well Control Incident Lesson Sharing
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Wrong Operation of MPD Choke Results in Influx

IOGP Well Control Incident Lesson Sharing #23-12

During complex Pressurized Mud Cap Drilling (PMCD) on a HPHT well, a bridge plug was being run into a well for P&A when the MPD choke was mistakenly opened resulting in an influx. This event highlights the special attention and effort required to reduce errors associated with human factors during complex operations.

The IOGP Wells Expert Committee/Well Control Incident Subcommittee believes that this incident description contains sufficient lessons to be shared with the industry. We further encourage the recipients of this mail to share it further within their organization.

Water Depth 2800m, end of the well - P&A. Operations are managed in PMCD mode (Pressurized Mud Cap Drilling) maintaining an equivalent mud weight above pore pressure with 300 psi SBP (Surface Back Pressure) and a constant injection of seawater at 600 lpm to mitigate any gas migration.

In order to isolate the open hole, a bridge plug was RIH on DP in the riser down to above BOP - well is shut-in on BSR with constant injection via the KL. The MPD-RCD (Rotating Control Device) was installed on top of the riser. The well and riser were pressure-equalized (300 psi) and BOP was opened to RIH plug to 3100m across BOP.

Closed Annular BOP and performed a pressure test to 800 psi of the riser + RCD - OK. Equalized back to 300 psi and re-opened BOP, ready to RIH. As the next step was supposed to switch injection lines (from Kill line to MPD line), the MPD choke was wrongly operated fully open (not part of the procedure), releasing the surface back pressure.

This had the immediate effect to underbalance the system, authorizing the well to flow at a significant rate - 10 to 14 m3/min.

Realizing the MPD operator mistake, the driller shut-in the well on Annular resulting on an estimated 35m3 influx Shut in pressure (KL) at 1050 psi.

After analysis and assessment of the situation, the influx was fully bull-headed back to formation down the KL pumped 1,5 casing volume at 1200 lpm. Circulated riser volume and swept BOP no gas show.

Re-established PMCD initial parameters - OK.

What Went Wrong?:

  • Misunderstanding of the well situation by key players, confusion in the sequence. PMCD operations on floating rig (Ultra Deep Water) are relatively complex and not frequent.
  • Lack of detailed procedure for this specific sequence.
  • Lack of supervision on rig floor for this sensitive sequence.
  • No stop work authority, the MPD choke operator opened the well despite being conscious of the consequences.
  • Operation not considered less sensitive, had been performed already 7 times, less focus from the team.
  • Alarms not correctly set on flow out (Coriolis) which affected Driller's reaction to shut-in.

Corrective Actions and Recommendations:

  • Formalize a Chain of Command and communication channel for MPD instructions on rig floor.
  • Identify sensitive sequences which deserves a reinforced supervision.
  • Reinforce stop work authority.
  • Detail procedures with P&ID for sensitive/complex sequences. Make sure focus is maintained on standard alarms (flow return & pit volume) despite a non-conventional drilling mode (PMCD with no return).
  • Share the incident and reinforce PMCD simulator training for crews.

safety alert number: 361
IOGP Well Control Incident Lesson Sharing


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