Loss of well control, hole and drill string due to poor communication and well monitoring.
Drilling 8"1/2 x 9"1/2 hole with 1.30SG mud weight (MW) at 2248m - this mud density is used to drill the top section shales for borehole stability purpose.
Crossed an identified sands layer which was expected to be sub-hydrostatic (0.5SG). Observed a connection gas reading up to 60% + pack off tendency. Increased mud weight by step to 1.35SG but gas reading were still high.
Decided to Shut the well in and observed pressure in the well SIDP 400 psi - SICP 510 psi.
A Gain of +/- 10m3 was estimated later (by postmortem analysis of the previous pipe connection and pump-off logs).
Performed Driller's Method and killed the well by displacing 1.51 SG kill mud. Open hole collapsed during circulation with the consequence of string getting stuck and kick zone isolated.
What Went Wrong?:
The reservoir was expected to be depleted. This part of the field was artificially overpressurized by a water injector well. This was not identified during the well preparation phase and the risk not transmitted to the drilling teams.
Lack of crew vigilance. Poor well monitoring during DP connections. The high connection gas observed at surface were the result of a crude contamination in the mud system. Significant gain volumes were taken during the previous pipe connections without being detected.
Corrective Actions and Recommendations:
Shared the experience and reinforce the well preparation process with a rigorous risk identification - hazard related to a continuous injection in a mature field to be emphasized.
Re-inforce well monitoring - specifically during pipe connections.
Review mapping of injection on the field.
safety alert number: 301
IOGP Well Control Incident Lesson Sharing http://safetyzone.iogp.org/
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