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Misunderstood pore pressure, lack of vigilance and empowerment cause Well Control Incident.
Drilling 6" hole - just entering an identified reservoir - with 1.40SG mud weight (MW). The formation pressure expected was not well understood and a large uncertainty remained between a depleted reservoir scenario or a pressurized case (water injection on a mature field).
On the first stand into the reservoir, a circulation was performed in order to assess the gas level and the stability of the well, a maximum of 7% was observed. No flowcheck performed but a conclusion was made on a depleted scenario case. A drill pipe (DP) connection was then performed to continue drilling. 7m3 of gain were taken during the connection without been noticed. Drilling was resumed for a few more metres and significant flow increase & gain in active system was observed.
Drilling was stopped but the well was not shut in immediately. It took 5 more minutes to investigate the anomaly.
Shut in drill pipe pressure (SIDP) 450psi - shut-in casing pressure (SICP) 1160psi - 25m3 total estimated gain.
Significant gain volume generated serious difficulties to control the well.
Well was finally killed using driller's method with kill mud weight (KMW) 1.64SG.
What Went Wrong?:
Misunderstanding of the pore pressure prediction (high uncertainty expected between 0.98 to 1.51SG).
Wrong pore pressure diagnosis while based on non-valid gas criteria - the gas% criteria was not a pump-off event.
No flowcheck performed and anticipated in the drilling strategy to enter that reservoir.
Lack of crew vigilance, poor well monitoring during DP connections - first kick during connection not identified.
Basic well control procedure not properly implemented for kick detection and well shut-in. Driller not empowered to shut the well in without authorization.
Corrective Actions and Recommendations:
Enhance geoscience/drilling communication for a better understanding of the formation pressures and risks identified.
Review personnel competency.
Reinforce well monitoring and shut-in procedures with crew.
Review and identify risk of reservoirs artificially pressurized (suspect reservoir pressure boosted by water injection well in vicinity).
Figure 1: Well architecture
Figure 2: Late kick detection sequence of events
safety alert number: 300
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