Well control incident due to pore pressure uncertainties
IOGP Well Control Incident Lesson Sharing #23-10
During the drilling of an appraisal well, an influx occurred due to pore pressure uncertainties within the region. The use of a classical Early Kick Detection system (EKD) proved invaluable for detecting an influx very early and thereby minimizing its size. Mud weight drilling margins were anticipated to be narrow in this hole section. However, as a result of the higher than anticipated pore pressures encountered, the drilling mud weight margin was reduced lower than expected. Lessons learned and applied to subsequent regional exploration wells included the use of a Managed Pressure Drilling (MPD) system which significantly reduced well control incidents in the lower mud weight margins related to pore pressure uncertainties.
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.
While drilling 12-1/4" section with 1.56 SG SBM (CC), EKD system (including a Coriolis type mass-flow meter integrated to the flowline through a bypass) detected abnormal flow-out at 4382mMD. Stopped circulation and shut in the well on UAP.
Observed SICP stabilizing around 285psi, difficulties to assess SIDPP due to presence of motor and non-ported float valve.
Gain volume estimated around 2bbl. Tried to initiate circulation - no return. Injection suspected at shoe at 2850m. Decided to spot kill mud cap in open hole by injection.
The Kill Weight fluid spotted was calculated on the basis of the frac pressure at shoe observed.
What Went Wrong?:
It was a Narrow Mud Weight Window environment by design. The reservoir pressure encountered while drilling was found to be above the prognosis (exploration uncertainties) therefore reducing even more of the drilling window.
Appraisal well - uncertainties.
Corrective Actions and Recommendations:
A MPD system was implemented on the following wells in order to be able to drill with a reduced mud weight margin and be able to accommodate any further pressure uncertainties. The strategy was successful with a significant reduction of well control events on that exploration prospect with a very high pressure uncertainty.
Capture and transfer experience to new wells in surrounding area.
safety alert number: 359
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