Drilling through a carbonate formation characterized bymassive vugulars and fracturescreates a difficult and high-risk operation when using conventional drilling methods. Total lost of circulation, accompanied byinflux or kicks may be encountered when drilling through high volumes of pores and numerous fracture zones. Using managed pressure drilling (MPD) techniques when drilling through vugs and severely fractured formations can reduce non-productive time in combatting lost circulation problems and preventingformation fluid influxes from reaching the surface during drilling operation. The application of a pressurized mud cap drilling (PMCD) variant of MPD in a recent project in Indonesia was key in allowing the operator to reach their targeted depth.

PMCD is a non-conventional drilling technique designed to maintain annular wellbore pressure when total loss of circulation occurs. With the utilization of MPD equipment and PMCD techniques, mitigation of drilling hazards caused by lost circulation or influx was successfully performed in this project without stopping the drilling operation. The decision of calling out the MPD to the rig site saved the operator drilling days followingunsuccessful attempts to cure the lost circulation by pumping cement for several days.

With PMCD, the proper bullheading sequence was carried outin order to control influxes which weremigrating through the mud-cap to the surface. With applications and actual operations on location, drilling operations successfully reachedthe targeted depth with minimum NPT and optimum hazard and cost reduction.

This paper will describethe planning phase, operational aspects, and results of drilling with the PMCD technique through challenging carbonate formations in a Southern Sumatra oil and gas field.

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