--> Geomechanics Wellbore Stability Analysis in Successful Drilling of A Challenging HPHT Exploration well in North Sumatra

AAPG Asia Pacific Technical Symposium

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Geomechanics Wellbore Stability Analysis in Successful Drilling of A Challenging HPHT Exploration well in North Sumatra

Abstract

Pertamina exploration block in North Sumatra indicates High Pressure and High Temperature (HPHT) field. The pore pressure indicated up to 2.2 g/cm3 and the temperature is more than 350°F, even the well kick documented from the existing well. Over pressured layer recognized as Lower-Baong Shale Formation which sitting above the reservoir Carbonate Belumai Formation. Drilling challenge in this field is to encounter the over pressured overburden shale, with also narrow mud weight window prior entering the reservoir. Several wells had been drilled in the surrounding field since 2003 and the exploration study is continuing in this field. Drilling campaign for the upcoming wells is set with targeted the same formation and probably will encounter the similar geomechanics issue as the existing wells. To achieve the drilling target, pre-drill geomechanics model must be conducted which provide pore pressure prediction, breakout collapse pressure, fracture gradient or losses pressure, and breakdown pressure. Pre-drill geomechanics analysis is the predicted model generated based on the 1D geomechanics Mechanical Earth Model (MEM) in the existing wells near to the upcoming wells. 1D geomechanics Mechanical Earth Model (MEM) constructed based on well-logs data and drilling reports in the existing wells. Drilling reports were reviewed carefully to capture drilling problems and guide the MEM computation. The availability of uniaxial core laboratory test is another important data to validate and update the MEM computed from the well-logs data after drilling. With the validation of the laboratory test, the MEM results validated enough to be propagated to another formation vertically over the well sections. Validated geomechanics MEM in the existing wells in this field, will deliver accurate pre-drill wellbore stability analysis for the upcoming exploration wells. Wellbore stability analysis is used in the well planning to define the borehole trajectory, casing design and mud-weight recommendation. The accuracy of the geomechanics model is of extremely importance to minimize the drilling risk and enable accurate planning and mitigation.