GEOMECHANICS WELLBORE STABILITY ANALYSIS IN SUCCESSFUL DRILLING OF A CHALLENGING HPHT EXPLORATION WELL IN NORTH SUMATRA

摘要

One of the Pertamina exploration wells in North Sumatra targeted an unconventional shale gas reservoir, the Lower-Baong Formation. Based on offset wells data, this formation is High Pressure and High Temperature (HPHT). The pore pressure indicated up to 2.2 g/cm3 and the temperature is more than 350°F. A kick event occurred at one of the offset wells during drilling in 2004. A risk of kick and possible narrow mud weight window are predicted for this exploration well. To achieve drilling target, the pre-drill geomechanics model which provides pore pressure, breakout, fracture gradient and breakdown mud weight analysis was conducted. Since narrow mud weight is suspected, accuracy of the pre-drill geomechanics model is of paramount importance in order to minimize the drilling risk and enable accurate planning and mitigation. A 1D Mechanical Earth Model (MEM) is constructed based on offset wells data. Drilling reports of the offset wells were reviewed carefully and drilling problems are captured and analyzed. 1D MEM from offset wells is propagated to planned well based on formation tops and subsequently wellbore stability analysis is conducted to estimate safe and stable mud weight for the planned well. From the wellbore stability analysis results, it shows that indeed the planned well has narrow mud weight window and even very narrow up to 0.07 g/cm3 (SG) (0.6 ppg) window in 12.25" hole Lower-Baong Formation which makes impossible to drill safely using conventional drilling. Hence, the original drilling plan was revised significantly and the conventional drilling plan was switched to Manage Pressure Drilling (MPD). Casing point was also revised in order to capture revised plan mud weight and to stop right at the top of conventional Belumai carbonate in 8 3/8" hole. With the proper geomechanics analysis and real-time monitoring, the well successfully drilled safely to the formation target without a significant drilling problem.