Publications

Although East Java basin has been drilled since before World War II, drilling issues are still often encountered. These are mainly associated with wellbore instability and lost circulation even in vertical wells. These problems are generally due to the presence of overpressure shale with clay-rich mudstones, normal pressure in Ngrayong Formation but a pressure ramp below it in Tawun Formation, and shallow salt-water flow. In the recently drilled N-1 well by Pertamina EP Cepu ADK, drilling through normal pressure zones encased by higher pressure zones, overpressured Tuban Shale, were key technical challenges to overcome. Shales usually have heterogeneous mechanical behavior due to thinly bedded mudstones with varying rock strength and horizontal stresses. Additionally, presence of natural fractures in the wellbore can cause hole collapse resulting in hole pack-off and stuck pipe incidents. Overpressure shale must be anticipated in the drilling program to prevent blowout, minimize reservoir damage, and mitigate lost circulation in weak zones. To overcome these problems, a geomechanics analysis with Mechanical Earth Model (MEM) comprised of pore pressure prediction, rock strength elastic properties, and in-situ stress profiles was conducted, by integrating offset wells, drilling reports, seismic data, and analog information to provide an optimum mud weight window through wellbore stability analysis. The model provided recommendation of mud weight window consisting of kick, borehole breakout or stress-induced hole enlargement, losses and breakdown of mud weight range. Integrated with utilization of high-performance water-based drilling fluid containing inhibitive chemicals to prevent reaction with clay minerals especially in near Tuban Formation, the geomechanics model has helped the company to accomplish the goals of the drilling program fficiently. The model allowed the integrated drilling team to monitor borehole conditions, make adjustments in the program to keep wellbore integrity and stability, and reach the exploration target safely and on schedule.