Pertamina EP recently executed an exploration drilling project in Sumatera area. The objective was to evaluate the oil reserves contained in the fractured volcanic basement reservoir zone. The very hard (26,000 psi average compressive strength, with spikes of up to 50,000 psi) and abrasive nature brought drilling challenges, in particular, how to effectively and efficiently fail and fracture the rock. In a well in the previous drilling campaign, 10 runs of bit were required to complete the 294-m interval hole section containing basement formation. Various roller cone and PDC bit designs were utilized, with a resulting maximum penetration rate capability of 1.25 m/h and maximum run interval capability of 56.44 m. Another well with a basement interval of 463 m consumed 11 bit runs with maximum run drilling interval and penetration rate of 64 m and 1.56 m/h, respectively. A conical diamond element (CDE) bit was proposed to achieve more efficient drilling in the basement. The 3D geometry of the conical diamond elements, which are placed across the bit blade, provided both a superior impact and wear resistance and concentrated point loading onto the formation; these characteristics enable the bit to cause failure and fracturing of the hard-to-drill rock more efficiently. A diamond layer in the bit is twice as thick as that of a conventional PDC cutter; this enhances impact strength and improves overall bit durability. CDE bits were implemented to penetrate the basement formation in two different wells in Sumatera area. The wells were designed as directional J-type wells with maximum inclination of 70°. In the first well, eight-bladed and seven-bladed CDE bits were run with an excellent result of 131-m and 176 m of drilling interval achieved, respectively, yielding up to 211% of drilling meterage capability improvement (compared to the previous well). The rates of penetration (ROP) generated by CDE bits were also large; up to 4.17 m/h ROP was achieved (instantaneous on-bottom ROP reached up to 20 m/h), which was equivalent to 333% improvement. In the second well, the seven-bladed CDE bit generated 230 m of drilling interval with 3.27 m/h ROP, improving the offset well performance by 259% (in terms of interval) and 109% (in terms of ROP). The CDE bit significantly saved operating time and cost by improving the drilling speed and, most importantly, eliminating unnecessary trips to change the bit. The estimated potential savings from CDE bit utilization in these two wells reached more than USD 1.1 million. The drilling optimization in Sumatera yielded insights into CDE design and features appropriate for basement application, and the engineering study behind the successful execution, actual drilling operation, and run performance comparison provided information for future such operations.

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A depleted reservoir in the Tapian Timur field in South Kalimantan, Indonesia, is located at 1,400-mTVD with a 400-psi formation pressure. Drilling through it presents potential threats to operations, including lost circulation during drilling and cementing and borehole instability. Two wells previously drilled and completed in the area suffered long NPT due to stuck pipe and bad cement bond quality. Even though the field has been explored and developed for 43 years, a thorough study of the reservoir characteristics was never done. Improper knowledge of a reservoir like this one cause’s unnecessary lost time during drilling and poor zonal isolation of the target zone, can potentially jeopardize the economic feasibility of a project. For this reservoir, a geomechanical study was applied, and a mechanical earth model (MEM) was built. The model utilized multidisciplinary data across the field (drilling and logging data, core, pressure history) to evaluate wellbore stability and improve drilling practices. The MEM study provided key recommendations for the drilling process and cementing operation. Decisions made based on the study include changes in casing point, mud-weight optimization, and utilization of high-performance lightweight slurry with fiber. A subsequent well drilled in the area applying the recommendation from the study was drilled and completed faster. Cement bond logs from the section also indicated excellent zonal isolation across the reservoir. The recommendations from the study are currently integrated into the standard operation procedures.