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.