Integrated Drill String Design Elevates Drilling Efficiency in Challenging Basement Formation

Abstract

Exploration in the basement could uncover the potential oil and gas resources in the South Sumatera area. However, drilling in the basement always brings different challenges compared to shale or sandstone formation. These challenges include low penetration rate due to hard lithology, deteriorating drilling bit and tools due to abrasive environment and high vibration, and unpredicted loss circulation zone that can increase mud consumption. In long interval sections, multiple bit runs are inevitable, causing higher open hole exposure time that can increase the risk of well integrity and well control. All challenges will have a significant effect on increasing drilling cost. Pertamina EP planned to run 4 - 6 bits based on the last two wells drilled through basement formation. Comprehensive steps were taken to minimize the drilling risks and optimize the drilling operation In terms of drilling assembly, including bit and Bottom Hole Assembly (BHA), enhanced design was selected based on the lesson learned from previous campaigns. The bit equipped with durability proven conical diamond element with other features to compromise basement formation. BHA was designed by considering motor selection, stabilizer placement, and collar combination. To optimize drill string stability, several BHA options, combined with a detailed bit model, were analyzed in advanced Finite Element Analysis (FEA) drilling dynamic simulation. The process generated the most stable bit-BHA configuration, ROP-bit durability prediction and drilling parameter roadmap. Extensive drilling strategy planning was taken in place to set the interval, ROP target, and parameter management. To have a step change from the previous basement drilling campaign, Pertamina EP set the target to drill 880-m basement intervals in 8 ½-in sections by maximum two bit runs with minimum on-bottom ROP of 5 m/h. By implementing all engineering preparation, basement formation in PLD-001 exploration well was successfully drilled in one run to section TD from 2,690 m to 3,573 m. The engineered bit-BHA combination drilled metamorphic basement rock with 10 – 15 klbs of WOB, 40 – 60 SRPM, and 550 – 595 gpm of flow rate. Close supervision was performed to monitor drilling dynamics, with no significant shock and vibration was observed. In overall, the bit generated 6.2 m/hr of on bottom ROP, 24% higher than the initial target. After drilling, the bit was pulled out of hole with considerably good condition despite heavy drilling in the 880 m of basement.