Hybrid Solution Improves ROP and Borehole Quality in Challenging Large Diameter Holes

Abstract

In the prolific Burgan field, South East Kuwait, new well profiles are being employed which require the 16 inch hole section to be drilled deeper. This new well profile requires drilling through interbedded abrasive sandstones and sticky shale which were previously drilled in 12 ¼ inch hole size. Drilling becomes increasingly challenging using larger diameter bits where torque fluctuations, down-hole conditions and durability limit performance. As the casing profiles changed, each section's drilling solution was revisited as drilling dynamics and performance diminished drastically when drilled in the larger hole size. What was once a viable solution to drill the section in 12 ¼ inch becomes no longer suitable when drilled in 16 inch. The purpose of this paper is to demonstrate the engineering analysis for large diameter hole application and the selection of an optimized hybrid drill bit design, roller cone combined with Polycrystalline Diamond Compact (PDC), to maximize drilling performance where standard PDC or roller cone bits alone fail to provide an optimal solution. The 12 ¼ inch section was typically drilled using a rotary bottom hole assembly with a roller cone bit or using a performance mud motor with a PDC bit. Drilling parameters were reduced in the upper layers especially with PDC bits as they involve sticky shale which can induce bit balling. While in the lower layers the abrasive formations tend to wear down roller cone bits. Combining the benefits of both bit types, a 16 inch hybrid bit was proposed to answer the challenge of drilling the section as fast as a PDC bit while providing the stability, torque control and smooth hole condition generated by a roller cone. Selecting an appropriate hybrid design to match the application resulted in an initial trial of a durable hybrid bit that finished the interval with virtually no wear on the bit and significantly improved borehole quality. The second trial of the hybrid bit with a modified, more aggressive design approximately doubled the ROP of a PDC bit while maintaining a smooth hole profile and minimizing torque. PDC bits generally provide faster rate of penetration than roller cone bits while generating higher torque fluctuations as they drill by shearing bottom hole formation. Due to their high torque demands, PDC bits require an additional mud motor to drive them efficiently in larger hole sizes. Roller cones crush and gouge the bottom hole formation and due to their mechanical rollers result in reduced reactive torque and a smoother hole profile. Hybrid drill bit technology proven the capability of achieving the benefits of both designs in specific selected applications.