Innovative Dual-Diameter PDC Bit Design Provides Unique Solution to Successful Drilling of Challenging Volcanic Interval

摘要

Drilling hard, abrasive volcanic rock has presented significant technical challenges to bit manufacturers over the years, creating a dilemma for operators designing a strategy to efficiently drill difficult volcanic intervals. This paper will focus on the development of a unique concentric two-stage, dual-diameter PDC bit coupled with a low-speed, high-torque motor, providing an alternative solution for drilling a challenging section featuring a hard volcanic layer for a major international operator in a remote location West of Shetland. The paper reviews the design concepts of the bit that allows an initial smaller pilot hole to be drilled, relieving the stress of the surrounding rock. The larger reamer section subsequently enlarges the hole to the desired diameter through stress-relieved rock, thus requiring less energy to remove a comparative volume of rock. The unique design concept was tested in an in-house pressurized drilling laboratory with basalt samples of varying compressive strengths to evaluate its performance in these challenging rock types. Alternate bit designs with a variety of cutter geometries and cutter layouts were also evaluated and an optimum design was selected for the field test to deliver lower vibrations and increased ROP. A strategy was formulated to drill the interval in a single run with the optimal dual diameter PDC bit and a low-speed, high- torque mud motor. Downhole dynamics data recorders were placed in the BHA to capture downhole dynamic activity to evaluate performance and provide information to help further optimize performance through the interval. On the subject well, the new dual-diameter PDC design successfully drilled the entire 12 1/4-in hole section in a single run, including 67 m of volcanics. The design achieved a record ROP for the section, saving five days of drilling time and resulting in significant cost savings for the operator. In addition to providing measurable stability and drilling efficiency benefits in conventional applications, the unique concentric dual-diameter PDC bit also provides a technical solution to operators looking at more cost-effective means of drilling these challenging formations in a high-cost drilling environment.