A New Method of Producing Laterally Stable PDC Drill Bits

Abstract

Summary Bit whirl is well documented as a major cause of damage to polycrystalline-diamond-compact (PDC) drill bits and results in short runs, low rate of penetration (ROP), high cost per foot, poor hole quality, and downhole-tool damage. Hence, consistent lateral stability is highly desirable in PDC bits. This paper presents a new method of producing PDC drill bits that reduce or eliminate bit whirl. Traditionally, attempts to design laterally stable PDC bits have assumed that the forces generated by the bit during stable drilling cause it to begin whirling. The new approach assumes that it is the response of the bit to forced motion off its center that causes whirl. The new approach has been validated theoretically, in the laboratory, and in the field. Test results from a full-scale drilling rig indicate that this method is superior to existing methods at minimizing bit whirl. This new method was validated successfully through an extensive field-test program undertaken in the northwest USA, involving high-speed downhole measurements from both conventional 7⅞-in.-diameter PDC drill bits and bits designed using this new method. The results demonstrated that the conventional PDC drill bits exhibited bit whirl throughout the run. The bits designed using the new method eliminated bit whirl completely. The performance benefits of this new method were demonstrated in a 16-in.-bit section in a Middle Eastern field, where lateral vibrations are a significant problem. Before testing the new-method PDC bit, bit optimization led to the use of high imbalance force designs to minimize lateral vibration. The first test of the new method laterally stable PDC bit set field records for ROP, footage, and cost per foot compared with more than 50 runs with "antiwhirl" (Warren et al. 1990) designs, resulting in savings of more than USD 50,000 per run. This paper demonstrates how use of the new method can eliminate whirl in both laboratory and field environments and deliver significant performance improvements over existing stability techniques. This new method represents a significant step forward in the design of PDC bits to mitigate bit whirl, and it has been proved to reduce bit damage, increase run lengths, increase ROP, and deliver sizeable savings for operators.