New Bit Design and Cutter Technology Extend PDC Applications to Hard-Rock Drilling

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 91840, “New Bit Design, Cutter Technology Extend PDC Applications to Hard-Rock Drilling,” by Robert Clayton, SPE, Shilin Chen, SPE, and Guy Lefort, HES-Security DBS, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23–25 February. An advanced series of polycrystalline-diamond compact (PDC) drill bits incorporating a new highly abrasion-resistant PDC cutter has extended effective PDC-bit application to hard-rock drilling. In offset comparisons, the advanced series of PDC bits fitted with the new cutters delivered significant increases in footage drilled and rate of penetration (ROP). The bit design is globally balanced to optimize axial, lateral, and torsional forces and can be modified by adjusting features such as profile shape, cutter rake angles, impact arrestors, and cutter type to optimize bit performance when drilling in hard and transitional environments. Introduction One of the greatest challenges that any PDC bit manufacturer faces today is the extension of PDC-bit application into hard-rock drilling, where impact, heat damage, and abrasive wear of PDC cutters limits performance. Research and development have been focused on better understanding of cutter/formation interaction, cutter performance, bit dynamics, and bottomhole-assembly (BHA) dynamics. Since the first modeling studies in the late 1980s, there have been many analyses of the interaction between the cutting elements of a PDC drill bit and the formation. One of the predominant developments from these early investigations was the first reliable kinematics cutter-force and -wear prediction model. These models helped bit manufacturers to understand the mechanism of cutter/formation interaction better and to design the cutter layout to balance load and cutter wear over the bit face. Laboratory tests demonstrated that conventional PDC bits whirl backward during drilling, and backward whirl was a primary cause of PDC-cutter damage. Bit-dynamics models, including BHA-dynamics models, were developed and were able to repeat the backward-whirl phenomenon. However these dynamics models rarely were used by bit manufacturers in the bit-design process because of their complexity and limited ability to consider the effects of cutter layout on bit dynamics. Since introduction of antiwhirl technologies in the late 1980s, PDC bits have made significant inroads into roller-cone markets, but have faltered when drilling hard rock. In these applications, where roller-cone bits suffer short bit life and slow ROP and risk loss of cones, PDC bits typically suffer short life as a result of high-impact damage, large vibrations, and abrasion. The force-balanced PDC-bit design principle was developed on the basis of an understanding of PDC-bit dynamics and cutter/formation interaction. The cutters on a force-balanced PDC bit are arranged so a net resultant radial force is minimized or balanced. Use of tracking cutters provides a restoring force to keep the bit rotating around hole center. The design of a force-balanced PDC bit allows a higher density of cutters on the gauge, which is usually required in hard-formation drilling. The use of force balancing, tracking cutters, and asymmetrical spiraled blades improved bit performance significantly and expanded the range of applications for PDC bits.