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

Abstract

Abstract An advanced series of PDC drill bits incorporating a new highly abrasion-resistant PDC cutter has extended effective PDC bit application to hard rock drilling. In direct offset comparisons, the advanced series of PDC bits fitted with the new cutters delivered significant increases in footage drilled and rate of penetration. To achieve an optimum match in drilling efficiency and bit life to lower costs and mitigate risk in hard rock environments, the series is designed using a combination of advanced modeling capabilities and sophisticated analytical tools. These tools allow the designs to be "customized" for specific applications, optimizing cutting efficiency and durability according to specific rock properties and drilling parameters. A transitional drilling model simulation allows evaluation of how cutting forces are affected during transitional drilling, common in "hard rock" environments. 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. In addition, recognition of a third dimension of PDC performance - Thermal Mechanical Integrity (TMI) — has lead to development of a new PDC cutter that provides 13.5 times the abrasion resistance of the industry standard, without sacrificing impact resistance. This improved understanding of PDC cutter failure provides a different way of looking at the traditional characteristics of abrasion and impact, enabling cutter durability to be optimized in both abrasive and hard, inter-bedded formations. The paper discusses the science behind the advanced series of bits, including the impact of TMI on cutter performance. New laboratory capabilities and testing results are described, and actual field case histories presented to demonstrate performance improvements of these PDC bits in hard rock applications. 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 damage, 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 BHA dynamics. Since the first modeling studies conducted by Sandia Laboratories in the late 1980s, analysis of the interaction between the cutting elements of a PDC drill bit and the formation it is drilling has been widely investigated. One of the predominant developments from these early investigations was the first reliable kinematics cutter force and wear prediction model. These models were helpful for bit manufacturers to better understand the mechanism of cutter/formation interaction and to design the cutter layout of a PDC bit so load and wear of cutters over the bit face can be balanced. Perhaps the most significant advancements in understanding how this interaction affects bit performance were the result of research conducted by the Amoco Research Center during the late 80's.1 Laboratory tests demonstrated that conventional PDC bits whirl backwards during drilling, and backward whirl was a primary cause of PDC cutter damage. This important finding led to extensive studies in bit dynamics and drillstring dynamics. Bit dynamics models, including BHA dynamics models were developed and were able to repeat the backward whirl phenomenon under specific conditions. However these dynamics models were rarely used by bit manufacturers in the bit design process due to their complexity and limited ability to consider the effects of cutter layout on bit dynamics. There were two design principles identified; namely, antiwhirl PDC bit design and force balanced PDC bit design. The anti-whirl PDC bit design principle1,2 incorporated a specific design concept, namely low friction gauge, to minimize the effect of bit backward whirl. According to this design principle, cutters were so arranged over the bit face so that a net resultant radial force (around 12% of weight on bit) was directed toward a specified portion of the bit with less friction.