Faster and Longer Bit Runs With New-Generation PDC Cutter

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 102067, "Faster, Longer, and More-Reliable Bit Runs With New-Generation PDC Cutter," by J. Clegg, SPE, ReedHycalog, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. A new process for manufacturing polycrystalline-diamond compact (PDC) cutters creates a wrapped thermostable region across the face and around the periphery of the PDC cutter. This differs significantly from existing first-generation thermostable product because the treated region has complex geometry in three dimensions. The full-length paper describes the new cutter geometry and shows how the thermostable region supports the cutter as it slowly wears. Introduction Historically, there has been a trade-off between abrasion and impact resistance of PDC cutters. Typically, impact resistance was achievable by use of a larger diamond grain size but at the expense of abrasion resistance. The first attempts to extend the envelope of PDC-bit and -cutter performance occurred in the mid-1990s when multimodal polycrystalline diamond was introduced. This used a mixture of grit sizes from 50 to 2 µm in diameter, allowing the smaller grits to fill the voids left between the larger particles, resulting in a much denser polycrystalline-diamond layer that improved abrasion resistance. Additional improvement was achieved by use of nonplanar interfaces. The move from planar to nonplanar interfaces was driven by a desire to reduce stresses at the interface between the polycrystalline diamond and its tungsten carbide substrate. The introduction of the nonplanar interface combined with the multimodal polycrystalline-diamond mix provided significant improvement in both relative toughness and relative wear life over planar multimodal materials. The thicker diamond edge that resulted improved relative abrasion life. Thermostable PDC Cutters Cobalt normally is present in the creation of polycrystalline diamond in the PDC press. It is introduced in the tungsten carbide support, where it acts as a cement. At the elevated temperature and pressure where diamond-to-diamond bonding occurs, the cobalt migrates into the diamond grit and helps to catalyze the bonding process. It also forms a bond with the tungsten carbide substrate and ensures that the PDC is one integral component. However, cobalt significantly reduces the thermal stability of the PDC because it has a greater coefficient of thermal expansion than the surrounding diamond particles. Between 700 and 760°C, the cobalt expands and forces the diamond-to-diamond bonds apart. This results in the rapid breakdown of the compact. What appears to be abrasive wear when a dull PDC bit is analyzed often can be the result of the breakdown of the diamond-to-diamond bonds because of overheating. Therefore, it is important that the temperature of the PDC cutter is maintained below 700°C to avoid thermal breakdown. Unfortunately, the cutting-tip temperature often exceeds this critical limit.