Unconventional Polycrystalline-Diamond-Bit Design Allows Significant Performance Gains

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 156961, ’Departure From the Norm in Polycrystalline-Diamond-Bit Design Allows Significant Performance Gains in Highly Erosive and Abrasive Formations,’ by Aron Deen, Jason Maw, Craig Knull, and John Clegg, SPE, Ulterra Drilling Technologies, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 8-10 October. The paper has not been peer reviewed. This paper discusses the design, manufacture, and application of a contiguous blade of polycrystalline diamond (PCD). This bit has full-length, contiguous PCD coverage, without gaps between cutters on each blade, as in a conventional PCD-compact (PDC) bit design. The implementation of this bit represents a step change in bit design. Drill-Bit Damage The traditional view has been that PCD cutters are the most important part of the PDC bit because they are required to engage the formation and maintain their geometrical properties long enough for shear drilling to be economical. The bit body, then, is necessary only in order to hold the cutters in their arranged layout, and to convey mechanical and hydraulic power to remove the rock in their path. PCD cutters are very hard and resistant to abrasion, but are susceptible to damage from their relative lack of toughness. Bit-body materials, whether made of steel or tungsten carbide composite hard facing or infiltrated matrix, are more prone to failure in the drilling environment. One drill-bit design limitation is a requirement for sufficient body material to exist to create a pocket to which the PCD cutter will be brazed. In the case of the webbing be-tween the cutters, depicted in Fig. 1, this puts the body material in a position where it is likely to engage either the formation or high-velocity fluid flow. Invention and Hypothesis The premise that there has to be enough body material to constitute a braze pocket leads to the conclusion that there has to be a webbing between the cutters. The resulting engineering problem was to optimize this webbing, on the basis of the need for cutter exposure and material strength of the body material. But does a braze pocket need to be constructed entirely from body material? Erosion and Abrasion. The first obvious benefit of this new design philosophy should be that, given that there is no weak point of webbing subjected to abrasive or erosive wear, intercutter or cross-blade abrasion and erosion will be mitigated. The webbing between the cutters is the area most susceptible to this mechanism along the fluid path. Abrasion occurs because of the same design weakness—the webbing—but be-cause of a different mechanism. As illustrated in Fig. 2, abrasion is more uniform because of the rotation of the bit. It occurs when the bit body contacts the formation and is worn away. The result, though visibly different from erosion, shares a similarity in terms of location.