Status of Polycrystalline-Diamond-Compact Bits: Part I Development(includes associated papers 19032 and 19900 )
Authors: Feenstra R. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Polycrystalline-diamond-compact (PDC)bits have become anestablished new generation of oilfield bits particularly for drillingsoft-to-medium-hard, nonabrasive formations. The development of these bits isstill at an exciting stage. The state of the art in PDC cutters and bits isdiscussed and illustrated by a rich source of fairly recent literature. A lookinto the future is attempted in light of the bits'potential.

Introduction

In the past, roller-cone bits dominated rotary drilling and diamond bitscoring and turbodrilling. The manufacturers of bits made either roller bits ordiamond bits. During the 1980's, this picture changed markedly because of thesuccess of the PDC bit and more recently the collapse of oil prices. PDC bitshave taken an important share of the drilling. coring and motor-andturbodrilling, market, and are now produced by both diamond-bit androller-cone-bit manufacturers as a new generation of bits. The trend among themanufacturers is clearly toward fewer, more integrated companies that sell allthree types of bits in addition to other tools for downhole use.

This paper focuses on PDC bit development through the 1980's and highlightsthe state of the art in the PDC cutters and bits.

PDC Cutters

Most characteristics of the PDC bit stem from those of the PDC cutter, whichhas been in use in the oil field for roughly 10 years. The development of PDCcutters lies mainly in the hands of cutter manufacturers. The originator ofthese cutters has maintained a leading, role but competing companies haveplayed a stimulating role in bringing, about a steady improvement in thequality and variety of the cutters. Cutters and bits currently are manufacturedby separate companies but a trend is apparent that bit manufacturers areexploring the idea of integration. The strength of the bit manufacturers isthat they can get firsthand, rapid feedback on cutter performance in the field, whereas cutter manufacturers must rely on their clients' responses-i.e., thebit manufacturers. In addition. for cutter manufacturers the oilfield is onlyone of many application of diamond technology, albeit an interesting one.

This section provides some understanding, of the status of today's cutter, as developed for oilfield use.

Nomenclature.

A selective number of terms are used to identify PDC bits."PDC" refers to the cutter (Fig. 1a) that is mounted in PDC bits and isdiscussed here. Such generic terms as "shear bits" and "polycrystallinediamond" (PCD) are not limited to the specific diamond compact with atungsten-carbide substrate identified in Fig. 1a. The term "Stratapax bit"isdying out; it referred to the trade name used by the originator of the diamondcompact.

A new product, the thermally stable polycrystalline (TSP) diamond, is asynthetic diamond, not a diamond compact, and therefore more related to diamondbits than to PDC bits. A TSP diamond (Fig. 2) is used to replace crystallinenatural diamonds in diamond bits, TSP diamonds will be discussed briefly forthe sake of completeness.

Characteristics. A characteristic of the PDC cutter is the thin(=0.5-mm[=-0.02-in.]) PCD layer on top of a thicker (=-3-mm [-=0.12-in.])tungsten-carbide substrate (see Fig. 1a). This characteristic is of paramountimportance in the PDC bit: the cutter is self-sharpening because the tungstencarbide wears at least an order of magnitude faster than diamond and hence theweight on bit (WOB) is carried by the diamond layers. This explains why PDCbits can drill so rapidly at light bit loads, a feature exploited by many ofthe applications to be highlighted in "Status ofPolycrystalline-Diamond-Compact Bits: Part 2-Application" (scheduled for theJuly 1988 Journal of Petroleum Technology).

The cutter-set at a negative rake of 0.26 rad [15 degrees] (Figs. 1b and1d)-acts initially as an extremely sharp cutter. The cutter wears with time anda thin flat develops on the diamond, which is why the bit load needs to begradually increased to maintain penetration. This is sometimes overlooked whenPDC bits are being tested in the laboratory: test results for a brand-new bitmay not be representative of an average bit run under conditions prevailingdownhole.

The advent of the PDC cutter has in fact given rise to what is probably thefirst self-sharpening drag bit. Such a bit suffers from less chip holddown thandoes a roller-cone bit and much less than a diamond bit. This feature isanother reason for its rapid drilling characteristic downhole in the absence ofballing phenomena.