PDC Drill Bit Design and Field Application Evolution
Authors: Kerr Callin Joe 
Published: December 2007

This paper traces the development of polycrystalline diamond compact (PDC) bits from their introduction in 1973. Such design features as body materials, crown profiles, cutter density, and cutter exposure and their effect on bit performance are discussed. In addition, the paper reviews various aspects of bit applications engineering, including bit hydraulics, drilling fluids, directional behavior, and formation types.

Introduction

The introduction of PDC cutters in 1973 facilitated the development of the first drill bit that used synthetic diamonds as cutting elements. The development process has progressed so that today a large amount of footage is drilled with PDC bits. Through this process, several design features that affect bit performance have become clear. Specifically, the effects of bit body material, cutter placement and density, bit profile, back rake, side rake, and hydraulic horsepower on application and performance have been delineated. The industry has seen the introduction of fine steel-body, then tungsten-carbide-matrix-body drill bits. Cutter placement and density were initially determined by way of an equal-volume-percutter calculation. The state-of-the-art PDC drill bits feature com-puterized cutter placement and density based on an equal work rate per cutter. Also incorporated is a torque-balancing function and a vector resolution that considers back rake, side rake, and bit profile. Hydraulic layouts are now optimized with the aid of a flow-visualition chamber.

Extensive field testing, in conjunction with engineering design evaluation, has defined the current application zones and limits of PDC bits. The availability of new cutter shapes and sizes, as well as improved thermal stability and mechanical toughness of the PDC cutter, will enable further evolution of PDC bit designs. Currently, large-diameter PDC cutters providing increased exposure and shaped cutters featuring a higher point loading per cutter are some of the technological advancements being tested. This influx of improved superhard materials, coupled with hydraulically and mechanically more efficient bit designs, will expand the PDC application zone of the future.

Initial Designs

Initial PDC bits differed quite drastically from the present catalog models of manufacturers. Ear]Y Prototypes used concepts very similar to those of natural diamond bits (Fig. 1).

The profiles used were basically the same as those of conventional natural diamond bits, simply replacing the natural diamonds with the PDC cutting element. Various flow arrangements were investigated, including the conventional crowsfoot, waterways, and fixed nozzles.

Additionally, there were changes in the PDC cutting element. Initial cutters were 0.323 in. [8.2 mm] in diameter vs. the standard 0. 524-in. [ 13.3-mm] diameter cutter, which is most commonly used today. Early PDC cutters were available in both a 180 and a 360 deg. [3.14- and 6.28-rad) blank affixed to a stud. Early bits used steel, carbide, and hardfaced steel studs, and combinations of both the 180 and 360deg. [3.14- and 6.28-rad] blank.

Field Application

An early field test of a petroleum drill bit that used PDC cutters took place in Nov. 1973 in Colorado. From these tests, numerous inherent design weaknesses were apparent. The problems are thoroughly documented by Walker et al. Basically, the problems were that the small-diameter (0.323-in. [8.2-mm) blanks did not provide sufficient area to allow a competent braze to the stud. Furthermore, the small blank did not provide sufficient exposure necessary for high rates of penetration (ROP's). Problems were also encountered with impact breakage of the studs, cutter loss caused by braze failures, and poor hydraulic configuration to provide adequate cleaning.

Improved Designs

After discouraging early field results, bit designers realized that to make use of this new cutting element, the trend of g would have to deviate completely from conventional natural-diamond-bit designs. The contrast between the grinding and plowing cutting mechanism of a natural diamond and the shearing action of rock by PDC cutters necessitated that a workable bit have a radically different design for efficient drilling. This realization led to the first PDC bit style, which resembles what is commonly found today in manufacturer catalogs (Fig. 2). This bit features a bladed-type ar-rangement, tungsten-carbide-matrix body, waterways, and several nozzles (actual number depending on the particular design). Performance was marginal but encouraging, with the typical problems being plugged nozzles and cutter loss.

In 1978, the PDC cutter became available with an additional piece of tungsten carbide attached. This provided a larger area to braze to the bit body, and thereby a stronger bond (Fig. 3).

Field testing proceeded, and the cutter-loss problem was not completely solved until a new brazing process was developed in 1979. The success of field tests during this time was sporadic, because the Teaming curve of application and operating parameters of these bits was in the very early stages. However, some runs showed sensational performance and savings.

By this time, operators and manufacturers had begun to realize, the potential of PDC bits, and development work received new attention in the following time period. Advancement in design, operating parameters, and formation application proceeded at a rapid pace.

Note that the bulk of the early bits mentioned here are the tungsten-carbide-matrix body bits with brazed cutters. Stud cutter used in steel-body bits, however, were also being investigated in parallel. By now the basics of PDC bit designs were apparent, and what remained was the determination of the specifics and the effects on bit performance.

Design Features Affecting Bit Performance

Bit-Body Material. Currently, the two aforementioned materials, tungsten-carbide matrix and steel, are being used in PDC drill bit (Figs. 4 and 5). No conclusions are drawn as to which design is more efficient, but some characteristics of each have become apparent.

Steel-body bits use a stud cutter (Fig. 6) that is interference-fitted into a receptacle on the bit body.

JPT

P. 327^