Evolution of Polycrystalline Diamond Compact Bit Designs for Rocky Mountain Drilling
Authors: Pain D.D. , Schieck B.E. 
Published: December 2007

Summary
The Rocky Mountain region of the U.S. has proved to be a good area for polycrystalline diamond compact (PDC) bits in selected formations. Lower costs per foot as a result of higher penetration rates and longer bit lives have been realized in many applications. PDC bits are used routinely in Wyoming in the Green River and Powder River basins.
Simply using a PDC bit in these areas does not necessarily ensure an economical run. Care must be taken in choosing the correct bit design for each application to obtain the lowest cost per foot. Since the first PDC bit run, there has been an evolution of designs to enhance penetration rates, and thus to lower drilling cost per foot. This evolution has included changes in bit profile, cutter density, cutter exposure, cutter side rake, and cutter shape. When optimally combined, these features have increased penetration rates by well over 100% in many formations.

Introduction
PDC drill bits have proved to be economical tools in certain drilling applications in the Rocky Mountain region. Two areas in the Rockies found to be particularly suited to the application of PDC bits are the Green River and the Powder River basins.
To compete in these areas, a PDC bit must be capable of high penetration rates as well as longer bit life to justify the added cost of the PDC bit compared to conventional roller-cone bits. These are the two bit performance factors that determine drilling cost per foot - the most accurate measurement of drilling efficiency.1 Given an adequate bit life, any parameter that can be altered to improve the penetration rate of a drill bit will further improve the drilling efficiency and result in lower drilling costs.
This paper discusses design features that can be incorporated into PDC bits to improve penetration rates. Bits with these design features were tested in the Powder River and Green River basins, and field results are reported in this paper.

Bit Design Features
The design features incorporated into the PDC bits that were tested in the Rocky Mountains include variations in bit profile, cutter density, cutter exposure, cutter side rake, and cutter shape. Although these are not all the possible design criteria for PDC bits, they are among the most important. All bits used in this test were steel-bodied, stud-cutter bits, with 20° back rake on the cutters - with back rake defined as the angle of the cutter in relation to the axis of the stud to which the cutter is attached.

Bit Profiles
The first bit profile evaluated is shown in Fig. 1. This is a short-tapered profile with a concave, shallow-cone center, referred to as Style A. This profile was originally chosen to attempt PDC drilling in the area because of its durability. Among the earliest PDC bit profiles, it has stood the test of time in attempting new PDC bit applications. With this profile, the cutting action of the bit relieves the confining formation pressures over a broad area, beginning near the bit's outside diameter. Also, the short-tapered profile allows for variations in cutter density, cutter profile, and cutter orientation without losing bottomhole coverage.
Fig. 2 (Style B) shows a more aggressive bit profile because of its longer taper. The concave, shallow-cone center is still present on this style, but the cutter placement and cutting mechanism are different from Style A. The longer taper on this bit provides fewer cutters on the leading edge. Thus, the unit load per cutter is higher and there are fewer cutters acting to relieve the confining formation pressures. These factors lead to a bit that will drill faster than Style A; however, more care should be taken in application selection because higher unit cutter loads can cause rapid failures in hard or abrasive formations.
Fig. 3 (Style C) shows an even more aggressive profile because of the absence of the center cone. This bit has a convex center with the same taper as Style B. There are even fewer cutters on the leading edge of this profile; thus, it is the most aggressive of three profiles discussed. For this reason, care should be taken in selecting a suitable application for this profile because the bit dulls quickly when a hard formation is encountered.

Bit Profiles
The first bit profile evaluated is shown in Fig. 1. This is a short-tapered profile with a concave, shallow-cone center, referred to as Style A. This profile was originally chosen to attempt PDC drilling in the area because of its durability. Among the earliest PDC bit profiles, it has stood the test of time in attempting new PDC bit applications. With this profile, the cutting action of the bit relieves the confining formation pressures over a broad area, beginning near the bit's outside diameter. Also, the short-tapered profile allows for variations in cutter density, cutter profile, and cutter orientation without losing bottomhole coverage.
Fig. 2 (Style B) shows a more aggressive bit profile because of its longer taper. The concave, shallow-cone center is still present on this style, but the cutter placement and cutting mechanism are different from Style A. The longer taper on this bit provides fewer cutters on the leading edge. Thus, the unit load per cutter is higher and there are fewer cutters acting to relieve the confining formation pressures. These factors lead to a bit that will drill faster than Style A; however, more care should be taken in application selection because higher unit cutter loads can cause rapid failures in hard or abrasive formations.
Fig. 3 (Style C) shows an even more aggressive profile because of the absence of the center cone. This bit has a convex center with the same taper as Style B. There are even fewer cutters on the leading edge of this profile; thus, it is the most aggressive of three profiles discussed. For this reason, care should be taken in selecting a suitable application for this profile because the bit dulls quickly when a hard formation is encountered.