Laboratory Evaluation of PDC Drill Bits Under High-Speed and High-Wear Conditions
Authors: Hoover Eddie R. , Middleton John N. 
Published: December 2007

Summary
Recently, five experimental polycrystalline diamond compact (PDC) bit designs were tested in the laboratory, at 100 and 500 rpm in three different types of rock: Nugget sandstone. Crab Orchard sandstone, and Sierra White granite. This paper describes the testing procedures, summarizes bit performance and wear procedures, summarizes bit performance and wear characteristics, and correlates these experimental results with specific design options such as rake angle, bit profile, and material selection. profile, and material selection.

Introduction
The significant improvements in turbodrill technology over the past few years have generated a great deal of interest in the development of drill bits capable of operating at the high speeds typically associated with these downhole motors. Because of severe wear of moving parts such as bearings and seals, tricone rock bits generally have performed poorly on high-speed turbines. PDC drag bits, on the other hand, have no moving parts and have been used successfully on turbines in the North Sea and elsewhere overseas. In 1979, Sandia Natl. Laboratories initiated a program to investigate the limitations of PDC drill bits as applied to high-speed, high-temperature geothermal drilling.Recently, five different experimental PDC bit designs were tested in the laboratory under atmospheric conditions at both 100 and 500 rpm. Four of the designs were developed by commercial bit manufacturers, while the fifth was developed at Sandia. In general, the designs were very diverse and used different bit profiles, cutter rake angles, cutter positioning strategies, material selection rationales, etc. This diversity allowed the comparison of different design concepts.

General Description
Two 6.75-in.-diameter PDC drill bits were procured from each of the following commercial bit manufacturers: American Coldset Corp., NL Hycalog, Smith Intl. Inc., and Dresser Industries Inc./Security Div. In a parallel effort, two 6.5-in.-diameter PDC bits were parallel effort, two 6.5-in.-diameter PDC bits were designed and built at Sandia. All of these experimental drill bits were developed specifically for high-speed operation (greater than 300 rpm). The five different designs are pictured in Figs. 1 and 2.A typical PDC cutter assembly (Fig. 3) consists of a PDC drill blank bonded to a support post or stud using PDC drill blank bonded to a support post or stud using either General Electric Co.'s high-temperature LS-brazing process or a diffusion-bonding technique developed at Sandia. For purposes of comparison, the 10 drill bits fabricated for this experiment used both bonding techniques. In addition, both steel and tungsten carbide support posts or studs were used. Six of the bits had all LS-brazed carbide cutters (a PDC drill blank attached to a tungsten carbide stud using GE's LS-brazing process), one had all diffusion-bonded steel cutters, and the process), one had all diffusion-bonded steel cutters, and the other three bits had some combination of LS-brazed carbide cutters, diffusion-bonded steel cutters, and diffusion-bonded carbide cutters. All 10 bits are described in detail in Table 1.
The full-scale drilling tests were conducted at the Drilling Research Laboratory in Salt Lake City under atmospheric conditions and using water as the drilling fluid. The flow rate was held constant at 200 gal/min for all tests. For these PDC bits, the hydraulic pumpoff effect was found to be insignificant with respect to the bit weights used. The hits were tested in Nugget sandstone, Crab Orchard sandstone, and Sierra White granite at two different rotary speeds: 100 and 500 rpm. The unconfined compressive strength of these three types of rock was found to be 18,000, 20,800, and 24,000 psi, respectively. Depending on the type of rock, rotary speed, and bit design used, the weight or vertical force on each cutter varied from 30 to 1,500 lbf.
JPT
P. 2316