The Design And Use Of Polycrystalline Diamond Compact Drag Bits In The Geothermal Environment
Authors: Varnado S.G. , Huff C.F. , Yarrington P. 
Published: December 2007

This work was supported by the U.S. Department of Energy.

Abstract
The potential for utilizing polycrystalline diamond compact (PDC) cutters to provide high performance bits has been recognized by the drilling industry. New bit designs suitable for geothermal drilling are being developed based on the results of single cutter laboratory tests and analytical analyses. A new bonding technique for attaching the cutters to the bit body has been developed. Bits using this new technique have been built and tested with promising results.

Introduction
Drilling for geothermal resources is an extremely expensive procedure, due in part to the low penetration rates obtainable with conventional drilling bits in these hard formations. Bits utilizing PDC cutters have the potential for increasing penetration rate. potential for increasing penetration rate. The Department of Energy, through Sandia Laboratories, is performing research and development directed toward reducing the cost of drilling and completing geothermal wells. Under this program, Sandia Laboratories conducts both in-house research and external research at a number of industrial firms. As part of this program, the design of bits part of this program, the design of bits utilizing PDC cutters has received attention, both at Sandia and at the General Electric Corporate Research and Development Center. These activities have proceeded from single cutter tests to the design and testing of full-scale PDC bits.
Recently, bit designs using a full face configuration of PDC cutters have been tested in the laboratory and under field conditions in a geothermal well. In addition, a hybrid bit design utilizing PDC cutters with a conventional roller cone has also been tested with encouraging results in the Panhandle Lime formations in Texas.
This paper describes the results of single cutter tests, the bit design philosophy, the bonding process for attaching the cutters to the bit body, and the results of laboratory and field tests of these new bit designs.

SINGLE-CUTTER TESTS
A comprehensive series of cutting experiments utilizing single PDC cutters has been performed by Hibbs, et al. The PDC cutters were bonded to tungsten carbide tool posts for the tests. Resultant forces on the cutter were measured for a range of rake angles from -5 degrees to -30 degrees. Marble, granite, and sandstone rocks were used. The compressive strengths of these rocks ranged from 10,000 to 50,000 psi. The results of these experiments, performed under atmospheric conditions, were used to design full-scale bits and to predict the horsepower requirements and penetration rates for the full-scale bits, The bits designed by GE were 8-3/4" in diameter and utilized a rake angle of 015 degrees. The bits are designed such that each cutter experienced equal horsepower during rotation of the bit. These bits were fabricated and tested in the drilling laboratory at Tulsa University. The results indicated that the bit performance at atmospheric pressure could be predicted with a reasonable degree of accuracy using the results of the single cutter experiments. During the testing, however, some cutters were lost because of a failure in the bond between the cutters and the tungsten carbide tool posts. These tests indicated the need to develop a reliable attachment technique in order to properly utilize the PDC cutters in high performance bit design.
The cutters on these bits were attached using a conventional low-temperature brazing technique.