Investigation of Percussion Drills for Geothermal Applications
Authors: Finger John T. 
Published: December 2007

Summary
A series of tests was conducted to provide data for an economic evaluation of percussion drilling in geothermal reservoirs. Penetration rate (ROP), operation on aqueous foam, and high-temperature vulnerabilities of downhole percussion tools are described. Percussion drilling with solid-head bits can offer significant performance increases and economic benefits compared with conventional rotary drilling, but there are technical obstacles to widespread percussion use.

Introduction
Part of our program management for the U.S. DOE in geothermal drilling and completions is an attempt to identify advanced drilling systems that could drastically reduce well costs. One of the candidate systems to be evaluated is percussion drilling, which has the advantages of high ROP in brittle rock, light weight on bit (WOB) for straighter holes, and ability to use low-density fluids. The general method of evaluating a system is to quantify its changes in performance and equipment requirements compared with a standard model and then to examine the effect of those changes on well cost.
The term "percussion drilling," especially in the mining industry, sometimes means a technique that uses a stationary pneumatic or hydraulic drive unit to transmit rotation, static thrust, and cyclic impact to the bit through a long shaft called the "drill steel." The work described in this paper concerns only downhole motors that use the drilling fluid to drive a reciprocating piston against a bit or bit sub.
The results of this investigation are in five parts:comparison of ROP's of different tools in uniform rock samples,demonstration that hammers can operate at high temperature,demonstration that hammers can operate with stable aqueous foam as a drilling fluid,identification of other problems that constrain percussion's use in geothermal drilling, anda brief analysis of percussion drilling economics in generic geothermal wells.
Each of these parts is described in detail.

ROP Comparison
All ROP tests were done in Sierra White granite, a rock representative of the Roosevelt Hot Springs, UT, geothermal area. This is a fairly strong granodiorite that has an unconfined compressive strength of 28,200 psi [194 MPa], porosity of 1 %, and quartz content of about 25 %. The tests were done at Drilling Research Laboratory in Salt Lake City with a drill rig instrumented to measure WOB, torque. rotary speed, ROP, drilling fluid flow rate and pressure, and temperature at several points in the test item. All holes drilled were approximately 8 in. [203 mm] in diameter. The baseline for ROP comparisons was a set of data for a 7 7/8-in. [200-mm], tungsten-carbide-insert, roller-cone mining bit operated over a range of WOB's from 10,00 to 40,000 lbf [44.5 to 178 kN] and a range of rotary speeds from 40 to 100 rpm (Fig. 1). Although this unsealed roller bearing bit could be run at higher WOB's and rotary speeds, the values chosen realistically represent field practice.
ROKs were measured for two basic types of percussion tools-those with roller bits and those with solid-head bits. The hammers that used the roller bit from the baseline tests are commonly called "oilfield" hammers, and are rented, not bought, from service companies. The hammers that use solid bits with hemispherical tungsten carbide inserts are called "mining" or "industrial" hammers and usually are bought from one of the several manufacturers. Fig. 2 gives a comparison of the two bit types.
Since the bits imposed different constraints on the tools, the two types were tested with different variables. The oilfield tools were operated at a constant rotary speed with varying WOB and fluid supply pressure. These results (Fig. 3) demonstrate that the ROP is relatively insensitive to WOB and strongly influenced by supply pressure. This is reasonable, since (1) the changes in WOB are small compared to the force of the hammer blows, and (2) it has been shown theoretically that power input to the rock should increase as the 3/2 power of the fluid supply pressure. Three tools were tested with the roller-cone bit-two rental air-powered hammers and a prototype liquid-powered hammer. The latter tool was designed, built, and field tested by Pan American Petroleum Corp. (now Amoco Production Research Center) but was never built commercially because of the unfavorable economics. It has been operated with drilling mud as the driving fluid, but in these laboratory tests clear water was used.
In general, the performance is about the same for the air tools and the liquid tool, but the costs of using them may be quite different. The incompressible mud requires a valve design for the liquid tool that is more complex, more expensive. and shorter lived than the equivalent valve in the air tool. The air tool, however, may require a higher-pressure compressor than a normal air-drilled well, while the liquid hammer driving pressure is more likely to be within the capacity of a normal mud system. The air hammer doesn't increase the flow rate requirements since more air is needed to raise cuttings than to drive the hammer, but the extra pressure drop may require a booster on the compressor.
JPT
P. 2128^