Auto Indexer for Percussive Hammers Final Report

Abstract

Geothermal energy has been underutilized in the U.S., primarily due to the high cost of drilling in the harsh environments encountered during the development of geothermal resources. Drilling depths can approach 5,000 m with temperatures reaching 170 C. In situ geothermal fluids are up to ten times more saline than seawater and highly corrosive, and hard rock formations often exceed 240 MPa compressive strength. This combination of extreme conditions pushes the limits of most conventional drilling equipment. Furthermore, enhanced geothermal systems are expected to reach depths of 10,000 m and temperatures more than 300 C. To address these drilling challenges, Sandia developed a proof-of-concept tool called the auto indexer under an annual operating plan task funded by the Geothermal Technologies Program (GTP) of the U.S. Department of Energy Geothermal Technologies Office. The auto indexer is a relatively simple, elastomer-free motor that was shown previously to be compatible with pneumatic hammers in bench-top testing. Pneumatic hammers can improve penetration rates and potentially reduce drilling costs when deployed in appropriate conditions. The current effort, also funded by DOE GTP, increased the technology readiness level of the auto indexer, producing a scaled prototype for drilling larger diameter boreholes using pneumatic hammers. The results presented herein include design details, modeling and simulation results, and testing results, as well as background on percussive hammers and downhole rotation. EXECUTIVE SUMMARY Geothermal resources provide opportunities for clean, renewable baseload energy in the U.S. and worldwide. However, current utilization of available resources is only a small fraction of the potential, primarily due to the high cost of drilling in these harsh environments. It is not uncommon for drilling depths to approach 5,000 m and temperatures to reach 170 C during geothermal exploration and resource development. Furthermore, hard rock formations often exceed 240 MPa in compressive strength, and geothermal waters can be an order of magnitude more saline than seawater, creating a highly corrosive drilling environment. Enhanced geothermal systems are even more extreme, with potential to reach depths of 10,000 m and temperatures greater than 300 C. These extreme conditions not only cause most conventional drilling equipment to wear prematurely but also consume valuable time servicing and replacing damaged parts. Sandia National Laboratories (Sandia) has been addressing these challenges with the development of new tools and technologies to enable more efficient, cost-effective drilling in harsh geothermal environments. Previous Sandia research funded by an external customer (DARPA) resulted in a harsh-environment downhole rotation proof-of-concept for a tool called the auto indexer. This relatively simple motor operates without elastomers, which tend to fail at high temperatures. For example, positive displacement motors, are limited in geothermal applications because they require elastomers. As part of this proof-of-concept, Sandia tested the auto indexer in a bench-top environment and demonstrated success with 4.0 in. (~10 cm) diameter tools and can provide the rotation required to operate a pneumatic hammer during drilling. Demonstrating that the auto indexer is compatible with pneumatic, down-the-hole-hammers (DTHH) is significant because percussive technologies improve penetration rates in hard rock and can potentially reduce drilling costs associated with geothermal development. Pneumatic DTHH are used extensively in the mining, construction, and water well drilling industries and are arguably the highest performance drilling technology for hard rock drilling. Established research has shown that percussive devices have among the lowest mechanical specific energies?energy required to remove a given volume of rock?of any drilling method and a reputation in the industry for reliable hard rock drilling. Under this current project, funded by DOE GTP, Sandia developed a scaled prototype of the auto indexer to drill larger diameter boreholes common to geothermal resource development, thereby increasing the technology readiness level of the tool. The ability to conduct drilling operations successfully, at appropriate hole diameters, and cost-effectively in harsh geothermal environments using a percussive hammer is the value proposition offered by this research. Downhole rotation, coupled with percussive hammers, will improve penetration rates in hard rock formations and furthers the pathway to directional drilling, both of which will improve access to geothermal resources. This report documents the development of the auto indexer, design details, modeling and simulation results, and testing results, as well as a discussion on percussive hammers and downhole rotation options.