High Temperature Triaxial Direct-Shear Testing for FORGE and Field Scale Implications

ABSTRACT: Rock fractures are the most important element of Enhanced Geothermal Systems (EGS) but we lack the necessary data to predict the combined effect of temperature, flow, mechanics, and chemistry on the forecasted effectiveness of these fractures for heat extraction and power production. Here, we seek to quantify the strength, deformation, and hydraulic conductivity of rock fractures before and after shear slip, at geothermal reservoir conditions. This information enables evaluation of the likelihood that hydraulically induced shear slip will improve the performance of EGS. This data can also aid risk forecasting for injection induced seismicity. Concurrent effluent analysis yields insights regarding the chemical reactivity of fresh shear fractures. Our measurements were obtained using triaxial direct-shear experiments conducted at conditions replicating well 16A(78)-32 at the Frontier Observatory for Research in Geothermal Energy (FORGE) in Milford, Utah. We also implement these measurements into reservoir models to evaluate their impact on predicted reservoir performance relative to prior estimates. 1. INTRODUCTION Geothermal energy retains promise to supply clean stable electrical power and heat (Hamm et al., 2018), as well as flexible power to accommodate the intermittency of wind and solar energy (Ricks et al., 2024). Predicting the behavior of rock fractures is key to expansion, especially for Enhanced Geothermal Systems (EGS) because of its requirement for fractures to facilitate fluid flow and heat extraction (Tester et al., 2006; Brown et al., 2012). However, the properties of fractures at hot and deep conditions are not well known, especially if the effects from stimulation, chemistry, and mechanics are to be considered. The Frontier Observatory for Research in Geothermal Energy (FORGE) is an opportunity to better understand such processes by comprehensive scientific assessment of a hot dry rock (HDR) geothermal resource (Podgorney et al., 2023), along with the performance of technologies needed to access and harvest the heat. Here, we present laboratory experiments to characterize thermal, hydraulic, mechanical, and chemical (THMC) coupled processes related to the stimulation of and flow of fluid through shear-induced fractures. Results from this work reveal insights into the potential of hydraulic shear stimulation to enhance flow and quantify key properties for fractures, including strength, frictional properties, dilation tendencies, and chemical reactivity; the details of which are included herein.