To inform well layout and stimulation designs at Experimental 2 site (4100 Level) for the EGS Collab project, we measured a suite of geomechanical and hydrological properties for the amphibolite and rhyolite geologic units near the Sanford Underground Research Facility (SURF) 4100 Battery Alcove. Our measurements used a triaxial direct-shear method that fractures samples along targeted planes to study natural fracture properties. Measurements included intact and residual strength, permeability, stress-dependent aperture, and rock dilation with shear displacement. Here we present results from two specimens that were extracted from core from the site. The first is from a Yates amphibolite and rhyolite contact (YA02-01). The second is a weak natural fracture from the rhyolite (YA02-05). Results show that the initial permeability of the weak rhyolite fracture was around 0.3 mD and that hydroshearing could be successful along this fracture. After hydroshearing, permeability increased up to 0.7 mD and it was retainable. For the Yates-rhyolite contact, hydroshearing was not successful so we instead used mechanical shear to fracture the specimen. Permeability increased significantly right after the shear fracture is initiated, but further mechanical shear resulted in permeability reduction. This work confirms that hydroshearing is possible at the Experiment 2 (4100 Level) site, but it requires a very weak preexisting fracture in addition to a shear-favorable orientation. Most of the observable fractures do not meet this criteria.

1. Introduction

Geothermal energy is a key technology for a clean and sustainable energy future. When applied using Enhanced Geothermal Systems (EGS), the United States alone could generate an additional hundreds of GW of electricity by 2050 (Bromley et al. 2010; Dobson et al. 2017; Fan 2020). However, technical barriers still exist for EGS, including a poor understanding of in-situ stimulation performance, challenges managing reservoir fluid flow, and effective zonal isolation to mitigate short-circuiting and improve long-term performance (Dobson et al. 2017).

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