Measurement of Geomechanical and Hydrological Properties of EGS-Collab Geothermal Rocks

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.