Hydro‐Mechanical Measurements of Sheared Crystalline Rock Fractures With Applications for EGS Collab Experiments 1 and 2

We present hydro‐mechanical measurements that characterize shear on natural fractures in schist, amphibolite, and rhyolite specimens from the enhanced geothermal system (EGS) Collab Project's Experiment 1 and 2 sites (E1 and E2) at the Sanford Underground Research Facility. We employed a triaxial direct shear method augmented with X‐ray imaging to perform hydroshearing (injection‐induced shearing) and mechanical shearing on naturally fractured specimens at in situ stress conditions. Measurements included fracture permeability, strength, stress‐dependent aperture, shear dilation, and frictional strength. Results reveal that in situ natural fractures must be permeable, weak, and shear‐oriented to be hydrosheared, and only a subset of the observable in situ fractures were suitable. When sheared, the fracture permeability typically increased by a factor of 10 or more and this increase was retained over time. However, shear slip did not always result in permeability increase. High phyllosilicate content associated with exceptionally weak fractures exhibited poor or even decreased permeability after stimulation. These measurements in combination with site data were used to conduct a slip‐tendency analysis for different fracture sets, and we selected the top candidate natural fractures for hydroshearing at the EGS Collab sites. We also found that the lower in situ shear stress and stronger fractures at the E2 site make hydroshearing more challenging than at the E1 site. Overall, the methods and analysis used in our work can be applied to any geothermal project to identify in situ joint sets that are best suited for hydroshearing, which in turn can help to optimize well placement and energy production. Plain Language Summary Geothermal energy is attractive because it is green, clean, and “always on”. However, technical challenges and high‐capital costs historically hindered its development. Among the technical challenges, is a need to reliably increase the rock permeability to promote productive geothermal wells. One leading method is called “hydroshearing”, where fluid is injected to encourage shear slip along pre‐existing natural fractures and thus open the fractures to increase permeability. Our research focuses on the enhanced geothermal system (EGS) Collab Project's field sites where we characterize actual natural fractures so that we can predict shear slip via hydroshearing and then identify the best locations for drilling wells suitable for hydroshearing. T