Experimental investigation of shear in granite fractures at Utah FORGE: Implications for EGS reservoir stimulation

Abstract

Efficient heat transfer from hot dry rock to a working fluid requires the large surface area provided by fractures. These fractures are likely to include both tensile hydraulic fractures and natural shear fractures and faults. Maintaining flow through these fractures is vital for the performance of Enhanced Geothermal Systems (EGS). Among numerous prior studies, there remains a lack of laboratory measurements to quantify shear fracture evolution in the coupled thermal, hydraulic, mechanical, and chemical (THMC) environment of a geothermal reservoir. To address this, we conducted triaxial direct-shear tests on crystalline, granitic rock samples from the Utah Frontier Observatory for Research in Geothermal Energy (Utah-FORGE) site in Milford, Utah. We measured fracture permeability, aperture, strength, deformation, and effluent chemistry before and after shear slip under in-situ conditions at the Utah-FORGE site – replicating stress, pressure, temperature, minerology, and injectate water chemistry. Our results show that shear displacement can increase fracture permeability by up to an order of magnitude (factor of ∼10); however, in some cases, permeability decreased by up to two orders of magnitude (factor of ∼0.01), due to gouge formation, chemical alteration, stress cycling, and changes in surface roughness. Our tests also indicate in-situ shear is likely to produce smooth-planar shear surfaces (e.g., dilation angles <7°), akin to slickensides, which reduces the benefit of shear fracture stimulation. Effluent analysis confirms rapid silicate and halite mineral dissolution and magnesium precipitation on FORGE samples, especially after shear stimulation. Our work provides key new measurements for modelling Utah-FORGE and similar granitic geothermal prospects.