We conducted high-temperature triaxial direct-shear (TDS) experiments to investigate the coupled thermo–hydro–mechanical–chemical (THMC) processes controlling fracture creation, deformation, permeability evolution, and geochemical reactivity under conditions representative of the Utah FORGE enhanced geothermal system (EGS) reservoir. Rock specimens were collected from the FORGE well 16A(78)-32 and tested at confining pressures of 36.1 MPa, pore pressures of 22 MPa, and temperatures up to 210 ± 15 °C. The experiments enabled in-situ fracture creation, permeability measurements, mechanical characterization, and effluent geochemical sampling and analysis. Results show that shear-induced fracturing can increase bulk permeability by several orders of magnitude, although permeability evolution remains highly sensitive to shear displacement, stress cycling, temperature, and chemical reactions. High-temperature tests generate shear fractures with low fracture dilation angles (3°–7°), substantially smaller than those measured in comparable low-temperature experiments, suggesting a strong thermal control on shear fracture deformation behavior. Geochemical observations indicate rapid mineral dissolution following fracture creation and evidence for secondary mineral precipitation. Together, these results provide critical insights on fracture flow behavior, THMC coupling, and reservoir simulation at the FORGE site. This work also provides detailed documentation of experimental methods and data interpretation, supporting the dataset archived in the Geothermal Data Repository (GDR) (Frash et al., 2023).

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