Enhanced Geothermal Systems (EGS) rely on the coupled chemical and mechanical behavior of natural and stimulated fractures to sustain permeability and ensure long-term reservoir stability. At the Utah FORGE site, granitic rocks contain mineral-filled natural fractures whose evolving properties under high-temperature conditions critically influence stimulation performance. This study integrates hydrothermal exposure experiments and micro-scale mechanical characterization to investigate how geochemical alteration modifies the micromechanical behavior of fracture minerals and interfaces, with the objective of deriving fracture-scale constraints relevant to proppant performance in the Utah FORGE reservoir. A representative granitic core sample extracted from a depth of 9,843 ft was subjected to hydrothermal exposure at 250 °C to simulate EGS reservoir conditions. Nanoindentation testing was performed before and after hydrothermal treatment to quantify changes in hardness and elastic modulus, while X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS) were used to characterize mineralogical and microstructural evolution along fracture surfaces. Results indicate that hydrothermal interaction produces spatially heterogeneous chemical and mechanical modification of fracture walls, including the development of aluminosilicate alteration coatings, Fe–Al–rich reaction layers, and localized secondary precipitates. These alterations lead to systematic reductions in hardness and elastic modulus in fracture-proximal domains, while relatively unaltered quartz- and feldspar-rich regions retain higher stiffness and brittleness. Such fracture-scale heterogeneity implies non-uniform proppant–fracture contact conditions and evolving load-transfer behavior under EGS operating conditions. Rather than prescribing specific proppant materials, the results provide mineral- and alteration-dependent mechanical constraints that inform proppant design considerations for Utah FORGE, including resistance to embedment in chemically softened fracture zones, tolerance to variable surface compliance, and mechanical robustness against contact with brittle, quartz-dominated domains. By linking fracture geochemistry and micromechanical evolution, this study establishes a mechanistic basis for evaluating proppant performance in crystalline EGS reservoirs.

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