The geochemistry of flowback and produced waters at Utah FORGE were obtained from stimulation-circulation tests between 2022 and 2024, involving a hot dry rock reservoir made of crystalline granitoid and gneiss at 200 to 220 deg C. The collection points for analyzed samples include injection well 16A(78)-32 and production well 16B(78)-32. The dissolution of halide minerals accounts for sharp increases in Na, K, and Cl that were measured in all produced waters, but the size of the increase appears to be diminishing over time as salts are progressively removed from fracture-controlled flow paths. Early interwell connection was established and sustained, although deposition of calcium-bearing minerals (mainly carbonates) in the reservoir due to heating of injected water likely occurred. Temperature-dependent quartz and anhydrite solubilities appear to control the silica and sulfate concentrations in produced waters, which makes these parameters useful for chemical geothermometry. Pipe scale deposition of carbonate minerals and amorphous silica appears to be minimal. Deeply derived gases infiltrate the reservoir, induce weak acidification of produced water, and yield CO2/Ar and H2/Ar equilibration temperatures consistent with reservoir conditions.

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