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The study of fluid-rock interactions provides insight into subsurface geologic processes, such as diagenesis, hydrothermal alteration and metamorphism. Understanding and predicting these interactions also helps us assess the geologic impact of hydrocarbon recovery and geothermal production and injection. Therefore, the study of fluid-rock interactions has both geologic and economic impact. At the Dixie Valley geothermal field, NV, precipitated calcite and aragonite within a production well trapped boiling fluids in fluid inclusions. The trapped gases were analyzed and shown to be compositionally different than those sampled at the well head. The inclusions trapped a greater ratio of light gases CH4 and H2 to CO2 than those sampled at the well head. This result indicates that the fluid inclusions trapped the initial steam fraction during boiling. Declining performance of injection wells at the Coso and Salton Sea geothermal fields, CA, were found to result from mineral deposition in the near-wellbore environment during fluid injection. At Coso, opal-A and minor calcite scale mineral precipitates were found in cuttings from wells drilled near previously operating injection wells. At the Salton Sea, cuttings from a deepened injection well contained banded barite, fluorite, amorphous silica, and minor anhydrite scales. Mineral precipitation was modeled and predicted with the non-isothermal reactive transport modeling code TOUGHREACT. Geochemical simulations were also performed to predict the consequences of injecting H2SO4 modified fluid for mitigating silica precipitation at Coso using TOUGHREACT. The models predict that silica precipitation will be reduced significantly by maintaining pH of 5 or less. This can be accommodated in the models by reducing the kinetic rate constant for silica precipitation. TOUGHREACT simulations also predict that corundum proppants will be chemically stable under geothermal conditions. The formation of talc in the outer aureole of the Alta Stock does not define a regular isogradic surface, unlike isograds in the inner aureole. Examination of mineral and fluid stabilities in the H2O-CO2-NaCl system shows that several fluid evolution scenarios, including fluid immiscibility, may produce the observed talc heterogeneity.