Fluid-rock interactions in CO2-saturated, granite-hosted geothermal systems: Implications for natural and engineered systems from geochemical experiments and models

Abstract Hydrothermal experiments were conducted and geochemical models constructed to evaluate the geochemical and mineralogical response of fractured granite and granite + epidote in contact with thermal water, with and without supercritical CO 2 , at 250 °C and 25–45 MPa. Illite ± smectite ± zeolite(?) precipitate as secondary minerals at the expense of K-feldspar, oligoclase, and epidote. Illite precipitates in experiments reacting granite and granite + epidote with water; metastable smectite forms in the experiments injected with supercritical CO 2 . Waters are supersaturated with respect to quartz and saturated with respect to chalcedony in CO 2 -charged experiments, but neither mineral formed. Carbonate formation is predicted for experiments injected with supercritical CO 2 , but carbonate only formed during cooling and degassing of the granite + epidote + CO 2 experiment. Experimental results provide insight into the buffering capacity of granites as well as the drivers of clay formation. Metastable smectite in the experiments is attributed to high water–rock ratios, high silica activities, and high CO 2 and magnesium–iron concentrations. Smectite precipitation in supercritical CO 2 -bearing geothermal systems may affect reservoir permeability. Silicate formation may create or thicken caps within or on the edges of geothermal reservoirs. Carbonate formation, as desired for carbon sequestration projects coinciding with geothermal systems, may require extended periods of time; cooling and degassing of CO 2 -saturated waters leads to carbonate precipitation, potentially plugging near-surface production pathways.