CARBON DIOXIDE-WATER-ROCK INTERACTIONS IN GRANITE-HOSTED GEOTHERMAL SYSTEMS: EXPERIMENTAL INSIGHTS INTO THE INFLUENCE OF SELECT ALTERATION MINERALS

Supercritical CO₂ is proposed as a working fluid in enhanced geothermal systems (EGS) due to large expansivity and low viscosity as compared to water. Hydrothermal experiments evaluate the effect of supercritical CO₂ on water-rock interactions in granite-hosted geothermal systems that have been extensively altered to chlorite and calcite. Dilute groundwater, granite, and two different alteration assemblages (chlorite and calcite) are reacted at approximately 250°C and 250 bars in a rocking autoclave and Au-Ti reaction cell (water:rock ratio ~20:1) for a minimum of approximately 28 days. Injection of supercritical CO₂ into the ongoing reaction initially increases pressure; CO₂ subsequently dissolves into the water and pressure decreases until a steady state pressure of ~360 bars is achieved within approximately 15 hours. Excess CO₂ is injected to produce a separate supercritical fluid phase, ensuring CO₂ saturation of the water (2.9 to 3.0 molal) for the duration of each experiment. CO₂-water-rock reaction continues for at least an additional ~28 days. The granite consists of sub-equal amounts of quartz, perthitic K-feldspar (~25 wt% albite and 75 wt% K-feldspar), oligoclase (An₂₃), and 4 wt% Fe-rich biotite. Alteration minerals consist of calcite and chlorite (chlinoclore). Each experiment uses mineral powders (75 wt% of rock mass, ground to <45 μm) to increase reactivity and also mineral pieces (0.1 to 0.7 cm in size) to promote petrologic evaluation of mineral reactions. The water (I ≈ 0.12 molal) contains approximately 0.1 molal Na and Cl and millimolal to molal quantities of SiO₂, Al, Ca, Mg, K, SO₄, and HCO₃, and is calculated to be saturated with respect to minerals present at the start of each experiment. In-situ water samples are collected nine times during each experiment and analyzed for total dissolved carbon by coulometric titration, anions by ion chromatography, and major, minor, and trace cations by ICP-OES and -MS. Bench pH values are paired with aqueous analyses to calculate in-situ pH. Minerals are evaluated by SEM-EDS, XRD, and/or bulk chemical analysis before and after each experiment. Analytical data and geochemical calculations are integrated to evaluate CO₂-water-rock interactions of the altered granite.