Experimental study of static and dynamic interactions between supercritical CO2/water and Australian granites

Recent research in Enhanced Geothermal Systems (EGS) have given rise to the interests of a CO2-based EGS concept due to the unique thermo-physical properties of supercritical carbon dioxide (scCO2) in EGS applications. However, available studies related to CO2-based EGS are mostly theoretical investigations and relevant experimental study is highly scarce. To support the development of the new concept, this study conducts both static and dynamic fluid-rock interaction experiments between scCO2/water and three different Australian granites. A tailored fluid-rock integration apparatus was designed to conduct the above investigation. The pulverised granites were exposed to scCO2/water for up to 15 days at the simulated reservoir temperatures of 200 °C, 250 °C and pressures of 20 MPa and 35 MPa. The results of static fluid-rock interactions show that the elements of Na, Si, K, Ca, Mg, Fe, Al were found dissolved into the scCO2-rich geofluid at an average rate of 4.5, 2.7, 1.6, 0.5, 0.3, 0.2, and 0.1 ppm/day, respectively. The dynamic fluid-rock interactions shows that the average rate of mineral dissolution in the pure water was around 183 ppm/day of Si, 14 ppm/day of Na, 12 ppm/day of Al, and 4 ppm/day of K, while only 0.4-2.5 ppm/day of Si, 0.4-1.6 ppm/day of Na, and 0.1-0.3 ppm/day of K for using scCO2-rich stream as the geofluid. The typical composition of the trace elements dissolved in both pure water and scCO2-rich geofluids were also identified. Fluid-rock equilibrium analyses shows that the geofluids obtained after the 15 days of static fluid-rock interaction may have reached/were approaching geochemical equilibrium for some elements (e.g. Si), whilst for the flow-through experiments the reacted geofluids were far from geochemical equilibrium. The examination of the fluid-rock interaction using the three Australian granites highlighted the importance of mineral composition to fluid-rock interaction. The research provides valuable experimental data and insights for understanding the CO2-based EGS system.