Injection of supercritical CO2 for geothermal exploitation from sandstone and carbonate reservoirs: CO2-water-rock interactions and their effects

CO2 can be injected into geothermal reservoirs to exploit geothermal energy. It is of concern that complex geochemical reactions induced by CO2 can result in change of the reservoir porosity and affect the fluid flow and heat mining rate. In this study, laboratory experiments on CO2-water-rock interactions were conducted to investigate the geochemical reactions using rock samples from typical sandstone and carbonate reservoirs. Based on the experimental results, 3D reactive transport simulation models for sandstone and carbonate reservoirs were established to simulate the geochemical reactions and their effects on heat mining rate during geothermal exploitation using CO2. The potential of CO2 storage in the heat mining process in different geothermal reservoirs was also assessed. The experimental results show that, for the sandstone tested, the presence of CO2 can lead to the dissolution of ankerite and clay minerals and the precipitation of plagioclase, which can result in the increase of Ca2+ and Mg2+ in formation water. For the carbonate tested, CO2 can induce the dissolution of dolomite and the precipitation of ankerite and calcite. The numerical simulation results indicate that the influence of the geochemical reactions on flow behavior and heat mining rate is dependent on the reservoir type and mineral compositions. For the sandstone reservoir, the reduction of the porosity caused by minerals precipitation has a negative effect on heat mining rate, while for the carbonate reservoir, the dissolution of dolomite and clay minerals can overshadow the precipitation effect of calcite and silicate minerals and increase the heat mining rate.