Experimental study of the change of pore structure and strength of granite after fluid-rock interaction in CO2-EGS

Deep geothermal energy plays an essential role in the future renewable energy supply. To investigate the mechanism of fluid-rock interaction on the pore structure and the strength of granite in various zones of CO2-EGS, this paper carried out experiments on the fluid-rock interaction at 240 °C and 37 MPa for the three zones of CO2-EGS. Experiments were conducted to test and analyze the elemental composition, surface micromorphology, pore structure, P-wave velocity, and mechanical strength of the granite before and after the fluid-rock interaction. The results show that the water/ScCO2 dissolves the albite and potassium feldspar in the granite, and the carbon in ScCO2 may be sequestered in rock as carbonate mineral precipitates. The injection of ScCO2 fluids increases the pore volume of the rock by 43%, 65%, and 45% in the presence of no water, a small amount of water, and a large amount of water, respectively. There is no chemical reaction, but only physical interaction, between ScCO2 and granite when no water is involved in the core zone of CO2-EGS. The dissolution and precipitation are slight when there is limited water in the rock of surrounding zone, and the P-wave velocity and uniaxial compressive strength also only decrease by ignorable amounts. Only with sufficient water in the outer zone, the ScCO2 fluid can significantly change the uniaxial compressive strength and elastic modulus of rock sample. The findings indicate that the CO2-EGS won't suffer from the blockage of mineral precipitation, decrease of permeability and short service lifetime of geothermal reservoir, which are big problems in conventional water based EGS.