Numerical study of enhanced geothermal systems with supercritical CO2 injection considering reservoir changes

The injection of supercritical CO2 (ScCO2) into the enhanced geothermal systems (EGS) is a highly intricate thermohydromechanical (THM) coupled process. To improve the realism and accuracy of simulating convective heat transfer characteristics in CO2-EGS, a coupled model based on discrete fracture network (DFN) theory is established. In this model, the dynamic evolution laws of reservoir porosity and permeability, as well as the nonlinear variations of thermophysical parameters for ScCO2 under the influences of temperature and pressure are considered. The validity of the numerical results was confirmed through a comparative analysis with the analytical solution. The result shows that the ScCO2 can rapidly extracts heat from both the fractures and the adjacent rock matrix. The heat of the rock matrix far away from the fractures cannot be compensated to the fractures immediately, resulting in the high temperature peak effect of the local rock matrix. The injection of ScCO2 can increase the porosity and permeability of reservoir, which is induced by the weakened thermal expansion effect of rock. The alteration of reservoir porosity and permeability also leads to the change of the flow characteristics of ScCO2. Additionally, the thermal parameters of ScCO2 changes with the temperature and pressure conditions of the reservoir. The change of flow rate and heat extraction rate of ScCO2 in EGS is dominated by the decrease of density, specific heat capacity, porosity, permeability, and thermal conductivity.