Influences of directional aperture heterogeneity on the performances of two-phase enhanced geothermal system considering the CO2 buoyant force

The technology of Enhanced Geothermal System (EGS) using CO2 as working fluid is believed to be a promising method for harnessing geothermal energy, contributing to the reduction of atmospheric greenhouse gas emissions and decreasing the dependency on traditional fossil fuels. Due to the buoyant force of CO2 in water-saturated EGS, the effects of aperture heterogeneity on fluid flow and heat transfer may be directional and differ in the two-phase flow system from the traditional single-phase EGS. In this case, we employed a thermal-hydraulic coupled model, considering a structured fracture geometry to comprehensively investigate the effect of directional distributed aperture combined with buoyant force on the EGS energy performances. The Monte Carlo method is applied to ensure that the simulation results reach statistical equilibrium. Results show that the vertical aperture heterogeneity leads to poorer energy output and carbon sequestration in a two-phase EGS, compared to the horizontal aperture heterogeneity. On the other hand, the negative effects of directional aperture heterogeneity on the total EGS energy output become more significant with the decreasing fracture density. Finally, compared to the EGS using water as the working fluid, those employing CO2 exhibits greater sensitivity to directional aperture heterogeneity for both high and low fracture-density networks due to the buoyant force of CO2.