Multiphysics-Coupled Numerical Simulation of CO ₂ –H ₂ O Hybrid Working Fluid in Enhanced Geothermal Systems

This study focuses on the numerical simulation of the mixed working medium-enhanced geothermal system (EGS). We have developed a thermo-hydro (TH) solver that couples the component transport, phase equilibrium, and heat transfer processes. The solver was effectively verified through benchmark tests of experimental data and comparisons with analytical solutions. Sensitivity analysis indicates that an increase in the molar fraction of the aqueous phase will reduce the effective storage amount of CO₂ and have a nonlinear impact on the system lifetime and energy extraction efficiency. Although increasing the injection pressure can enhance the CO₂ retention rate and power output, it will accelerate heat loss. It is worth noting that a higher injection temperature usually has a positive effect on all of the key parameters. Furthermore, increasing the permeability of the matrix can significantly enhance the reservoir productivity and thermal recovery rate, confirming the rationality of expanding the scale of reservoir fracturing. Meanwhile, by controlling the attenuation rate of the fracture permeability, the service life of the system can be effectively prolonged. The results of this study provide important guidance for optimizing the CO₂ storage efficiency, energy extraction power, and system durability of geothermal carbon sustainable utilization projects.