Simulation of buoyancy effect on the efficiency of geothermal development in supercritical CO₂-based EGS

Supercritical carbon dioxide (CO₂) is used as a heat transmission fluid for the geothermal development in enhanced geothermal systems (EGS), aiming at improving the efficiency of heat mining and alleviating the emission of the world greenhouse gas. Buoyancy effects contributed by the density difference between water and carbon dioxide pose a great impact on the operation of this CO₂-based EGS. A numerical simulation tool based on the unified pipe-network method (UPM) is developed in the current study to analyze the complex mechanism of multi-phase and multi-physical coupled process in the geothermal development. The reliability of the current numerical model solved by a sequential implicit time scheme is verified against results obtained using a fully implicit time scheme. Case studies are performed for the development of CO₂-based EGS using typical properties of geothermal reservoirs embedded with fracture networks. Higher values of the CO₂ storage volume and the net heat extraction rate are achieved when the CO₂ buoyancy effect is considered in the same operation condition. The influences of different operation strategies, including varying production pressure differentials and injection flow rates, on the performance of the heat extraction and the carbon dioxide sequestration are also analyzed.