CO₂ injection for geothermal development associated with EGR and geological storage in depleted high-temperature gas reservoirs

High-temperature gas reservoirs (HTGR) come with significant geothermal potentials. Supercritical CO₂has been considered as one of the best heat transmission fluids for geothermal production. In this study, a novel concept technology of CO₂-HTGR system has been proposed to develop the geothermal energy in the depleted high-temperature gas reservoirs through CO₂injection associated with EGR and geological storage. CO₂enhanced gas recovery (EGR) with pressure build-up should be conducted first to establish a CO₂gas reservoir with a low methane content, then the hot CO₂gas in the reservoir is produced for heat utilization and then injected back. Finally, the CO₂gas reservoir is shut down for permanent geological storage. Reservoir numerical simulation has been conducted to study the fundamental processes of this concept technology including the establishment of a CO₂gas reservoir through EGR and pure injection, and the heat mining performance of CO₂gas cycling in the created reservoir. The simulation results indicate that a high injection-production ratio during CO₂EGR can shorten the time of establishing a CO₂gas reservoir. The purity of the established CO₂gas reservoir has a significant influence on the heat mining performance of cyclic CO₂gas. When the CO₂purity in the gas reservoir is higher than 90%, the damage of the remaining methane to the heat mining rate of supercritical CO₂can be controlled within 9.5%. The integrated process of CO₂injection for geothermal development associated with EGR and geological storage is more attractive than the conventional CO₂geothermal system and has a stronger on-site feasibility.