CO₂ storage with enhanced gas recovery (CSEGR): A review of experimental and numerical studies

CO₂ emission mitigation is one of the most critical research frontiers. As a promising option of carbon capture, utilization and storage (CCUS), CO₂ storage with enhanced gas recovery (CSEGR) can reduce CO₂ emission by sequestrating it into gas reservoirs and simultaneously enhance natural gas production. Over the past decades, the displacement behaviour of CO₂–natural gas has been extensively studied and demonstrated to play a key role on both CO₂ geologic storage and gas recovery performance. This work thoroughly and critically reviews the experimental and numerical simulation studies of CO₂ displacing natural gas, along with both CSEGR research and demonstration projects at various scales. The physical property difference between CO₂ and natural gas, especially density and viscosity, lays the foundation of CSEGR. Previous experiments on displacement behaviour and dispersion characteristics of CO₂/natural gas revealed the fundamental mixing characteristics in porous media, which is one key factor of gas recovery efficiency and warrants further study. Preliminary numerical simulations demonstrated that it is technically and economically feasible to apply CSEGR in depleted gas reservoirs. However, CO₂ preferential flow pathways are easy to form (due to reservoir heterogeneity) and thus adversely compromise CSEGR performance. This preferential flow can be slowed down by connate or injected water. Additionally, the optimization of CO₂ injection strategies is essential for improving gas recovery and CO₂ storage, which needs further study. The successful K12–B pilot project provides insightful field-scale knowledge and experience, which paves a good foundation for commercial application. More experiments, simulations, research and demonstration projects are needed to facilitate the maturation of the CSEGR technology.