Optimization of CO ₂ Injection for EOR and Storage in Low-Permeability Reservoirs

Based on the issue of low efficiency of CO2 displacement and sequestration in different low-permeability reservoirs, the impact of different CO2 injection methods (continuous CO2 injection, water-alternating-gas (WAG), and Huff and Puff (HnP)) on the microscopic oil production and CO2 storage in low-permeable reservoirs are studied by conducting several experiments, aiming to select the optimal CO2 injection and storage strategy. Nuclear magnetic resonance (NMR) was employed to analyze the microscopic oil recovery and CO2 storage within different scales of pores of cores. The results indicate that CO2 miscible injection enhances mass transfer and reduces interfacial tension (IFT) between CO2 and crude oil, expands the CO2 sweep volume and oil washing efficiency in reservoir cores, and ultimately improves the oil recovery. CO2 miscible WAG (CO2-M-WAG) with a gas–water ratio of 1:1 and a single water or CO2 slug size of 0.1 pore volume (PV) of cores achieved the highest oil recovery and CO2 storage percentage after displacing the cores, followed by continuous CO2 miscible flooding (CO2-MF). In contrast, the use of a CO2 miscible Huff and Puff (CO2-M-HnP) resulted in relatively low oil recovery and CO2 storage for the cores. During the process of CO2-M-WAG, the mass transfer between CO2 and the oil phase is promoted, and the premature breakthrough at the core outlet end is also mitigated by stabilizing the CO2 displacement front. Otherwise, relative permeability hysteresis occurs during the CO2 WAG process, leading to some CO2 being isolated into clusters in certain small pores and throats, thereby enhancing CO2 storage. Therefore, to simultaneously Enhance Oil Recovery (EOR) and CO2 storage in low-permeable reservoirs where CO2 gas channeling is prone to occur prematurely, the optimal CO2 injection method may be the CO2-M-WAG with an optimized gas–water ratio.