Potential Evaluation of Enhanced Oil Recovery and CO2 Storage in Fractured Tight Reservoirs Using CO2-WAG Technology Based on Core-Scale Experiments and Numerical Simulation

Abstract The synergistic development of enhanced oil recovery (EOR) and Carbon Capture, Utilization, and Storage (CCUS) by water-alternating-gas (WAG) technology in fractured tight reservoirs was researched. As a key component of CCUS, continuous CO2 injection (CCI) faces inherent limitations in tight reservoirs with well-developed natural fractures, including mitigating gas breakthrough and poor sweep efficiency. While WAG injection improves mobility control, the multiphase flow mechanisms and CO2-water-oil-rock interactions in fractured systems remain insufficiently understood. The CCI and WAG injection experiments were conducted by utilizing natural tight core samples from the Ordos Basin with core artificial manufracturing technology and high-precision nuclear magnetic resonance (NMR) monitoring technology, and the in-situ characterization of dynamic fluid distributions were characterized. Experimental results demonstrate that compared to continuous flooding, CO2-WAG increases flow resistance by 3-8 times while enhancing oil recovery by 5.1%-9.0% and storage efficiency by 4.3%-5.6%. The optimal segment plug ratios of 1:1 was identified due to the highest comprehensive CO2 utilization-storage factor. Furthermore, for the fractured core with 50% fracture penetration ratio, compared to the no fractured cores, oil recovery was decrease by 16.5 % and CO2 storage efficiency was reduced by 7.4 %, Further analysis integrated with NMR imaging results, it revealed that the injected water during CO2-WAG can preferentially occupy the dominant flow pathways or fractures, thereby suppressing gas breakthrough through the Jamin effects. Integrated with component numerical simulation results, it was identified that substantial flow resistance was established by high-water-content slugs within porous media, while high-gas-content slugs effectively facilitate CO2 wave propagation and diffusion effects. These findings demonstrate strong agreement with core-scale experimental observations. The high-precision NMR scanning and evaluation method and component numerical simulation proposed in this paper provide theoretical support for CO2-WAG enhanced oil recovery and storage synergistic development in tight reservoirs, and the application of CCUS in unconventional reservoirs was promoted.