Mechanistic Investigation of Enhanced Oil Recovery via CO2 Synchronous Huff-and-Puff and Asynchronous Injection–Production in Low-Permeability Reservoirs

CO2 injection for enhanced oil recovery and carbon sequestration in low-permeability reservoirs has become a major research focus, driven by growing global energy demand and carbon-emission reduction targets. Among available development strategies, synchronous huff-and-puff and asynchronous injection–production show considerable field application potential; however, large-scale physical simulation experiments to validate these approaches remain lacking. In this study, large-scale high-temperature, high-pressure, two-dimensional physical simulation experiments were conducted under CO2 miscible flooding conditions to compare the displacement mechanisms of these two strategies in a low-permeability reservoir. Numerical simulation was employed to history-match the experimental results, confirming their accuracy and reliability. The results show that CO2 exhibited strong adaptability under the investigated reservoir conditions. It effectively replenished formation energy and, upon dissolution in crude oil, induced pronounced swelling and viscosity-reduction effects that enhanced oil mobility. Under miscible conditions, interfacial tension was significantly reduced, further improving displacement efficiency. Compared with synchronous huff-and-puff, asynchronous injection–production established a unidirectional pressure gradient by injecting CO2 into the low-permeability zone while producing from the high-permeability zone, substantially enlarging the swept volume and mobilizing residual oil. The final oil recovery under this mode reached 41.56%, an improvement of 21.78% over synchronous huff-and-puff. Reservoir heterogeneity was identified as the key factor controlling CO2 flooding effectiveness and residual oil distribution. The high-permeability zone served as the preferential CO2 migration pathway, while the low-permeability zone retained considerable residual oil. Therefore, rational optimization of the injection–production direction and pressure regime is essential for overcoming heterogeneity constraints and improving overall recovery performance.