Mechanism and Synergistic Parameter Optimization of CO ₂ Huff-n-Puff Technology for Enhanced Oil Recovery in Shale Reservoirs

Addressing the development challenges of shale reservoirs with low permeability and significant nonhomogeneity, this study systematically reveals the mechanisms for enhancing recovery through CO2 huff-n-puff technology. This is achieved through high-pressure PVT experiments, core-tube substitution, and orthogonal design huff-n-puff experiments. The experimental results indicate that CO2 injection significantly reduces crude oil viscosity (by 52%) and enhances mobility through a mixed-phase drive (with a minimum mixed-phase pressure (MMP) of 29–30 MPa) and crude oil volume expansion (with the volume coefficient increasing up to 1.39). Orthogonal experiments combined with Gray correlation analysis demonstrate that the number of huff-n-puff rounds (correlation coefficient of 0.93) and injection pressure (0.85) are critically sensitive parameters. Optimizing the injection pressure to 30 MPa and the duration of well soaking to 48 h could raise the recovery rate to 46.8%. Furthermore, the NMR T2 spectrum analysis revealed distinct oil mobilization patterns across different pore sizes, confirming the critical role of injection parameters in accessing both free and adsorbed oil. The co-optimization parameter model proposed in this study provides a theoretical and experimental foundation for CO2 huff-n-puff technology in shale reservoirs.