Synergistic Mechanisms and Long-Core Experimental Insights of CO₂ Foam Alternating Flooding for Enhanced Oil Recovery in Low-Permeability Reservoirs

CCUS technologies for enhanced oil and gas recovery rely on the unique properties of CO₂ to enable efficient displacement and secure geological storage, presenting a cost-effective and practical approach to mitigate greenhouse gas emissions and support global carbon neutrality objectives. This study systematically evaluates the development effects of different CO₂ flooding methods in low-permeability reservoirs through 1 m artificial long-core physical simulation experiments and reveals the synergistic enhancement mechanisms of foam-CO₂ alternating flooding. The core model is constructed by the harmonic permeability sorting method (permeability 2.6-3.9 mD, porosity 6.35-11.13%), and the displacement characteristics of four schemes are compared and analyzed, including continuous CO₂ gas flooding, water-gas alternating flooding, foam-assisted continuous flooding, and foam-CO₂ alternating flooding. The results show that the foam-CO₂ alternating flooding (0.15 PV slug) performs optimally, with a recovery efficiency of 60.75%, which is 23.69 and 7.49% higher than that of continuous gas flooding and water-gas alternating flooding, respectively, and the gas channeling breakthrough time is delayed to 0.48 PV (29.7% later than traditional gas flooding). Its synergistic enhancement mechanisms include: (1) the Jamin effect of foam reduces gas mobility by 73%, forcing the fluid to divert to medium-low permeability regions; (2) the compound surfactant (0.5% AEO-9 + 0.3% CHSB) works synergistically with CO₂ to significantly enhance the microscale oil displacement efficiency; (3) the coupling analysis of pressure response and production dynamics confirms a three-level synergistic mechanism of "mobility control-interfacial regulation-sweep volume expansion". Furthermore, the core construction method based on harmonic permeability effectively reduces the interference of heterogeneity on experimental results, providing an experimental basis for the optimization of CO₂ flooding injection parameters in low-permeability reservoirs and the adaptability evaluation of CCUS technologies. The research results provide theoretical support and technical paths for the dual goals of efficient development of low-permeability reservoirs and carbon emission reduction.