Synergistic Mechanisms and Long-Core Experimental Insights of CO 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 CO 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 CO flooding methods in low-permeability reservoirs through 1 m artificial long-core physical simulation experiments and reveals the synergistic enhancement mechanisms of foam-CO 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 CO gas flooding, water-gas alternating flooding, foam-assisted continuous flooding, and foam-CO alternating flooding. The results show that the foam-CO 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 CO 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 CO 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.