Advancing CCUS-EOR in low-permeability reservoirs with surfactant-enhanced carbonated water and CO₂ alternating flooding: An integrated experimental and numerical investigation

Surfactant-enhanced carbonated water alternating with CO₂ (SCWAG) flooding, which integrates the advantages of surfactants, carbonated water (CW), and CO₂, has demonstrated significant potential for the development of low-permeability reservoirs. Nonetheless, the underlying mechanisms of SCWAG-enhanced oil recovery require further elucidation. Its CO₂ storage performance and pore-scale oil displacement characteristics have not been thoroughly investigated, and the influence of various factors on SCWAG performance remains poorly understood. This study, for the first time, investigates the pore-scale oil displacement characteristics and CO₂ storage performance of SCWAG by integrating core flooding experiments and nuclear magnetic resonance scanning. An innovative core-scale 3D heterogeneous numerical model was developed using computed tomography scanning and refined via history matching, thereby enabling reliable SCWAG simulation and facilitating reservoir-scale analysis of factors affecting SCWAG performance. The results demonstrate that SCWAG notably improves both sweep efficiency and oil displacement efficiency, achieving higher recovery and CO₂ storage efficiency than other methods. The total recovery reached 76.99%, with individual recoveries of 56.35%, 76.85%, and 87.96% for micropores, mesopores, and macropores, respectively, while the CO₂ storage efficiency is 57.22%. Permeability contrast exerts a significant effect on recovery, whereas CO₂ storage efficiency was primarily influenced by the injection rate and water-to-gas ratio. Moreover, the interaction between the water-to-gas ratio and permeability contrast exerts a substantial impact on both recovery and CO₂ storage efficiency. This study provides novel insights and an in-depth analysis of the SCWAG process, offering practical guidelines for its application in low-permeability reservoirs.