Synergistic effects of carbonated water flooding coupled with surfactant for EOR and CO₂ storage: Pore-scale mechanisms and influencing factors analysis

Active carbonated water (ACW) flooding combines the benefits of carbonated water (CW) and surfactant flooding, simultaneously enhancing oil recovery and facilitating CO₂ storage. Nevertheless, the enhanced oil recovery (EOR) mechanisms of ACW remain insufficiently understood, and neither its CO₂ storage performance nor its pore-scale oil displacement characterization have yet to be investigated. Additionally, the influence of different factors on the performance of ACW has not been systematically studied. In this study, core flooding and nuclear magnetic resonance scanning experiments were conducted to elucidate the EOR mechanisms of ACW, clarify for the first time its CO₂ storage performance and pore-scale oil displacement characterization, and examine the influence of various factors on its effectiveness. The results demonstrated that CW flooding enhanced the oil recovery by 12.24% compared to water flooding (46.19%). By leveraging the synergistic effects of CW and surfactant, ACW further increased oil recovery to 64.55%, with pore-scale recovery of 72.22%, 50.77%, and 29.29% for macropores, mesopores, and micropores, respectively, representing increases of 15.77%, 15.05%, and 10.39% relative to water flooding. Furthermore, both CW and ACW demonstrated effective CO₂ storage, with dissolution trapping emerging as a critical mechanism. Compared to the CO₂ storage efficiency of CW at 50.57%, ACW showed a higher efficiency of 52.84%. Within the parameter ranges studied, the oil recovery and CO₂ storage efficiency of ACW both increased with decreasing injection rate and with increasing permeability and surfactant concentration. Correlation analysis revealed that for both metrics, permeability had the most significant influence, followed by surfactant concentration and injection rate. This work provides new insights into the potential of ACW flooding for simultaneous EOR and CO₂ storage, offering practical guidance for its application in low-permeability reservoirs.