Experimental study on the mass transfer and microscopic distribution characteristics of remaining oil and CO2 during water-miscible CO2 flooding

Accurate understanding and quantitative characterization of the microscopic distribution and mobilization mechanisms of remaining oil are crucial for further enhancing oil recovery during CO₂ flooding. In this study, miscible CO₂ flooding experiments after water flooding were performed to investigate the mass transfer and microscopic distribution characteristics of remaining oil and CO₂ using micro-computed tomography technique. Additionally, the distribution characteristics of multiphase fluids (oil, water and CO₂) in the pores were comprehensively investigated. Furthermore, the mass transfer effects during the miscible CO₂ flooding process, as well as the CO₂ sequestration ratio (SR) and sequestration factor (SF) were systematically examined. The experimental results show that the ultimate oil recovery factor after miscible CO₂ flooding were 80.9 %, 70.7 %, and 64.7 %, respectively. During the water flooding stage, the produced oil mainly consisted of light hydrocarbons (C₅–C₁₆), followed by medium hydrocarbons (C₁₇–C₂₇). The remaining oil was mainly in cluster and network pattern, followed by multiple and oil film pattern. After miscible CO₂ flooding, the produced gas mainly consisted of CO₂ and CH₄, with a significant increase in the mass fraction of light hydrocarbons (C₅–C₁₆) in the produced oil. The remaining oil was mainly in multiple and singlet pattern, followed by network and film pattern, with a small portion in cluster pattern. The higher the displacement rate, the smaller the SR, but most of the injected CO₂ (SR > 70 %) was retained in the porous media, demonstrating the feasibility of CO₂ geological sequestration during the miscible flooding process.