Investigation on Non-uniform Miscibility of CO2-Oil in Porous Media and Its Influence on EOR and CO2 Storage

The miscibility of CO2-oil is critical to the successful implementation of CO2 miscible flooding on oil reservoirs. At present, the research on the miscibility regulation mainly focused on bulk oil, and the influence of complex pores on CO2-oil miscibility is not considered. In this study, the experiments of continuous CO2 miscible flooding and water alternating CO2 flooding (WAG) were carried out at pore and core scales, respectively. Based on microfluidic experiments, remaining oil after water flooding in the porous media was clarified in five types: film oil, dead-end oil, columnar oil, unswept oil, and cluster oil. During miscible CO2 injection, CO2 connected the dispersed oil. The dispersed oil became continuous and was carried out of the chip. WAG resulted in a more dispersed CO2, water, and oil distribution. During WAG, CO2 contacted the oil in the water privilege channel, and the oil volume expanded. The dispersed oil was trapped at the pore throat, enhancing the Jiamin effect. Thus, the swept area of water and CO2 in the porous media through WAG was larger than continuous CO2 flooding. Because of water separation, dead-end pores, and low convective velocity, apparent non-uniform miscibility of CO2-oil system in porous media was observed. After WAG, more water was trapped in the porous medium than continuous miscible CO2 flooding, and more oil and CO2 were separated. Therefore, more oil and CO2 were immiscible in the middle section of the chip. The results of core flooding experiments showed that the oil recoveries of continuous CO2 miscible flooding and WAG were 71.3% and 75.3%, respectively. WAG produced 21.4% more CO2 than the continuous CO2 injection after water flooding, with less CO2 stored in core samples. This study provides valuable experimental data and theoretical support for applying CO2 flooding and CO2 storage in oil fields.