Effects of CO2 injection volume and formation of in-situ new phase on oil phase behavior during CO2 injection for enhanced oil recovery (EOR) in tight oil reservoirs

CO2 injection in tight oil reservoirs is a proved method for enhanced oil recovery. The mass transfer between CO2 and oil makes the reservoir fluid phase behaviors very complex, and directly affects the recovery process. In this paper, a comprehensive study of interactions between CO2 and live oil and their phase behaviors were conducted by CO2 visual injecting experiment. Based on the calculation of phase equilibrium and entropy, the mass transfer of CO2 and live oil was divided into four stages, and the mechanism of CO2-live oil interactions elucidated. The results indicate that after CO2 injection, the oil will expand immediately, and the expansion of the oil volume is proportional to the pressure increase. In the early stage of CO2 injection, the mass transfer between CO2 and live oil will form an in-situ gas phase. The reduction in light oil components is mainly due to the CO2 dissolution and replacement, supplemented by the CO2 extraction, which resulted in a rapid decrease in oil volume. With the increase in CO2 injection, its ability to dissolve in the oil and replace the oil components weaken, and CO2 extraction becomes the main controlling factor for the reduction of light components of oil. Increasing pressure promotes the extraction capacity of CO2 for light oil components. Entropy change can accurately reflect the mass transfer process between CO2 and oil, during CO2 injection. These results can furnish an in-depth understanding of the complicated mechanisms and phases behavior in CO2 EOR in tight reservoirs.