Experimental study on phase behavior of the CO2–oil–saltwater system in a high gas–oil ratio offshore carbonate reservoir

The application of seawater alternating CO2 miscible flooding technology in offshore carbonate reservoirs shows promising potential. However, the impact of saltwater on the interactions between CO2 and oil remains unclear. In this paper, the multi-component dynamic mass transfer experiments of the CO2–oil–saltwater three-phases system were conducted by using the research method of pressure-volume-temperature analyzer, gas chromatography and high-resolution microscope. The influence of oil–water ratio and salinity on the component exchange law between oil and gas miscible multilayer transition zones is explored. The influence mechanism of saltwater on mass transfer between CO2 and oil is revealed. The results show that the ability of oil to dissolve CO2, the degree of oil expansion and gas injection viscosity reduction under bubble point pressure decrease with the increase in water content under constant total volume oil and water. Minimum miscible pressure (MMP) is increased due to the increase in water content. The extraction and mass transfer capacity of CO2 is reduced. Conversely, under a constant oil–water ratio, the ability of oil to dissolve CO2, the degree of oil expansion and gas injection viscosity reduction under bubble point pressure are enhanced with the increase in salinity. MMP is reduced due to the increase in salinity. The ability of CO2 extraction and mass transfer is enhanced. The mechanism of saltwater affecting the mass transfer of CO2–oil includes water resistance, CO2 consumption, and space competition between inorganic salt ions and CO2 molecules. The above research provides theoretical guidance for optimizing injection and production parameters of seawater alternating CO2 miscible flooding to improve oil recovery.