Phase behaviors of CO2-enriched oil system with aqueous phase under high-pressure conditions: An integrated experimental and theoretical study

Under the globally advocated initiative of carbon capture, utilization, and storage, CO2-enhanced oil recovery has emerged as the preferred injection strategy for tight or shale oil reservoirs. The CO2–oil–water interactions induce complex multiphase behavior in reservoirs, yet previous studies regarded the aqueous phase as an inert phase, largely neglecting or underestimating its impact on CO2–oil systems. This study designed and conducted pressure–volume–temperature cross-experiments with varying CO2-to-water ratios to simulate reservoir multiphase conditions, elucidating aqueous phase effects on CO2–oil systems by phase behavior perspective. The phase behavior modeling of water-bearing CO2–oil systems was achieved by coupling the Soave–Redlich–Kwong equation of state (SRK EOS) with the Cubic Plus Association (CPA) term and Huron–Vidal (H–V) mixing rules. The results show that the SRK-CPA EOS model achieves satisfactory accuracy in describing the phase behavior of water-bearing oil systems compared to the classical SRK EOS. CO2 injection exhibits greater potential for viscosity reduction due to its higher solubility. However, this mechanism is suppressed when the aqueous phase is considered due to the partitioning dissolution of CO2 in water. In CO2–oil–water systems, a rising water cut weakens CO2's viscosity-reducing effect, simultaneously resulting in greater solubility loss in water compared to CH4, a more pronounced decline in the three-phase hydrocarbon boundary line, and contraction of the three-phase region in the phase envelope. In contrast, the two-phase hydrocarbon boundary line contracts inward, which is completely opposite to that observed in oil–water systems in previous experiments. Finally, the critical water saturation threshold can be quantified by analyzing the rate of change in key parameters, such as saturation pressure. The findings of this study provide valuable insights for evaluating multiphase behavior, CO2 storage potential, injection parameter optimization, and solubility loss in water-bearing reservoirs during CO2 flooding.