CO2 injection induced thermodynamic shifts in continental and marine shale oils

As carbon capture, utilization, and storage initiatives gain momentum, understanding CO2-shale oil interactions is crucial for optimizing enhanced oil recovery (EOR) and maximizing CO2 sequestration. This study provides a comprehensive analysis of the phase behavior and thermodynamic responses of medium-high maturity continental (HMC), medium-low maturity continental (LMC), and marine (Bakken) shale oils under CO2 injection. Experimental data and phase behavior modeling reveal distinct trends in saturation pressure, molecular weight, volume expansion, viscosity, and the critical role of light-to-heavy component ratios. Key findings show that, generally, CO2 injection initially raises and then lowers saturation pressure, while the high methane content in HMC A induces a continuous decrease in saturation pressure, shifting from an oil-gas coexistence state to a pure oil phase. Increased CO2 results in significant reductions in viscosity and molecular weight, especially in LMC, and promotes volume expansion in HMC and Bakken oils. Light-to-heavy ratios significantly influence phase behavior, with higher methane content enhancing CO2 solubility. Furthermore, simulations indicate that achieving miscibility requires high pressures and CO2 concentrations, with HMC A exhibiting backward-contact miscibility in contrast to the forward-contact miscibility seen in other oils. This study underscores the need for tailored EOR strategies to account for compositional variations in shale oils, with methane and CO2 co-injection offering promising improvements in miscibility and recovery efficiency.