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.