Effect of confinement on the vapor-liquid-liquid three-phase equilibrium during CO2 utilization and sequestration in shale reservoirs

With the rising global energy demand, shale gas and oil emerge as pivotal resources. Recent innovations utilizing CO2 as an injectant can effectively enhance shale oil and gas recovery and facilitate CO2 storage within shale reservoirs. However, low-temperature CO2 injection may result in the coexistence of three hydrocarbon phases, while the abundant nanopores in shale formations also notably influence the phase behavior of reservoir fluids. To optimize shale oil recovery and CO2 sequestration in shale formations, it is a prerequisite for precisely capturing the effect of confinement on the phase behavior of reservoir fluids within nanopores during CO2 injection. In this work, we introduce a novel three-phase vapor-liquid-liquid equilibrium calculation algorithm, which is designed to handle the unique phase behavior challenges presented by CO2 utilization and storage in shale reservoirs. To improve the robustness and efficiency, the proposed algorithm integrates a trust region-based stability test with a hybrid flash calculation algorithm that combines the Newton-Raphson and trust-region methods. Our thermodynamic model incorporates the capillarity effect and shifts in the critical points due to molecule-wall interactions, which are essential for accurate phase behavior simulation under confinement. Initial validations against experimental bulk phase data show promising results, and further investigations indicate that confinement alters three-phase vapor-liquid-liquid equilibria by suppressing two-phase and three-phase regions and shifting boundaries in the phase diagrams. The proposed algorithm not only advances our understanding of multiphase equilibrium in nanoporous media but also enhances the practicality of CO2 sequestration and improved oil recovery strategies in shale formations. Document Type: Original article Cited as: Chen, Z., Li, R., Du, Y., Ma, S., Zhang, X., Shi, J. Effect of confinement on the vapor-liquid-liquid three-phase equilibrium during CO2 utilization and sequestration in shale reservoirs. Advances in Geo-Energy Research, 2025, 16(3): 199-210. https://doi.org/10.46690/ager.2025.06.02