Determination of Minimum Miscible Pressure and Phase Behavior of CO2–Shale Oil in Nanopores

Abstract

CO₂ injection into shale oil reservoirs has gained widespread attention as it not only enhances oil recovery but also facilitates the geological storage of CO₂. Shale oil reservoirs are characterized by numerous nanopores, and the resident fluid exhibits unique phase behavior due to nanoconfinement effects, resulting in the failure of the traditional theoretical model and microfluidic experiment in depicting the minimum miscible pressure (MMP) of CO₂–shale oil in nanopores. This study proposes a method for calculating the MMP of CO₂–shale oil in nanopores. It combines the three-phase equilibrium calculation method of gas phase–liquid phase–adsorbed phase with the multiple mixing cell (MMC) method. The method considers capillary pressure and corrects the critical properties influenced by the nano-limited domain effect. Using this approach, the phase behavior of CO₂–shale oil in nanopores and the MMP are investigated. It is found that as the pore radius decreases, the area of the two-phase region surrounded by the phase envelope of multicomponent fluids gradually shrinks. The smaller the pore radius, the more significant the shrinkage of the two-phase region. The entire phase envelope and the critical point shift to the left and down on the coordinates, and this phenomenon is more pronounced for pore radii less than 20 nm. The presence of CO₂ increases the system’s critical condensation pressure and decreases the critical condensation temperature. The MMP of the 6-component system of CO₂–shale oil for a pore radius of 5 nm in this paper is 63.7 bar at 101°C.