A novel model for phase equilibrium calculation of CO2-hydrocarbon fluids under nanopore confinement

Due to the widely existing nanopores in unconventional hydrocarbon (HC) reservoirs, the substantial capillary pressure and increased pore−fluid interactions would significantly affect multiphase fluid transport. This study aims to enhance the understanding of the effect of nanopore confinement on fluid phase behavior, especially for CO2−HC mixture, by introducing new terms related to nanopore confinement into the classic equation of state. It also establishes a new dimensionless relationship between the pore radius and critical property shifts. A novel hybrid algorithm is established to solve the phase equilibrium calculation, which is then validated against experimental data from the literature and calculated results by commercial software. A systematic comparison was also conducted on the influences of key factors, such as critical property shift, capillary pressure, and fluid−wall interactions. The results show that nanopores significantly affect the properties of each fluid phase when pore radii are below 100 nm. It would lead to a shrinkage of the two-phase region and suppression of bubble-point pressure. It also reduces the liquid-phase density and viscosity due to the enrichment of light HC components in the liquid phase. When pore radii fall below 50 nm, it would significantly lower the minimum miscibility pressure between CO2 and crude oil, which is critical for optimizing injection pressure for CO2-enhanced oil recovery in unconventional reservoirs.