Exploring the effects of four important factors on oil–CO2 interfacial properties and miscibility in nanopores

Abstract

In this paper, a modified Peng–Robinson equation of state (PR-EOS) coupled with the parachor model and a newly-developed diminishing interface method (DIM) are applied to predict the interfacial properties and minimum miscibility pressures (MMPs) of light oil–CO₂ systems in nanopores. First, the modified PR-EOS is used to calculate the vapour–liquid equilibrium by considering the effects of capillary pressure and shifts of critical temperature and pressure. Second, a thermodynamic formula of the interfacial thickness (IT) between two mutually soluble phases is derived, based on which the novel DIM is developed. The MMP is determined by extrapolating the derivative of the IT with respect to the pressure (∂δ/∂P)_T to zero. It is found that at the pore radius of 10 nm, all three quantities, the interfacial tensions (IFTs), ITs, and MMPs, show obvious increases with temperature. The effects of the initial oil composition on the three quantities are measurable but marginal and the MMP is more sensitive to the initial oil composition at a higher temperature. Moreover, the IFTs and ITs are weakly dependent but the determined MMPs are strongly dependent on the injection gas composition. The presence of CH₄ in the injection gas results in a substantial MMP increase in nanopores. At a constant temperature, the effects of the feed ratio of injection gas to oil on the IFTs, ITs, and MMPs are negligible with pure CO₂ injection, especially at low feed gas–oil ratios (less than 0.50 : 0.50 in mole fraction), whereas they become much stronger and cause the MMPs with impure CO₂ (0.65CO₂ + 0.35CH₄) injection to be considerably increased from 26.3 to 40.0 MPa by reducing the feed gas–oil ratio from 0.90 : 0.10 to 0.10 : 0.90 in mole fraction.