Nanoscale Miscibility Pressure Prediction Model With Critical Property Shifts and Adsorption Effects

Abstract

Abstract. Carbon dioxide flooding delivers dual benefits by increasing oil recovery and storing greenhouse gases, and shows strong application potential in unconventional reservoirs. These reservoirs contain abundant nanopores where strong wall fluid interactions and confinement effects substantially modify fluid thermodynamic properties. As a result, the conventional Peng Robinson (PR)-equation of state (EOS) exhibits noticeable deviations when predicting phase equilibrium at micro and nanoscale conditions. This study reviews CO2–crude oil behavior in nanopores and synthesizes current understanding of critical property shifts, phase envelope deformation, and reduction of minimum miscibility pressure. Existing approaches most often treat a single mechanism and lack integrated predictive models that couple multiple confinement effects. In this work, a dimensionless correlation between critical properties and pore size was developed on the basis of experimental observations and molecular simulations. This correlation was combined with adsorption-layer thickness adjustment, capillary pressure evaluation, and volume translation to construct a modified equation of state. The model was applied in multicomponent flash calculations together with the multiple mixing cell method to estimate minimum miscibility pressure under confinement. Validation indicates high accuracy and numerical stability across wide ranges of pore size, composition, and temperature, and the model successfully reproduces confinement-induced changes in saturation pressure and overall phase behavior. Results demonstrate that both critical temperature and critical pressure decrease nonlinearly with decreasing pore size, with the strongest variations occurring below about 10 nm. Phase behavior shifts toward lower pressure and temperature, and the two-phase region becomes narrower. Minimum miscibility pressure also decreases markedly as pore size is reduced. The study reveals the coupled multi-mechanism nature of CO2 flooding in unconventional reservoirs and provides a theoretical basis and technical guidance for optimizing injection strategies and evaluating storage potential.