Improved equation of state model for the phase behavior of CO2–hydrocarbon coupling nanopore confinements

In shale reservoirs, fluids are often confined within nanopores, leading to apparent effects on the properties and phase behavior of the fluid. However, previous studies have primarily focused on the effect of capillary pressure or adsorption on well performance, and only a very limited number of studies have researched the complex and coupled impact of confinement on capillarity, adsorption, and interactions between fluid molecules and pore walls. Therefore, in this study, an effective method is developed for evaluating the coupled effects of nanopore confinement on CO₂ injection performance. First, a comprehensive thermodynamic model that incorporates adsorption, capillary pressure, and molecule–wall interaction in nanopores by modifying the Peng-Robinson equation of state (PR-EOS) is proposed. Subsequently, the calculated critical properties of different components are validated against experimental measured data, illustrating that the developed model can accurately predict the properties of the components of CO₂–hydrocarbon systems. Numerical simulations of field-scale case studies were then performed and calibrated using a modified phase equilibrium model. Typical fluid properties were inputted to investigate the effect of nanopore confinement on the CO₂ injection performance. The results of this study show that the ultimate recovery factor increases by approximately 4.61 % at a pore size of 10 nm, indicating that nanopore confinement is advantageous to well performance. Light hydrocarbons undergo more intense mass transfer than heavy hydrocarbons. Furthermore, as the pore radius decreased from 100 nm to 10 nm, the CO₂ storage coefficient increased by 2.8 %. The findings of this study deepen the collective understanding of the effect of nanopore confinement on CO₂ displacement and storage, which has significant field-scale applications.