Critical shifts of fluids in shale nanopores under confinement effects using a modified Redlich Kwong equation of state

Shale oil or gas, as emerging focal points in current exploration and development activities, have garnered extensive attention from the global energy industry regarding their commercial exploitation. The widespread distribution of nano-scale pores and micro-scale fractures in shale reservoirs results in markedly heterogeneous and complex fluid distribution patterns and fluid properties within the reservoir. Accurate characterization of fluid phase behavior in shale reservoirs is crucial for optimizing exploration and development strategies. This study developed a modified RK equation of state (EOS) by incorporating the adsorbed layer thickness and reduced adsorption density to adjust the effective molar volume, which enables quantitative characterization of adsorption effects on critical temperature and pressure. Experimental and simulation data of critical temperature shift from literature were systematically collected to establish correlations between two correction coefficients and dimensionless pore diameter. Subsequently, the modified RK-EOS was solved, yielding analytical expressions for predicting critical property variations of confined fluids in nanopores. Finally, the evolution patterns of critical properties under different pore sizes were investigated. The results demonstrate that the modified RK-EOS model can accurately predict the influence of confinement effects on fluid critical properties, thereby providing a reliable theoretical foundation for numerical simulation of shale oil/gas reservoirs and optimization of development strategies.