Nanoscale Acentric Factor Framework for the Prediction of Confined Fluids’ Phase Behavior in Shale Reservoirs

Although numerous modifications to cubic equations of state (EOS) have been proposed to describe confined fluids, all of them still rely on bulk Pitzer acentric factors, leading to partial bulk-confined parametrization. This reliance represents a fundamental thermodynamic inconsistency as the acentric factor is intrinsically linked to the critical temperature and pressure, both of which shift markedly under nanoconfinement. To tackle this inconsistency, the nanoscale acentric factor was established through an analytical derivation within a free-energy framework constructed on the square-well intermolecular model as a function of pore radius and the potential well depth (ε). Comparison with experimental and molecular simulation data for pure, binary, and ternary mixtures relevant to shale reservoirs confirms substantially enhanced predictive accuracy: 1.30% for the critical temperature shift, 4.08% for interfacial tension, 6.59% for bubble point temperature, 4.15% for dew point pressure, 6.06% for bubble point pressure, and 4.81% for minimum miscibility pressure (MMP). Our findings underscore that incorporating a confinement-dependent acentric factor in EOS is indispensable for realistic phase envelope prediction in shale reservoirs.