Phase Behavior of Binary Mixtures in Shale Nanopores Using an EoS Framework Incorporating Capillary Pressure, Adsorption, and Critical Point Shifts

Abstract Shale gas is an important unconventional natural gas resource hosted in low-permeability formations containing abundant nanopores. Nanoconfinement in these formations can induce pronounced shifts and contractions of hydrocarbon phase envelopes relative to bulk behavior. In this work, an equation-of-state (EoS) framework incorporating capillary pressure, adsorption, and critical point (CP) shift is developed to investigate the phase behavior of CH4/C2H6 mixtures under nanoconfinement. First, the phase-equilibrium algorithm proposed by Sandoval et al. ( Sandoval, D. R.; et al.Energy Fuels2018, 32, 2819−283310.1021/acs.energyfuels.7b03274.) is reimplemented and verified against the reported results. However, comparisons with grand canonical Monte Carlo (GCMC) simulations reveal significant discrepancies in phase envelopes, while the predicted critical locus remains close to that of bulk conditions. This limitation arises because adsorption changes only the bulk fluid composition (in the middle of nanopores), while capillary pressure vanishes at the CP, leaving the critical locus unchanged. To address this issue, an extended phase-envelope algorithm is proposed by incorporating confinement-induced CP shifts. Different CP shift models are systematically evaluated. The results show that conventional CP shift models still exhibit notable deviations from GCMC simulations. In contrast, incorporating CP shifts derived directly from GCMC simulations significantly improves the prediction accuracy. With only minor adjustments to the critical properties, the vapor-phase saturation pressures predicted by the proposed EoS framework, accounting for capillary pressure, adsorption, and CP shift, show good agreement with the GCMC results. In addition, the results clarify that the reported GCMC phase envelope mainly represents the vapor-side condensation branch; thus, vapor- and liquid-phase pressures should be used to define dew- and bubble-point lines, respectively. This work provides a theoretical EoS framework for accurately describing the phase behavior of binary mixtures under nanoconfinement in shale nanopores.