Interfacial Tension Suppression of Confined Hydrocarbons in Nanopores: Insights from Grand Canonical Monte Carlo Simulations

Understanding interfacial tension (IFT) under nanoconfinement is critical for predicting fluid behavior in unconventional hydrocarbon reservoirs, subsurface CO2 sequestration scenarios, where caprocks are typically shale or high clay-content formations, and in nanoporous materials in general. In this study, we employ grand canonical Monte Carlo (GCMC) simulations combined with the Irving–Kirkwood approach to investigate the IFT of pure and multicomponent hydrocarbon systems confined within slit-shaped nanopores of varying widths. The results reveal a significant reduction in IFT with decreasing pore size, particularly when the pore width falls below 10 nm. In ultratight pores (2–4 nm), IFT is suppressed by up to 70% relative to bulk values, due to overlapping adsorption layers and reduced phase separation. We introduce a physically meaningful metric, the characteristic adsorption layer thickness, by correlating simulation results with the classical parachor model. This provides a framework for defining phase boundaries and estimating phase saturations within nanopores. Additionally, we present a computationally efficient extrapolation method based on the inverse relationship between fluid density and GCMC iteration number, achieving accurate equilibrium property predictions with up to 80% reduction in simulation time. These findings offer new insights into the molecular origin of interfacial phenomena in confined systems and establish practical methodologies for the behavior of the fluids at the interfaces linking molecular simulations with reservoir-scale multiphase flow models.