Two-step phase transition behavior of hydrocarbons in undulated nanopores

Shale rocks contain numerous nanopores with complex geometries. The translational symmetry of these nanopores is broken not only in the direction perpendicular to the pore walls but also along the pore walls. In narrow parts of heterogenous nanopores, enhanced surface interactions induce early vapor–liquid phase transitions in fluids, forming local liquid-bridge phases. The liquid-bridge phase caused by pore heterogeneity may exert a key influence on the phase transition mechanism of shale hydrocarbon reservoirs. However, most existing studies focus on slit-like nanopores ignoring the pore heterogeneity perpendicular to the pore walls. Systematic investigation into the conditions and factors governing liquid-bridge phase formation in heterogeneous nanopores is still unclear. In this study, hybrid Molecular Dynamics and Grand Monte Carlo simulations were employed to explore the phase transition mechanism of alkane in undulated nanopores. We found that two-step phase transitions of hydrocarbon involving vapor, liquid-bridge, and liquid phases were observed in heterogeneous nanopores. There is a threshold average pore widths determining the phase transition mechanism. Below this value, the liquid-bridge phase cannot be stably existed in undulated nanopore. Notably, the relationship between pore heterogeneity (undulation, average radius, and carbon chain length) and the threshold average pore width was proposed. This helps classify the phase transition mechanism of confined hydrocarbons and determines if the liquid-bridge phase can remain stable. These findings offer new insights into how nanopore geometry affects hydrocarbon phase behavior and support shale reservoir development.