Molecular insights into the composition distribution and phase behavior of hydrocarbon mixtures in a multiscale system with mixed wettability

XD Qiu and YS Liu and SZ Zheng and H Yang, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 109, 648-659 (2025).

DOI: 10.1016/j.ijhydene.2025.02.137

The phase behavior of hydrocarbon mixtures in shale formations, characterized by pore size distribution (PSD) and diverse surface wettabilities, remains a complex subject requiring elucidation. In this study, the quartz (SiO2) surface was initially modified with two distinct functional groups, namely hydroxyl (-OH) and methyl (-CH3), to confer hydrophilic and intermediate-wet characteristics, respectively. Subsequently, the phase behavior of a binary methane/n-pentane (C1/nC5) mixture in a multiscale nanopore-fracture system with mixed-wettability was investigated for the first time utilizing molecular dynamics (MD) methodology. The results demonstrate that the vapor phase initially appears in the bulk (fracture) region, followed by the -OH and -CH3 pores in a stepwise manner. Furthermore, the local density and composition distribution of the hydrocarbon mixture in two nanopores were analyzed. The results indicate that compared to the lighter component (C1), the effect of surface wettability on the phase behavior of the heavier component (nC5) was more pronounced. Notably, an intriguing accumulation phenomenon (i.e., a localized density increase) of nC5 molecules was observed at the -CH3 pore center during the depressurization process, which was absent in the -OH pore. The findings revealed in this investigation aim to enhance our fundamental understanding of the multiphase equilibria of multicomponent hydrocarbons in multiscale systems that encompass various wettability properties, which is essential for optimizing hydrocarbon production and recovery from shale/tight formations.

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