(679c) Water-Lubricated CO2 and CH4 Transport in Crystalline Silica Mesopores:a Molecular Dynamics Study

The knowledge about gas transport under nano-confinement plays vital roles in various engineering applications, such as permeability prediction, gas capture and separation, etc. Particularly, pressure-driven flow behaviors of CO₂ and CH₄ in mesopores are important for geological carbon sequestration (GCS) and subsurface energy extraction in shale/tight formations where pore sizes range from a few nanometers to several micrometers. Specifically, crystalline silica with an orderly structure on its surface, which is one of the most abundant minerals in subsurface formations, typically exhibits strong hydrophilicity. In addition, water widely exists underground and tends to form water films on silica surfaces. Therefore, understanding CO₂ and CH₄ pressure-driven flow behaviors with the presence of water films in silica mesopores becomes imperative to GCS and gas extraction.

Generally, pressure-driven gas transport without nanoconfinement can be regarded as continuum flow, which can be described by the hydrodynamic Hagen-Poiseuille (HP) equation with no-slip boundary condition, assuming homogeneous fluid distributions. However, Holt et al. (Science 2006, 312 (5776), 1034-1037) reported that fluid flow enhancement over the HP equation with no-slip boundary condition can be several orders of magnitude in sub-2 nm carbon nanotubes (CNTs). Such enhancement is attributed to specular reflection thanks to the smooth surfaces, which is also observed by molecular dynamic (MD) simulation studies about slippage of CH₄ transport. Furthermore, Firouzi and Wilcox (The Journal of chemical physics 2013, 138, 064705) reported that CO₂ shows less significant slippage compared to CH₄ in carbon nanopores, because of stronger gas-surface interactions. Recently, our MD studies showed that CO₂ molecules on β-cristobalite surfaces can be “stuck” in the ring structures, which can hinder its transport, while CH₄ displays non-negligible slippage. On the other hand, Ho et al. (Nanoscale 2018, 10 (42), 19957-19963) (Physical Chemistry Chemical Physics 2019, 21 (24), 12777-12786) reported that the CO₂ layer can serve as a lubricant layer between water and kerogen, oil and muscovite, as well as oil and kerogen, while water film can reduce friction between oil and muscovite. Therefore, it is intriguing to study how water films influence CO₂ and CH₄ transport in crystalline silica nanopores (such as β-cristobalite). Although the lubricant effect of an atomistic layer on transport has been investigated before, how β-cristobalite surface structure and water films collectively influence CO₂ and CH₄ flow behaviors in terms of their molecular orientations and configurations still remains elusive.

To investigate pressure-driven CO₂ and CH₄ flow with water films in silica mesopores under a typical reservoir condition (323 K and 20 MPa), molecular dynamic (MD) simulations are conducted to study the collective effects of specific surface structures, fluid molecular characteristics, fluid-surface and fluid-fluid interactions. The pore size is kept at ~5 nm as a representative β-cristobalite mesopore. The number of water molecules is determined based on the monolayer configuration near the surface, while for comparison, pure CO₂ and pure CH₄ flow are also studied. The water films on surface generally deplete gas molecules, leading to substantial decreases in gas surface adsorption. In addition, due to the presence of water films, gas-surface friction coefficients are drastically reduced. In the presence of a water film, there is a reduction in the number of CO₂ molecules oriented perpendicularly to the surface. As a result, CO₂ topological accessible planes are less curved, which enhances the mobility of the interfacial CO₂ molecules. However, CH₄ topological accessible planes show an insignificant change with the water film. It suggests that CO₂ transport behaviors are more sensitive to the presence of water films on the β-cristobalite surfaces.

This study elucidates the lubricant effect of an atomistic water layer on gas transport, which is dictated by fluid-fluid and fluid-surface interactions as well as surface and fluid molecular characteristics, in β-cristobalite mesopores from molecular perspectives. This investigation can important insights into GCS and energy extraction in subsurface formations.