Abstract
Fundamental understandings of nanoconfined methane (CH4) are crucial to improving the exploitation of tight gas. In this study, diffusivity, one of the key transport properties of high-temperature and high-pressure methane gas, is examined under confinement in the silica nanochannels by using molecular dynamics simulations by employing Einstein diffusion equation. It was found that the diffusivity of nanoconfined methane is obviously anisotropic, namely, the perpendicular diffusion coefficient is lower than that in the longitudinal direction. The anisotropic diffusivity of nanoconfined methane is attributed to the restricted effect of potential interactions from the atoms of walls, which is verified by analyzing the diffusivity of methane molecules in the potential wells with Lagrangian dynamics. The diffusion coefficients of nanoconfined methane decrease with the increase of atomic potentials in the wall, which can be explained by the density distributions of methane in the nanochannels. Furthermore, we reveal the dependence of the diffusivity of nanoconfined methane on the channel height and confining effect of the wall on the diffusivity of methane molecules. The obtained results can provide a molecular insight into the transport properties of methane confined in nanospace and a theoretical guidance for the efficient extraction of tight gas.
Highlights
Natural gas is a high-quality, efficient, green, and clean energy, which occupies an important position in the world’s energy consumption
We adopt molecular dynamics (MD) simulations with the Einstein diffusion formula to study the diffusivity of high-temperature and high-pressure methane confined in silica nanochannels and reveal its underlying physical mechanisms
The results show that the diffusion coefficients in the z-direction are obviously lower than those in the x and y-directions, indicating that the diffusion coefficients of nanoconfined methane molecules are anisotropic
Summary
Natural gas is a high-quality, efficient, green, and clean energy, which occupies an important position in the world’s energy consumption. The proportion of natural gas in the energy structure of many countries, such as China, is much lower than the world’s average level. The development and utilization of natural gas is a major policy to build a safe, efficient, clean, and low-carbon energy system for those countries. Along with the additional exploratory development of natural gas, a large number of tight gas reservoirs (Zou et al, 2009) have been discovered all around the world. The existing data have indicated that there are a large number of pores in nanoscale in tight gas and shale gas reservoirs (Chalmers and Bustin, 2008; Curtis et al, 2012; Wang et al, 2014), meaning that the natural gas is confined in these nanoscale pores of the rocks. The transport properties of nanoconfined fluid (Sun et al, 2020) could present
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