Transport Property of Methane and Ethane in K-Illite Nanopores of Shale: Insights from Molecular Dynamic Simulations

L Zhang and C Liu and Y Liu and QB Li and QL Cheng and SY Cai, ENERGY & FUELS, 34, 1710-1719 (2020).

DOI: 10.1021/acs.energyfuels.9b04255

Shale gas is a multicomponent mixture stored in nanopores, which consists mainly of methane (CH4) and ethane (C2H6). The transport property of shale gas in clay nanopores is a fundamental issue not only for accelerating exploitation of shale gas but also for grasping the diffusivity mechanism of gas mixtures in nanostructures. In this work, the transport property of CH4 and C2H6 in K-illite nanopores of shale is investigated by molecular dynamics combined with Fick's first law. The equilibrium molecular dynamics (EMD) is utilized to calculate the binary Onsager coefficients. The thermodynamics factor, the self-, Maxwell- Stefan (MS), and transport diffusion coefficients of CH4-C2H6 are estimated, and the effects of pressure, temperature, and apertures are analyzed. The results show that the diffusion of gas in a confined space satisfies the linear law. The self-diffusion coefficient of CH4 is greater than that of C2H6, owing to the fact that surface diffusivity of C2H6 is lower than that of CH4. The MS and transport diffusion coefficients of CH4 are lower than those of C2H6. A high pressure can inhibit the diffusivity of alkanes. The larger aperture and higher temperature can enhance the diffusivity property. The reduction rate of transport diffusion selectivity for C2H6 over CH4 decreases with increasing aperture width. With increasing width of apertures, C2H6 has better diffusivity owing to the interaction of gas molecules and the wall surface. The competitive diffusion of C2H6 and CH4 is more sensitive to the formation conditions of temperature and pressure, and the transport selectivity of C2H6 over CH4 can be used to assess the diffusivity capacity between binary gas mixtures in nanopores. It is expected that this work can accommodate the secure and efficient exploitation of shale gas in nanopores with significant insights and quantitative predictions regarding the formation conditions of temperature and pressure.

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