Surface and confinement effects on the self-diffusion coefficients for methane-ethane mixtures within calcite nanopores

Abstract

The study of transport properties of unconventional oil reservoirs such as shale gas is crucial to optimize production, enhance oil extraction, and decrease costs and environmental hazards. One important dynamical property of fluids is diffusion, which is necessary to determine the oil mobility and rate of oil production from the reservoir, and can be obtained through the self-diffusion coefficient. To obtain the transport properties of fluids under confinement, one must take into account the surface and the confinement effects. Moreover, approaches that consider the position-dependency and the nonlinear particle distribution are required. Applying methods based on probability density function, the self-diffusion components of methane-ethane mixtures confined within calcite nanopores were calculated, using molecular dynamics simulations. The anisotropy between xx, yy, and zz components of the self-diffusion tensor has different effects depending on the position inside the nanopore. A surface effect causes an anisotropic behavior between parallel self-diffusion coefficients for this mixture close to the calcite walls. At the center of the pore, the confinement effect is manifested by lowering the value of the perpendicular self-diffusion coefficient when compared to the parallel ones. Mixture composition plays an important role, since a higher ethane content shows greater anisotropy between parallel components.