Hydrocarbon Self-Diffusion and Assessing the Validity of Graham's Law under Nanoporous Confinement in Shales

F Perez and D Devegowda, ENERGY & FUELS, 35, 10512-10518 (2021).

DOI: 10.1021/acs.energyfuels.1c00735

Due to the ultralow porosity and permeability of shales, molecular diffusion plays an important role in fluid migration and transport. Several studies have experimentally quantified diffusion coefficients in the bulk phase, but it is wellknown that confinement effects decrease diffusion by several orders of magnitude. Additionally, direct measurements of diffusion coefficients in porous media remain challenging and elusive. Using molecular dynamics simulations, this study quantifies the self-diffusion coefficients of a diverse set of hydrocarbons in shale organic nanopores at representative reservoir conditions. The hydrocarbons range from low to high molecular weight and from alkane chains to aromatic structures. The order of magnitude obtained for the self-diffusion coefficients is 10(-10) m(2)/s (one or two orders of magnitude smaller than their bulk values reported elsewhere) and corresponds very well to experimentally derived estimates. The scaling of the self-diffusion coefficients with molecular weight as derived by Thomas Graham is shown to be valid even under nanoporous confinement and insensitive to whether a specific species is adsorbed or in the free fluid phase at reservoir conditions. More importantly, this empirical, yet fundamental, scaling law initially derived for gases, is shown to be applicable to confined liquids. This makes the treatment of nonideal, multicomponent liquids straightforward and less subjective with applications in modeling oil primary production, solvent-oil interactions during enhanced oil recovery, and quantifying bypassed oil and produced fluid compositions in unconventional shales.

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