Pore-Size- and Composition-Dependent Self-Diffusion of CO2 and n -Decane in Hydrophobic Slit Nanopores

Abstract

Abstract Diffusion of CO2 and hydrocarbons in shale nanopores controls mass transfer during CO2-enhanced oil recovery. A finite-duration grand canonical Monte Carlo (GCMC)-assisted preparation protocol followed by canonical-ensemble molecular dynamics is used to investigate molecular distributions and in-plane self-diffusion of CO2/n-decane mixtures in 4, 7, and 10 nm hydrophobic slit pores at 344 K. Varying the relative exchange-attempt frequencies produced a protocol-defined composition series whose molecular populations are then held fixed. In-plane diffusion is obtained from two-dimensional molecular center-of-mass (COM) mean-square displacements relative to the mass-weighted COM of the entire mobile fluid; three independent trajectories are analyzed per condition. Both species exhibited pronounced interfacial layering. The fraction of n-decane united-atom interaction sites in the predefined wall regions decreased in the order 4 nm > 7 nm > 10 nm, reflecting the greater proportional contribution of wall-dominated regions in narrower pores. Fluid-COM referencing reduced the long-time mean-square displacements and between-trajectory variability, particularly for n-decane. CO2 diffused faster than n-decane under all conditions, and across the prepared composition series, the diffusivities of both species generally increased with pore width and decreased with increasing n-decane mass fraction. At the highest n-decane fractions, initially wall-associated populations are less mobile than interior populations in the 7 and 10 nm pores, whereas regional effective diffusivities are comparable within statistical uncertainty in the 4 nm pore, consistent with overlapping influences of the opposing walls. Overall, wider pores and CO2-rich prepared states are associated with greater molecular mobility, whereas narrow pores and wall-associated environments restricted n-decane transport under the hydrophobic surface condition considered here.