Nanoscale liquid hydrocarbon adsorption on clay minerals: A molecular dynamics simulation of shale oils

As adsorption can influence the phase behavior of shale oils, and consequently their transport, understanding the adsorption behavior of liquid hydrocarbons in shale is essential for the utilization of this valuable resource. In this work, the adsorption of n-nonadecane (saturated hydrocarbon), toluene and 3-methylphenanthrene (aromatic hydrocarbons), and porphyrin (resin) in the slits of clay minerals (montmorillonite, kaolinite, and illite with 10 nm slit apertures) at 358 K and 30 MPa was evaluated through molecular dynamics simulations. The single adsorption layer thicknesses of different hydrocarbons adsorbed on montmorillonite were 0.31–0.48 nm. The total thickness and volume proportion of the adsorption layers, which can be used as a measure of adsorption capacity, were 0.96–1.57 nm and 12.0–23.5%, respectively, and varied with the hydrocarbon component in the order of resin > aromatic > saturated hydrocarbon. The hydrocarbon adsorption capacity of montmorillonite is stronger than that of illite and kaolinite. The structural morphology of hydrocarbon molecules, in addition to the adsorbent pore size, strongly affects adsorption behavior. The adsorption of the long-chain alkane n-nonadecane occurred with minimal distinction between layers. This previously predicted disorder was seen to increase with distance from the mineral surface. The adsorption capacity increased with the polarity of the hydrocarbon component owing to the induction of van der Waals forces, which are reduced along with the Coulomb force at greater distances between the hydrocarbon and clay minerals. Adsorptive affinities are important for crude oil flow rates. Shale oil contains more high-adsorptive oil fractions, making it less fluid and difficult for production. Although this study profiles the nanoscale adsorption behavior of hydrocarbons, it can also function as a meaningful reference for the assessment of shale oil fluidity.