Kerogen, which is the main constituent of organic matter and plays an important role in hydrocarbon adsorption behavior in shale. In this paper, molecular dynamic (MD) simulation was performed to characterize the adsorption behavior of hydrocarbons in kerogen pores. This study showed that kerogen and hydrocarbon components have strong adsorption affinity. The structural morphology can affect their adsorption behavior and corresponding the distribution. The organic matter could make hydrocarbon fraction in shale different from that in reservoirs by inhibiting the fluidity of hydrocarbons. This study is beneficial for further studies on the transport of shale oil.
Commercial oil production from shales has not only changed the landscape of global demand and supply markets, but also demonstrated the resource potential in source rock systems globally. Although the kerogen represents only a few percentages of the total volume in shales, a large part of hydrocarbons contained in shales are stored in them. Adsorption behavior can influence the hydrocarbons transport in shales (Cao et al., 2021). Thus, it is of practical and fundamental importance to investigate the hydrocarbon adsorption and distribution within the kerogen pores in porous media.
Molecular dynamics (MD) simulations are beneficial for investigating the adsorption capacity of hydrocarbons and their occurrences in pores of organic matter or minerals in shales (Wang et al., 2016; Yang et al., 2020). In order to study the adsorption characteristics of hydrocarbon and kerogen, we chose different hydrocarbon molecules, which represented different hydrocarbon components, and carried out the simulation process, respectively.
Kerogen has a wide range of chemical compositions and physically complex molecular structures. Molecular model of type II kerogen (C234H263O14N5S2) in the middle of the oil window from Ungerer et al. (2015) was chosen for simulation. Besides, methane, n-octane (n-C8H18), n-octadecane (n-C18H38), toluene (C7H8), 3-methylphenanthrene (C15H12), and porphyrin (C20H32N4), representing various hydrocarbon components, were also collected (Figure 1). The cross-sectional size of the supercell models was set approximately 5 × 5 × 25 nm3 with kerogen matrixes (average density=0.98 g/cm3) packed in both side of the box along y direction. The aperture of the slit between the two kerogen bulks is approximately 10 nm. In order to investigate the micro adsorption characteristics, we also set an approximately 2.5 × 2.5 × 2.5 nm3 box with two kerogen molecules which were randomly distributed.