Insights into adsorption and diffusion behavior of shale oil in slit nanopores: A molecular dynamics simulation study

Nanoscale pores have been widely observed in shale plays. Within such a small scale pore, the interaction between fluid molecules and porewall surface becomes significant, and the adsorption state of shale fluids is quite different from that in conventional reservoirs. In this study, the method of molecular dynamics (MD) simulation is applied to address the adsorption and diffusion behavior of shale oil in slit nanopores. Firstly, two different models of organic graphene nanopore and inorganic quartz nanopore are constructed, and three typical components of alkanes (methane, propane, and n-octane) are selected to represent the shale oil fluids. Based on this model, the effects of fluid composition, nanopore type and porewall wettability on the adsorption and diffusion behavior of shale fluids are discussed. For the simulation method of hydrophilic porewall surface, in this study, a series of carbonyl functional group is grafted on graphene surface to enhance the hydrophilicity of porewall surface. Results indicate that the multicomponent alkanes in nanoscale pores can form a multilayer adsorption state. The heavy components will occupy the adsorption sites around the porewall surface. In contrast, the adsorbed amount of light components at porewall surface can be almost negligible. However, as the effect of multicomponent fluid is considered, a significant difference for the properties of adsorption layer and the mean square displacement (MSD) value can be observed compared with that of the pure component fluids. Similarly, shale fluids can also show a multilayer adsorption behavior of heavy components at the quartz nanopore wall surface, but the amount of adsorption is lower. Comparing the peak density of the first adsorption layer in organic matter pores with that in quartz nanopore, the former shows 1.87 times more than the latter for ternary mixture (C1-C3-C8). Furthermore, it can be also observed that the adsorption capacity of fluid in the nanopores with different wettabilities differs. As the O/C ratio in organic matter wall is above 15%, the peak density of adsorption layer is significantly increased with the carbonyl proportion increases. But this changing tendency is decreased as the O/C ratio reaches above 50.46%. Furthermore, from the simulation results, it is observed that with the carbonyl proportion increases, the fluid self-diffusion coefficient is gradually reduced. It indicates that the effect of rock wettability on the fluid diffusion behavior. This study can provide an important data support to understand the adsorption and diffusion behavior of shale oil in nanopores, which is important for the effective and efficient development of shale oil reservoirs.