The role of the potential field on occurrence and flow of octane in quartz nanopores

Occurrence and flow of hydrocarbons in nanopores are two important issues in the effective exploitation of shale oil reservoirs. In this study, molecular dynamics simulations are employed to investigate the mechanisms about occurrence and flow of octane in slit-shaped quartz nanopores. We show that the occurrence state of octane and, therefore, its flow behavior are profoundly affected by the potential field from quartz walls and adsorption layers if the nanopore width w becomes less than 50 Å. Two main adsorption layers are always formed, adjacent to the walls and independent of w, due to two potential wells generated by the attractive potentials of the walls. Each pair of symmetrical adsorption layers, each of which can be considered as a solid-like surface, forms a confined environment similar to a nano-slit. The attractive potentials from them are found to be the cause for the formation of the adsorption layers between them. The obvious bulk phase of octane is formed in the pore of w = 50 Å due to the wide zero potential barrier induced by the innermost two adsorption layers. The nonlinear dependence of flow rate on pressure gradient shows that Darcy's law fails to describe the flow in the nanopore. The non-Darcy behavior mainly arises from adsorption effects from the walls and the adsorption layers, slippage between octane and walls and between adjacent two adsorption layers, and the molecular exchange between adsorption layers. A modified microscopic model is established to predict the dependence of flow rate on potential field, pressure gradient and w, which is in a good agreement with our simulation results and verified by the dodecane flow through the nanopore. Our work can be of great importance for revealing the mechanisms of occurrence and transport and guiding the estimation and exploitation of shale oil resources.