Flow of Hydrocarbons in Nanocapillary: A Non-Equilibrium Molecular Dynamics Study
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AbstractUnlike flow of fluid in conventional reservoirs, the fluid transport in unconventional reservoirs involves kerogen pore structure with much smaller capillaries. This, in turn, leads to large capillary wall surface area and, consequently, to a significant physical adsorption effect. Measurements are needed to understand the nature of flow inside kerogen structures. However, direct measurements are difficult and have large uncertainties related to kerogen isolation. Non-equilibrium molecular dynamics simulation allow numerical study of fluid transport inside model nano-capillaries representative of kerogen matrices. In this work two different molecular simulation methods are used to study steady-state flow of methane and of methane-butane mixture: External Field Non-Equilibrium Molecular Dynamics (EF- NEMD), the main flow simulation method, and Dual Control Volume Grand Canonical Molecular Dynamics (DCV-GCMD), the simulation method used to verify the results. The flow inside the capillary is simulated under various conditions, e.g., capillary diameter, temperature, average pressure, fluid composition and capillary wall morphology. Based on the simulation results we observe that fluid flow velocity and mass flux are significant near the capillary surfaces where adsorption takes place. Hence, Hagen-Poiseuille equation based on the no-flow condition at the wall significantly under-estimates the fluid flow in nanocapillary. The dependence of surface transport velocity, as well as flow enhancement, are determined quantitatively. The results confirm a previous study by Riewchotisakul and Akkutlu (2015) indicating the presence of a mobile adsorbed-phase in the organic capillaries based on molecular simulation of steady-state methane flow using a moving piston model.
