Molecular insight into the boundary conditions of water flow in clay nanopores

Abstract

Understanding the underlying mechanisms of water flow in clay nanopores plays an important role in groundwater hydrology and shale production. In this work, we use molecular dynamic (MD) simulations to study water flow in various clay nanopores under a typical reservoir condition (323 K and 20 MPa) and investigate the boundary conditions to describe the water flow by using a hydrodynamic flow model. We found that only the first water adsorption layer is immobile, while other water molecules can flow with a slight slip velocity on the first water adsorption layer for a pressure-driven Poiseuille flow. In addition, the critical thickness is from 0.34 to 0.4 nm, which is almost half of the values applied in previous theoretical models. Such small difference in the critical thickness could result in large discrepancies on the water flow prediction. Our work should shed lights into water flow mechanisms in clay nanopores and provide reliable boundary conditions for the hydrodynamic flow models to predict water flow in unconventional reservoirs.