Two-Phase Transport Characteristic of Shale Gas and Water through Hydrophilic and Hydrophobic Nanopores

摘要

Previous attempts to characterize shale gas transport in nanopores are not fully successful due to the fact that the presence of water within shale reservoirs is generally overlooked. In addition, shale is known as a wettability-varying (hydrophilic and hydrophobic) rock depending on various components and maturity grades. Herein, toward this end, we performed a comprehensive study about two-phase transport characteristic of shale gas and water through hydrophilic and hydrophobic nanopores by integrating the molecular dynamics (MD) simulations and analytical models. Using MD simulations, we showed that water molecules prefer to accumulate at the walls (water film) in hydrophilic nanopores while form the water cluster at the center region of hydrophobic nanopores, which significantly alters the shale gas transport behavior. For hydrophilic nanopores, the existence of water film weakens the gas-walls collisions (slip effect), resulting in a viscosity dominant transport mechanism. In contrary, shale gas transport in hydrophobic nanopores is mainly contributed by slip effect where the gas-gas collisions (viscosity) is abated by the water cluster. On this basis, we proposed an analytical model to quantitatively depict the shale gas transport behavior in moist nanopores, which is well verified by MD simulations results. Particularly, according to our flow model, the gas transport capacity decreases to only 15% when mixing with 50% water molecules for both hydrophilic and hydrophobic nanopores, which would be greatly overestimated by traditional models neglecting the presence of water molecules. The deep insights gained in this work will further the exploitation and development of shale reservoirs.