Fluid Imbibition and Oil Displacement in Quartz Nanoporesduring the Shut-in Process: A Molecular Dynamics Perspective

Abstract In shale oil reservoirs, shut-in after hydraulic fracturing is commonly used to enhance recovery, with imbibition occurring in the nanopores playing a crucial role in this process. This study employs molecular dynamics (MD) simulations to investigate the microscopic mechanisms of water imbibition and oil displacement in quartz nanopores. The results reveal that the imbibition process is characterized by three distinct stages: an initial delay phase, during which entry resistance is overcome; a rapid advancement phase, where water infiltrates the pores and displaces the oil; and a final stabilization phase as the system reaches equilibrium. The duration of these stages is influenced by both pore size and pressure difference. Specifically, owing to the stronger water–solid interaction energy and enhanced hydrogen bonding, smaller pores exhibit faster water invasion rates and more efficient oil displacement, with the fitted imbibition-front slope decreasing by 21% as the pore size increases from 6 to 10 nm. Furthermore, increasing the pressure difference between the outlet and inlet weakens hydrogen bonding and the interactions, thereby delaying the imbibition process and diminishing oil displacement efficiency. As the pressure difference increases from 0 to 6 MPa, the fitted imbibition-front slope decreases by 35%, and the oil-displacement slope decreases by 24%. These findings provide valuable insights into the MD of fluid imbibition and oil recovery in nanopores.