Molecular dynamics study on mechanism of preformed particle gel transporting through nanopores: surface hydration

Abstract

Preformed particle gels (PPG) as a potential oil-displacement agent, composed of cross-linked partially hydrolyzed polyacrylamide, are being applied to promote the oil recovery ratio in several oil fields in China. At the molecular level, a molecular dynamics simulation of PPG transporting through nanopores was performed to investigate its propagation mechanisms during gel injection. Initially, a silica nanopore was modeled as a finite-length cylindrical pore, in which the inner surface was fully hydroxylated. Then, a swollen PPG with a smaller size was put in. After a long enough simulation, the hydration layer induced by silica pore surface was discussed to study the effect on the transport of the PPG. Steered molecular dynamics was then used to mimic the transport of the PPG under injection pressure. The results suggested that this hydration layer served as a physical and energy barrier that keeps the PPG away from the pore surface by analyzing radial number density distributions, orientational arrangement, dependence of the diffusive mobility, hydrogen bonding characteristics and potential of mean force. In addition, the lubrication of the hydration layer may reduce the resistance that the PPG has to overcome while transporting through nanopores. These factors will promote the propagation of the PPG within the nanopores and reduce the injection pressure. The simulated results are expected to provide molecular level insights into the mechanism of PPG transporting through nanoporous media or the molecular design of optimized PPG.