Displacement dynamics of trapped oil in rough channels driven by nanofluids

It is well accepted that nanofluids have great potential in enhanced oil recovery (EOR). However, the EOR mechanisms by nanofluids largely remain elusive. In the study, the displacement dynamics of residual oil trapped in rough channels by different nanofluids under varied injection pumping forces are investigated by atomistic modeling. Our results indicate that both hydrophilic nanoparticles (NPs) and Janus NPs have highly obvious oil displacement effects. Specifically, hydrophilic NPs increase the viscosity and enlarge the sweeping scope of injected fluid, while Janus NPs favor either staying at the oil-water interface to reduce the interfacial tension or adsorbing onto the convex surface. Under the drag of the injecting flux, Janus NPs displace trapped oil molecules and alter the local surface wettability by sliding along the surface. In contrast, hydrophobic NPs are prone to migrate into the oil phase, which not only reinforces the trapping effect of the oil molecules by the rough surface but also poses a risk of channel blockage. Despite that the oil displacement effect of all the injection fluids is found to be less significant with low pumping force, the Janus NPs are able to maintain a stable oil displacement performance under low pumping force thanks to their sufficiently long contact time with the oil phase. Furthermore, analysis on capillary number indicates that Janus NPs have outstanding application potentials in reservoirs under realistic flooding conditions. Our findings provide atomistic insights into the mechanism of nanofluids in EOR and shed light on the selection and optimization of NPs.