Visualizing oil displacement by nanofluids at pore scale: A concentration-dependent nanofluid spreading induced by structural disjoining pressure

Immiscible fluid displacement in porous media is governed by pore-scale behaviors, which can be manipulated by chemical additives to engineer the process toward a greater turn-out. Although recent advances in nanofluids have been reported to influence such a process, their interfacial phenomena are likely controversial and need independent cross-examinations. As non-energetically interfacial responsive nanoparticles, silica cores adorned with polyvinylpyrrolidone were examined for their direct contribution to crude oil displacement performance at relatively low concentrations, ranging from 10 to 500 ppm, in the current study. The crude oil displacement was experimented via water-wet borosilicate micromodel and visualized to elucidate pore-scale interfacial phenomena involved. Concentration-dependent property of nanofluids was found, evidenced by different pore-scale mechanisms observed. At low concentrations (10 and 50 ppm), wetting layer flow controlled the oil displacement and led to swelling into pore space, inducing snap-off events and hence high oil ganglia trapped (> 300). At higher concentrations (100 ppm), nanoparticle self-arrangement at the water wedge was more effective, which induced oscillatory structural disjoining pressures between the oil-aqueous and solid-aqueous interfaces leading to narrow nanofluid spreading. Hence, the spatiotemporal displacement performed differently at high concentrations (displacement efficiencies were 36.8% at 100 ppm and 43.1% at 500 ppm), with snap-off hardly observed. At 500 ppm, more stable and stronger nanofilm spreading was developed due to meniscus expansion, obtaining faster-displacing dynamics (54.9% per pore volume injected) and additional oil displaced (+5.7%) after breakthrough time. The findings amplify nanofluid contribution and emphasize its concentration dependence on immiscible fluid flow in porous media, a potential applicability to various fields including enhanced oil recovery and CO₂ geological storage.