# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 2 Principle of Enhanced Oil Recovery by Nanomaterials
### Reduce Oil-Water Interfacial Tension  
Water flooding will leave residual oil in pores, which will exist in the center of the pores in the form of oil droplets [36]. Figure 2 is a schematic diagram of oil droplets in a pore throat. The state of oil droplets in the pore throat is not the same under high oil-water interfacial tension and low oil-water interfacial tension. When the oil-water interfacial tension is high, the oil droplets cannot be retained through the pore throat; when the oil-water interfacial tension is low, the oil droplets deform and pass through the pore throat [37]. As one of the core mechanisms of enhanced oil recovery, the attenuation of oil-water interphase tension will lead to a significant increase in the value of capillary force. This physical response mechanism lays a theoretical foundation for the optimization of oil displacement efficiency. [PERSON] et al. [38] studied the effect of nanoparticles on interfacial tension through core displacement experiments. The experimental results show that when the mass fraction of Al\({}_{2}\)O\({}_{3}\) nanofluids increases from 0.02% to 0.1%, the interfacial tension between crude oil and nanofluids decreases from 2.25 mN/m to 1.5 mN/m, and the interfacial tension decreases by 33.33%. Under the same experimental conditions, when the mass fraction of SiO2 nanofluids increases from 0.02% to 0.1%, the interfacial tension between crude oil and nanofluids decreases from 2.15 mN/m to 1.25 mN/m, and the interfacial tension decreases by 41.86%. The authors believe that nanofluids can reduce the interfacial tension, and the reduction in interfacial tension will also make oil droplets more prone to deformation,  
Figure 2: State of oil droplets in pore throat [36]. which further reduces the difficulty of their movement in pores. The viscous resistance of the pores to the oil droplets is reduced, making it easier for oil to flow for exploitation. [PERSON] et al. [39] experimentally investigated the effect of aluminum nanoparticles coated with polyvinylpyrrolidone (PVP) on oil-water interfacial tension (IFT) in low-porosity sandstone and the enhanced oil recovery mechanism. It was found that when the injection volume was 0.52 pore volume (PV), the nanoparticles could reduce the oil-water interfacial tension from 92.68 N/m to 61.86 N/m and improve the fluidity. Nanomaterials enhance the migration ability of oil phases in porous media by destroying the adhesion between oil droplets and rock surfaces. This nanofluid system shows better oil displacement efficiency than traditional chemical flooding while maintaining lower injection costs, providing a new solution for tertiary oil recovery in high-salt reservoirs. [PERSON] et al. [40] used a high-pressure and high-temperature drop interfacial tension measuring instrument to measure the interfacial tension of oil droplets. A schematic diagram is shown in Figure 3. Based on the integrated analysis of multi-source experimental data, this study systematically explored the mechanism of action between interfacial tension attenuation and enhanced oil recovery and finally reached the following key conclusions: Interfacial tension is an important index to evaluate the energy stability of water-oil two-phase interface separation. When the interfacial tension is at a low level, the two-phase interface is easier to separate. In addition, the authors also found that the addition of effective surfactants can make the two phases mix to form an emulsion phase, which further reduces the interfacial tension. That is to say, although a single nanomaterial can reduce the surface tension, its effect is not very significant. Therefore, nanomaterials and surfactants can be synergistically compounded to significantly reduce the interfacial tension between oil and water phases and effectively improve oil recovery. The experiment of [PERSON] et al. [21] supports this view.