# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 2 Principle of Enhanced Oil Recovery by Nanomaterials
### Improve Rock Wettability  
The wettability of the reservoir rock wall plays a leading role in the flow and distribution of fluid in a reservoir and is the key factor to improve oil recovery [46]. As its core physical property, the wetting property of a material is mainly controlled by the synergistic regulation of its micro-morphology and chemical composition [47]. [PERSON] et al. [48] pointed out in their study of high permeability core flooding that SiO\({}_{2}\) nanoparticles can form a wedge structure to generate structural separation pressure and promote the diffusion of crude oil. At the same time, its adsorption on the rock surface makes the wettability change from oil-wet to strong water-wet, so that the contact angle decreases from 110\({}^{\circ}\) to 18\({}^{\circ}\), reducing the adsorption of crude oil. The synergistic effect of the two enhances oil recovery. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Original** & **Interfacial** & **Nanofluid Type** & **Interfacial** & **Interfacial** & **Increased Oil** \\
**Fluid Type** & **Tension (mN/m)** & & **Tension (mN/m)** & **Tension Reduction** & **Recovery Rate** \\ \hline
0.02\%/Al\({}_{2}\)O\({}_{3}\) & 2.25 & 0.1\%Al\({}_{2}\)O\({}_{3}\) & 1.5 & 33.33\% & 5.92\% \\ \hline
0.02\%/SiO\({}_{2}\) & 2.15 & 0.1\%SiO\({}_{2}\) & 1.25 & 41.86\% & 6.69\% \\ \hline \multirow{2}{*}{Saline} & \multirow{2}{*}{92684.7} & PVP nanoparticles coated with aluminum & \multirow{2}{*}{61,862.0} & \multirow{2}{*}{33.26\%} & \multirow{2}{*}{—} \\
60 \({}^{\circ}\)C oil and water & 26.8 & 1 wt\% active SiO\({}_{2}\) & 20.6 & 23.13\% & 10.1\% \\ \hline \multirow{2}{*}{60 \({}^{\circ}\)C oil and water} & \multirow{2}{*}{26.8} & 1 wt\% active SiO\({}_{2}\) & 7.0 & 73.8\% & 38.8\% \\ \cline{2-2} \cline{4-6}  & &
\begin{tabular}{c} Surfactant– \\ polymer fluid \\ \end{tabular} & — & 82.21\% & — \\ \hline \multirow{2}{*}{—} & \multirow{2}{*}{—} & Amphiphilic graphene oxide nanosheets & — & — & 15\% \\ \cline{2-2} \cline{4-6}  & & In situ emulsion nanofluids (ISNE) & 0.01–0.1 mN/m & — & 17.6\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the ability of nanofluids to reduce surface tension and increase oil recovery rate. [PERSON] et al. [32] verified the adsorption of ZrO\({}_{2}\) nanoparticles on rocks and the formation of nanostructures on rock surfaces. The surfactant molecules in the nano-oil displacement agent are adsorbed on the wet surface of the rock oil and are finally combined with the pollutant molecules in the crude oil to peel it off. By removing the contaminated layer on the rock surface, the original water-wet rock surface is exposed to achieve wettability transformation [49]. [PERSON] et al. [50] showed through experimental research that polycrystalline silicon nanomaterials can effectively regulate the surface characteristics of porous media through interface modification, induce the wetting behavior of rock to change from a hydrophobic state to a hydrophilic state, and then significantly improve the relative permeability characteristics and water injection recovery efficiency [51]. [PERSON] et al. [52]. simulated the penetration process of nanofluids through a microfluidic chip and found that the time-dependent wettability alteration effects exhibited nonlinear characteristics. [PERSON] et al. [53] studied the effect of Al\({}_{2}\)O\({}_{3}\) polymer nanofluids on the wettability of sandstone cores by contact angle measurement. Firstly, the sandstone core was treated as being in an oil-wet state, and the contact angle was 144.7\({}^{\circ}\).