# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 2 Principle of Enhanced Oil Recovery by Nanomaterials
### Improve Rock Wettability
Polyacrylamide (HPAM), SiO\({}_{2}\), and Al\({}_{2}\)O\({}_{3}\) nanofluids were treated at 27-90 \({}^{\circ}\)C. The experimental results in Figure 5 show that the contact angle decreases to 100.3\({}^{\circ}\), that of SiO\({}_{2}\) nanofluids to 78.6\({}^{\circ}\), and that of Al\({}_{2}\)O\({}_{3}\) nanofluids to 60.6\({}^{\circ}\) after HPAM treatment at 90 \({}^{\circ}\)C. At this time, the wettability of the core is converted from oil-wet to water-wet, and the change in wettability is attributed to the adsorption of nanoparticles on the core surface. Nanoparticles replace the oil film by structural separation pressure and hydrogen bonding, and the increase in temperature can enhance the adhesion efficiency between the particles and the core surface. [PERSON] et al. [54] also reported that differences in capillary force will affect wettability and that measurement of the contact angle can determine the wettability of rock. The contact angle of limestone was measured to be about 90\({}^{\circ}\) at 26 \({}^{\circ}\)C, and the \(\theta\) values in the presence of Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and SiO\({}_{2}\) nanofluids were 71\({}^{\circ}\), 57\({}^{\circ}\), and 26\({}^{\circ}\), respectively. The recovery rates of Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and SiO\({}_{2}\) nanofluids were 52.6\({}^{\circ}\), 50.9\({}^{\circ}\), and 48.7\({}^{\circ}\), respectively. The contact angles of Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and SiO\({}_{2}\) decrease with the increase in temperature. We can find that although SiO\({}_{2}\) has a higher adsorption capacity on limestone surfaces than Al\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\) nanofluids (a strong adsorption capacity means a smaller contact angle), the effect of enhanced oil recovery is inferior to that of Al\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\) nanofluids, that is to say, the oil displacement effect of nanofluids in strong hydrophilic  
Figure 5: Advancing air/water contact angle on oil-wet sandstone surface in varying temperature conditions [53]. reservoirs is not as good as that in neutral wetting reservoirs. This phenomenon underscores the critical relationship between contact angle and capillary forces: a reduction in contact angle directly lowers the capillary entry pressure according to the Young-Laplace equation. However, excessively low contact angles (e.g., \(<\)30\({}^{\circ}\)) in strongly water-wet systems may exacerbate water channeling and bypass residual oil, whereas neutral wettability (\(\theta\approx 90^{\circ}\)) is minimized, maximizing oil droplet mobilization efficiency. Thus, the optimal contact angle range observed in Al\({}_{2}\)O\({}_{3}\)/TiO\({}_{2}\) nanofluids directly correlates with higher recovery rates by balancing capillary-driven displacement and fluid mobility control. [PERSON] et al. 's [55] research can further illustrate this view. Studies have shown that some strategies to convert the wettability of rock from water to neutral can reduce the capillary force [56] and significantly improve the recovery rate. Hydrophilic SiO\({}_{2}\) fluid can change sandstone from water-wet to strongly water-wet, and the contact angle decreases from 36\({}^{\circ}\) to 10\({}^{\circ}\), which cannot improve oil recovery. However, the hydrophobic SiO\({}_{2}\) nanofluid can convert the wettability of the rock into neutral wettability, that is, the contact angle is about 90\({}^{\circ}\), and the oil recovery rate is significantly improved. Therefore, we can conclude that the change in rock wettability will significantly affect oil recovery, and wettability can be used as an important parameter to improve oil recovery. In summary, this section summarizes the changes in the contact angle, wettability, and recovery rate of rock treated by nanofluids, as shown in Table 2 below. Nanofluids change the surface wettability of rock by spreading and adsorbing on the surface of a reservoir rock mass. The wettability of rock can be expressed by measuring the contact angle. A small contact angle means stronger wettability.