# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 2 Principle of Enhanced Oil Recovery by Nanomaterials
### Lower Oil Viscosity  
Regulating the rheological properties of crude oil is one of the core mechanisms of enhanced oil recovery [57]. The mechanism is the spatial dispersion of nanoparticles, which can effectively inhibit the formation of macromolecular viscoelastic networks, thereby significantly weakening the fluid flow resistance [58]. [PERSON] et al. [59] pointed out that the hydroxyl groups in nanoparticles such as silicone interact with the polar groups in asphaltenes, destroy the hydrogen bonds between molecules, reduce the size of aggregates, and thus reduce the viscosity of heavy oil. In addition, metal nanoparticles (such as Fe) can catalyze the cleavage of chemical bonds in heavy oil aggregates at high temperatures, which promotes the cracking of macromolecules and further reduces the viscosity. [PERSON] et al. [60] studied the effects of SiO\({}_{2}\) nanoparticles, Al\({}_{2}\)O\({}_{3}\) nanoparticles, and acidic SiO\({}_{2}\) nanoparticles on the viscosity of heavy oil by adsorption and aggregation experiments on n-C7 asphaltene. Through experimental research, they found that acidic \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Rock Type** & **Surface Property** & **Contact Angle** & **Nanofluid Type** & \begin{tabular}{l} **Wettability of Rock After** \\ **Nanofluid Treatment** \\ \end{tabular} & \begin{tabular}{l} **Contact Angle** \\ **Recovery** \\ \end{tabular} \\ \hline Quartz & Lipophilicity & 131.2\({}^{\circ}\) & KH550-MoS\({}_{2}\) & Weakly wetting & 72.1\({}^{\circ}\) & Ascension: 14\% \\ \hline Limestone & Neuter wetting & 90\({}^{\circ}\) & \begin{tabular}{l} Al\({}_{2}\)O\({}_{3}\) \\ TiO\({}_{2}\) \\ SiO\({}_{2}\) \\ \end{tabular} & Ascension & 71\({}^{\circ}\) & 52.6\% \\ \hline Sandstone & Hygroscopicity & 36\({}^{\circ}\) & \
\begin{tabular}{l} Hydrophilicity SiO\({}_{2}\) \\ Hydrophobicity SiO\({}_{2}\) \\ \end{tabular} &
\begin{tabular}{l} Strong hygroscopicity \\ Neuter wetting \\ \end{tabular} & \(10^{\circ}\) & Reduction \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Changes in contact angle, wettability, and recovery rate of rock treated by nanofluids. SiO\({}_{2}\) can reduce the viscosity of heavy oil well, and there is a relationship between the viscosity reduction (DVR) of heavy oil and the addition of nanofluids, as shown in the Equation below, where \(\mu_{ref}\) and \(\mu_{namf}\) represent the viscosity before and after adding nanoparticles, respectively. \[\text{DVR}=\frac{\mu_{ref}-\mu_{namf}}{\mu_{ref}}\times 100\%\]  
Although this relationship was not fully confirmed in this study, the research of [PERSON] et al. [61] provides strong support for this view. They successfully prepared four kinds of silica nanofluids by combining silane precursors with them. The hydrodynamic diameters of these fluids were measured to be between 9 and 45 nm. In subsequent experiments, these four nanofluids were injected into heavy oil for simulation experiments. The results reveal that the number of siloxane and silanol groups on the surface of SiO\({}_{2}\) nanoparticles has a significant positive correlation with the adsorption capacity of asphaltene and DVR (residual oil saturation reduction rate), which is visually demonstrated in Figure 6. It can be clearly seen from the diagram that the maximum value of DVR reaches 30%. It can be concluded that the interaction between the active silicon hydroxyl group in the nano-silica fluid and asphaltene is an important factor leading to the decrease in oil viscosity, and the degree of adsorption of the nanoparticles to asphaltene is an important manifestation of their ability to reduce the viscosity of crude oil. [PERSON] et al. [62] studied the effect of alumina-based nanofluids (AlNPs) on improving oil recovery by reducing viscosity in cyclic steam stimulation.