# Progress on the Application of Nanomaterial Expansion in Oil Displacement
## 3 Nanomaterial Swelling Oil Displacement Mechanism
### Nano-Silica Oil Droplets Enhanced Expansion  
[PERSON] et al. [69] successfully prepared a new type of nanocomposite with the function of crude oil interface regulation and analyzed its molecular-level dynamic response mechanism with a hydrocarbon fluid based on a microfluidic chip platform system. The experimental results show that the hydrophilic silica (NPS) will cause the crude oil to expand sharply under the water-phase condition, as shown in Figure 7. This expansion effect is very significant: even the small oil droplets on the surface of the main channel will expand by nearly a thousand times. The swelling ratio increases with the decrease in salinity and the increase in NPS concentration in the environment. The expansion of crude oil droplets increases the flow resistance in the scanning area, and the improvement of scanning efficiency increases the recovery rate by about 11%. Negatively charged hydrophilic nanoparticles drive the polar components of crude oil into the oil phase through competitive adsorption. Low-salinity water drives water to diffuse to form spontaneous emulsification, causing the oil phase to expand and dewet, changing the relative permeability of oil and water, reducing the mobility ratio, and improving the oil sweeping efficiency. In [PERSON] et al. 's study [70], hydrophobic SiO\({}_{2}\) nanoparticles were treated at different pH levels and temperatures for different lengths of time, and the hydrophobicity of the nanoparticles was determined by the Washburn method [71]. It was concluded that the external environment had a great influence on the surface lipophilicity of SiO\({}_{2}\) nanoparticles. Hydrophobic SiO\({}_{2}\) nanoparticles have high salt toleranc  
Figure 7: Schematic diagram of closed-loop crude oil expansion in different periods [69]. of hydrophobic SiO\({}_{2}\) nanoparticles is more important for the improvement of reservoir exploitation efficiency, it is more suitable for use under high-temperature and high-alkaline conditions, but the use time should not be too long. [PERSON] et al. [72] studied the effect of nanofluid application on enhanced oil recovery of Berea sandstone cores. Silica, alumina, nickel oxide, and titanium oxide were used as nanoparticles, and they were added to different concentrations of water to form aqueous solutions. When deionized water entered the emulsion droplets, they formed reverse micelles, and emulsified water droplets gradually entered the original water droplets (W2), increasing the volume of the crude oil droplets. In the experiment, the emulsion droplets were wrapped by nanofluids to form W2/O/W1. A schematic diagram is shown in the following Figure 8. The double emulsion, in which the oil-soluble surfactant Span80 acts as a carrier, is the main factor for water transport in the double emulsion. The emulsion has the effect of expanding the swept volume to improve the harvesting efficiency [73]. The expansion of emulsion droplets is due to the following: Deionized water has a higher chemical potential than water in emulsion droplets. The molecular migration caused by chemical potential difference changes the volume and morphology of oil droplets. When they contact with each other, deionized water diffuses into emulsion droplets, so that part of the Span80 molecules in the emulsion droplets are adsorbed on the surface of water molecules, and the Span80 molecules are pushed closer to the original water droplets, which eventually increases the volume of the emulsion droplets; after adding negatively charged nanoparticles, while deionized water enters the interior of the emulsion droplet, due to adsorption, nanoparticles will be adsorbed on the nanofluid interface of the emulsion droplet, and more oil-soluble Span80 molecules will be topped to the interior of the droplet. The amphiphilicity of the Span80 molecule makes it adsorb on the surface of water molecules in the form of reverse micelles and form small water droplets. The  
Figure 8: Optical microscopic image of W1/O/W2 emulsion [74].