# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 2 Nanofluids
### Effect of Nanoparticles on Reservoir and Fluid Properties
#### 2.2.4 Effect on the Rock Wettability and Surface Wetting
The electrostatic repulsion between the particles rises as the size of the nanoparticles decreases, therefore increasing the structural  
Figure 11: Photomicrograph depicting particle layering of 10 vol % aqueous silica nanofluid with a particle diameter \(d=19\) nm on a solid surface (film size = 868 \(\upmu\)m). Figure reproduced from Ref. [87]. Figure 10: Scheme and photomicrograph depicting inner and outer contact lines and the nanofluid film region. Figure reproduced from Ref. [86]. disjoining pressure. The force is then maximum at the tip of the wedge (Figure 12). The spreading increases with a decrease of the film thickness (i.e., the number of particle layers in the film) [83] (Figures 11 and 13). Concerning the presence of salt, increasing its concentration will lower the repulsive forces between nanoparticles, and thus the pressure driving the wedge film will be reduced. Due to this, an increase in the TDS has a negative effect on oil removal when using nanofluids. A key factor in oil recovery is how oil droplets detach from solid surfaces. In order to study how nanofluids can improve this mechanism, a series of simulations via molecular dynamics techniques were performed with these droplets immersed in charged nanoparticle suspensions [88]. According to the results, suspensions of charged hydrophobic nanoparticles can be used as high-performance agents in detaching oil droplets from solid surfaces in EOR applications. The process of detachment can be divided into three stages [88; 89]: the contact line shrinks because of the decrease in the IFT, induced by the surfactant adsorption at the oil-water interface; the diffusion of water detaches the oil  
Figure 12: Pressure on the walls of a \(0.5^{\circ}\) contact angle wedge at the vertex as a function of the dimensionless radial distance. Particle volume fraction \(V_{m}=0.36\) and particle diameter \(\mathrm{d}=10\) nm. Figure reproduced from Ref. [84]. Figure 13: Scheme of the disjoining pressure acting on an oil droplet. Figure reproduced from Refs. [84; 85]. from the solid substrate/wall in the vicinity of the contact line; and finally, the droplet detaches when the contact radius becomes sufficiently small. Besides these analyses, the way in which different nanoparticles alter the rock wettability, modifying the contact angle and the relative permeability, has also been throughly discussed. [PERSON] [90] investigated the use of nanofluids to alter oil-wet carbonate rocks. Scanning electron microscope (SEM) (Figure 14) and atomic force microscope (AFM) images verified the adsorption process of the particles onto the calcite surface, forming larger clusters. This process was also evidenced by the significant change of the rugosity of the rock samples: the original calcite surface was flat, but the nanofluid changed its rugosity significantly, yielding an irregular coating layer spread along the surface. These results are consistent with AFM measurements on the treated calcite substrate, where higher surface roughnesses were found after the nanofluid treatment. Several kinds of nanoparticles have been reported in the literature as agents to modify the wettability of rock formations, but the most common material tested is PSNP [7; 77; 78; 90; 91; 92]. Besides polysilicon nanoparticles, the following materials were also tested: carbon-based fluorescent nanoparticles [77], oxides of different metals (e.g., Al, Zn, Mg, Fe, Zr, Ni, Ti) [92] (Figure 15), and polymer-coated nanoparticles (PNPs) [4]. Nanoparticles have also proved to have a potential use as scale inhibitors. Scale formation can cause the production channels to become blocked, leading to a significant reduction in productivity/recovery efficiency. Scale inhibitors are applied in order to prevent or lower the amount of scale formation. Silica nanoparticles can also be used as a scale inhibitor. An optimum amount of silica nanoparticles can reduce the scale deposition, which is still a challenging research topic in the oil industry, as is the design of environmentally friendly scale inhibitors [91].