# 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
#### 2.2.2 Effect on Rheology and Viscoelasticity  
Experiments have evidenced that the addition of nanoparticles into water increases the shear viscosity. Water molecules layered at the nanoparticles' surface decrease the fraction of adjacent mobile fluid molecules, increasing the shear viscosity and elastic properties [61; 62]. This effect becomes more noticeable in polymer/nanoparticle solutions in which the storage and loss moduli are strongly affected by the nanoparticle concentration. This enhances the viscoelastic properties of the sweeping agent and therefore increases the oil recovery factor [63; 64]. The former can be increased by either increasing the nanoparticle concentration, or by modifying the size of the particles [65]. [PERSON] was the first to study the influence of particles on the viscosity of a fluid at very low volume fractions (\(c<0.02\)), predicting a linear increase with the particle volume concentration. Since [PERSON]'s formula, new correlations have been published taking into account, among others, particle size and shape, higher volume concentration, temperature, pH, and size distribution [66; 67]. It is important to note that there are contradictory results in the literature, showing different trends with respect to the viscosity dependency on the particle size: both an increase and a decrease were observed with smaller particle sizes as well as an independence of the rheological properties on the particle size [66; 68]. However, in accordance with [PERSON] [66], it is considered that the increase in viscosity in nanofluids comes from two major sources: fluid-particle and particle-particle interactions. These are dependent on the overall surface area of the nanoparticles. At the same concentration, smaller particles show a larger surface area, and thus the viscosity should be higher. Nanoparticle aggregation is then one of the major causes of detriment in the rheological properties of nanofluids. The rheological properties of different nanofluids have been reported: organic/inorganic hybrid aqueous solutions composed by mixing silica nanoparticles and hydrophobically associating polyacrylamide (HAPAM) [8], hydrolyzed polyacrylamide (HPAM) [6; 7; 55], sulfonated polyacrylamide (SPAM) [6], acrylamide-styrene-AMPS (2-acrylamide-2-methylpropane sulfonic acid) copolymers (PA-S) [69], and aqueous solutions [56]. The use of nanoclays as nanoparticles for EOR purposes was also considered along with polymers and surfactant/polymer systems: using with HPAM [70] or HPAM-SDS (sodium dodecyl sulfate) [71]. Moreover, advanced alternating EOR processes were described in the literature using silica nanoparticles in water-alternating-gas techniques [72; 73]. Hybrid nanofluids (polymer/nanoparticles) exhibited improved viscosity properties and their tolerance to heat and saline environment (total dissolved solids or TDS \(>\) 120,000 ppm) was also enhanced, as was their long-term thermal stability (Figure 4a,b). In the flooding experiments, the nanofluid solutions exhibited better results than those obtained for water or polymer  
Figure 3: Mechanisms of nanoparticles’ entrapment in porous media. Figure adapted from Ref. [58]. solutions (Figure 4c). Research on biopolymers was also performed using xanthan gum, which is used in the upstream industry as a rheological enhancer, mixed with nanoparticles [8; 74]. These studies represent clear examples of the development of hybrid nanofluid technologies with different kinds of polymers, which have a potential use in the upstream oil and gas industry. Nevertheless, these hybrids might also be useful in different areas of science. The interactions between nanoparticles and polymers as a function of their concentrations play a major role in their viscosifying properties. For example, a reinforced three-dimensional network structure was formed in the HAPAM/silica system [8].