# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 3 Nanoemulsions
), flow assurance, deposit removal and clean-up, and also for EOR operations [95; 106]. The nanoparticles reported in the literature as stabilizers are: SWCNTs (single-wall carbon nanotubes)/silica nanoparticles [107], MWCNTs (multi-walled carbon nanotubes)/silica nanoparticles [108] (Figure 19), hydrophilic SiO\({}_{2}\) or partially hydrophobic clay [104; 109], spherical nanoparticles of cross-linked polystyrene [110], and zinc oxide (ZnO) or aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) [111]. Besides these stabilizers, the possibility of using surfactant along with the SWCNTs/silica nanohybrid was studied [107]. This system showed an improved dispersion stability at the expense of reducing the interfacial activity of the nanoparticles, causing them to stay in the aqueous phase (Figure 20). These could also be used to support a catalytic conversion of oil products in porous media (see **Nanocatalysts**). Figure 17: Scheme of a water-in-oil emulsion stabilized with surfactant and nanoparticles. Figure reproduced from Refs. [35; 103]. Figure 19: (**a**) SEM image of a nanohybrid structure with a resolution of 500 nm; and (**b**) Transmission electron microscope (TEM) image of the nanohybrid structure. The carbon nanotube structure as well as the silica particles are clearly distinguished Figure reproduced from Ref. [108]. Figure 18: Foam stability results in (**a**) SDS surfactant; (**b**) nanoparticle-stabilized CO\({}_{2}\) foam; (**c**) foam coalescence as a function of the bubble diameter; and (**d**) foam quality represented as a function of the bubble density (bubbles/mm\({}^{2}\)) (scale bars = 400 \(\upmu\)m). Figure reproduced from Ref. [104]. A major problem remaining to be tackled lies in the stability of nanofoams as the temperature increases [109]. However, nanofoams showed better stability and tolerance than SDS foams due to the presence of nanoparticles. The bubbles in the nanohybrid maintained their spherical or ellipsoidal shape because of the enhanced surface dilational viscoelasticity (Figure 21) [112]. This property was tested in oil recovery processes from dead pore ends. The SiO\({}_{2}\)/SDS foam recovered more oil due to the improved viscoelasticity properties as well as a reduction in the IFT (Figure 22). The nanoparticles on the bubble surface improved the stability resistance, mainly against Ostwald ripening and film rupture [109]. When the bubbles entered the dead-end pore, the microforces acting on the oil droplet also contributed to improve the sweeping efficiency (Figure 23). Figure 20: Scheme of nanohybrid particles carrying catalytic species in the oil and water phases (**top left** and **right**, respectively). (**bottom**) TEM image of Si nanoparticles and SWCNTs (single-wall carbon nanotubes) after ultrasonication in water for 3 hours. Figures (top left and right) adapted from Ref. [107]; Figure (bottom) reproduced from Ref. [99]. Figure 21: Behavior of foams at different temperatures and as a function of time: (**1**) SDS at 60 \({}^{\circ}\)C, (**2**) SiO\({}_{2}\)/SDS at 60 \({}^{\circ}\)C, (**3**) SDS at 80 \({}^{\circ}\)C, and (**4**) SiO\({}_{2}\)/SDS at 80 \({}^{\circ}\)C. Figure reproduced from Ref. [109]. Figure 22: Residual oil displacement in a dead-end being swept by different fluids: (**a**) brine, (**b**) SDS foam, and (**c**) SiO\({}_{2}\)/SDS foam flooding. Figure reproduced from Ref. [109]. In the flooding experiments, the nanoemulsions also exhibited better results than those obtained for standard waterflooding or EOR processes (Figure 24) [77; 111]. A different line of research involved the application of CO\({}_{2}\) foams stabilized with silica nanoparticles [113]. These nanoemulsions in EOR flooding processes showed that nanoparticle CO\({}_{2}\) foams increased the oil recovery as compared to the standard CO\({}_{2}\) gas flooding (Figure 25) techniques, achieving a better sweeping efficiency [104].