In the terminology of fluid flow in porous media, mobility of a fluid is defined as the ratio of relative permeability of the corresponding fluid to its viscosity. In EOR, mobility ratio is the mobility of the injected displacing fluid to that of the oil being displaced. Good mobility control is obtained when the viscosity of the injected fluid is higher than the viscosity of the oil in the reservoir and can lead to a piston-like displacement of the oil from the injection well to the production well, as shown schematically in Figure 1. However, poor mobility control due to a lower viscosity of the injected fluid can result in low recoveries due to viscous fingering.[49, 50] For instance, the viscous fingering effect may be observed if CO2 is injected as an oil-miscible solvent. Injected CO2 may find the path of less resistance to the production well and bypass most of the oil, leaving a huge portion of the oil in the reservoir behind.[51-54] Achieving good mobility control in combination with other mechanisms including low interfacial tension or wettability alteration is therefore essential for successful chemical EOR.[2, 53]
A method for achieving high viscosities of the injected phases and good mobility control is through generation of foams and emulsions, which can form in the presence of surfactants or nanoparticles. Foams and emulsions are dispersions of one fluid in a second immiscible fluid, and they typically exhibit high viscosities and shear-thinning rheological behaviors.[55, 56] The high viscosity of the injected phase can lead to improved mobility control. In addition, the shear-thinning behavior of the injected foam or emulsion is advantageous for achieving high injection rates into the reservoir.
Similar to surfactants, nanoparticles can be used to generate foams and emulsions to increase the viscosity of the injected phase. The stabilization of foams and emulsions using micron-sized particles was reported roughly 100 years ago by Ramsden and later by Pickering.[57, 58] Such emulsions are commonly known as Pickering emulsions. Unlike surfactants, nanoparticles have the advantage that they can irreversibly adsorb to a liquid-liquid or gas-liquid interface, forming very stable foams and emulsions. However, bare nanoparticles may be too hydrophobic or hydrophilic for stabilizing an interface. PNPs can be tailored for a specific interface and application. Below, we discuss the fundamental mechanisms involved in stabilization of foams and emulsions using PNPs and then discuss recent examples of their application for EOR. We begin by discussing surfactant-coated nanoparticles, which are closely related to PNPs and have been widely studied for EOR applications.
Foam and Emulsion Stabilization Using Surfactant- and PNPs
Surfactant-coated nanoparticles are closely related to PNPs and are prepared by blending surfactants and nanoparticles. Driven primarily by electrostatic interactions, the surfactant can form a monolayer on the nanoparticle surface, resulting in more hydrophobic particles. Figure 2 shows a schematic representation of surfactant adsorption onto a nanoparticle and examples of foams and emulsions stabilized by surfactant-coated nanoparticles. A number of studies have confirmed surfactant adsorption onto nanoparticles through contact angle measurements, adsorption isotherms of surfactants on nanoparticles, zeta potential measurements and dispersion stability measurements as a function of concentration of surfactant and nanoparticles.[59-61] Surfactant-coated nanoparticles generate stable foams and emulsions in some cases where precursor nanoparticles or surfactants separately do not.[59, 62-64]
Figure 2. (a) Foam as a viscous fluid is a dispersion of air in water and each air droplet is surrounded by surfactant-coated nanoparticles; (b) Cryo-SEM image of a foam with nanoparticles closed packed; (c) schematic representation of the effect of concentration ratio of nanoparticle and surfactant. Reproduced with permission from Ref.  with permission from the Royal Society of Chemistry and Reproduced with permission Ref.  from Wiley. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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The properties of surfactant-coated nanoparticles are dependent on the relative concentrations of surfactant and nanoparticle. If the concentration ratio of surfactant to nanoparticle is low, only a fraction of nanoparticle surface is coated with surfactant. However, at much greater concentration ratios, the surfactant can form a double layer on the nanoparticle surface, resulting in a hydrophilic nanoparticle surface. Stable foams and emulsions are formed at a concentration ratio that results in maximum nanoparticle flocculation. The most flocculated nanoparticle in this case corresponds to a low-charge, optimally hydrophobic nanoparticle, containing a monolayer of surfactant on the surface.[59-61] Further, single chain surfactants are believed to be a better choice for foam formation when mixed with nanoparticles since double chain surfactants may lead to formation of double layer adsorption on nanoparticle at concentrations lower than that of single chain surfactants.
The rheology of foams and emulsions formed by surfactant-coated nanoparticles is also influenced by the surfactant to nanoparticle concentration ratio. Viscoelastic behavior of the bulk is observed only over a range of concentration ratios. For instance, in a study of silica nanoparticles with a cationic surfactant (cetyl trimethylammonium bromide), Limage et al. find that if the molar concentration of CTAB to silica nanoparticles is about 0.03, viscoelastic behavior is observed. They also try to find a correlation between bulk rheology of nanoparticle and surfactant mixtures and that of the foam. Their rheological measurements are correlated with the structures forming at the interface using cryo-SEM imaging of the generated emulsions and foams.
Another role of the surfactant in this process is to lower the interfacial tension and form an initial dispersion of air/water or oil/water in case of foam or emulsion, respectively. Once this dispersion is formed due to shear and a decreased amount of interfacial tension, the stability of foam/emulsion is augmented by adsorption of nanoparticles at the interface.
Gonzenbach et al. provide a series of conditions which can result in formation of ultra-stable foams by means of surfactant-coated nanoparticles. Apart from reporting the condition of optimal ratio between concentration of surfactant and nanoparticle, they find that a lower particle size or higher concentration of nanoparticle and surfactant leads to generation of more foam. Also, by comparing long-term stability of the foams treated with different length of surfactants, they find that long-term stable foams can be made by using surfactants with a short chain length (n = 2−8) rather than long chain length.
Similar to surfactant-coated nanoparticles, PNPs can be used to stabilize foams and emulsions. PNPs can decrease the interfacial tension of oil and water or water and air, which can lead to more stable emulsions. For example, in 2005 Saleh et al. reported the use of silica nanoparticles coated with a polyelectrolyte to stabilize oil-in-water emulsions. More recently, Saigal et al. reported stable oil-in-water emulsions using silica nanoparticles coated with a pH responsive polymer, and they found that the most stable emulsions were formed at lower polymer chain grafting densities. Related studies on star polymers, bottlebrush polymers, and paramagnetic particles with adsorbed amphiphilic polymers found stable emulsions and reductions in the oil-water interfacial tension at relatively low (0.1 wt %) particle contents. Alvarez et al. evaluated the dynamic reduction in interfacial tension of air and water in the presence of PNPs while changing the grafting density of the polymer brushes and showed that the polymer coating is a key factor in reducing the interfacial tension of air and water using PNPs. PNPs with stimuli-responsive polymer chains have also been reported. PNPs can respond to temperature, pH, and light through a change in surface properties. Stimuli-responsive PNPs can potentially be used to design injectable fluids that respond to environmental changes before and after injection or in the presence of oil.
It should be noted that the reduction in interfacial tension by PNPs and star polymers is at most by one order of magnitude (from roughly 25 to 1 mN/m).[68-70] By comparison, surfactant additives can lead to much greater reductions in oil-water interfacial tension, down to 0.001 mN/m2 and below. Thus, irreversible PNP adsorption to the oil-water interface still plays a predominant role in emulsion stability with added PNPs, but the reduction in oil-water interfacial tension is modest compared with suitably chosen surfactant additives.
In addition to surface energy, entropy is important to the interfacial properties of PNPs. Polymers can exhibit conformational changes that influence the thermodynamics of PNP adsorption at the fluid-fluid interface.[73-76] However, there are only a handful of studies on the effect of polymer entropy on nanoparticle adsorption, although this has been studied more carefully in polymer-polymer blends and in polymer nanocomposites.
Surfactant- and PNPs for Mobility Control
Prior studies and field tests have relied on the mechanisms explained above to increase the viscosity of the displacing fluid and the recovery of oil.[79-85] Foams and/or emulsion formation in oil-rich porous media after injection of surfactant- or PNPs has been validated through CT-scans, an increased pressure drop across the core, and effluent analysis.[86-88]
Figure 3 shows the CT-scan of different cross sections of a Boise sandstone core after flooding with brine and CO2, both with and without PEG-coated silica nanoparticles. The difference in these two experiments is only the presence or absence of PNP, and the same core has been scanned at the same injected pore volume of CO2. The CT-scan results show greater sweep efficiency in the presence of PNP [Figure 3(b)], while with no PNP added, large regions of the core are bypassed due to viscous fingering [Figure 3(a)].
Figure 3. CT-scan of the cross section of a core flooded with CO2 and (a) 2% NaBr brine and (b) 2% NaBr brine and 5% PEG-coated silica nanoparticles; pure brine and CO2 are illustrated with red and blue, respectively. The scan is taken after 0.25 pore volume of CO2 injected and each slice is 1 cm apart longitudinally (Reproduced with permission from Ref.  from the Society of Petroleum Engineers). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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One practical challenge in the application of foam and emulsions from PNPs is the energy needed for foam and emulsion formation.[59, 60] There is a threshold shear rate needed for nanoparticles to start generating foams and emulsions. This threshold injection flow may be much greater than the practical injection rates in reservoirs. In addition, the pregeneration of foams and emulsions outside the reservoir before injection increases the cost and difficulty of injection into the reservoir.
It is noteworthy to mention that a type of polymeric nanoparticle with commercial name BrightWater was the first successfully field-tested nanoparticle to increase the sweep efficiency in an actual oil reservoir (Salema field, Campos Basin, Brazil). Recently, other tests have confirmed the successful application of these nanoparticles in other reservoirs. BrightWater is a polymeric nanoparticle that hydrolyzes at a specific temperature and expands to many times its original volume. By blocking the pores in the high-permeability regions of a reservoir, the injected flow will be directed toward low-permeability zones of the reservoir, which may have been previously untouched. Figure 4 illustrates the basic idea behind the application of these polymeric nanoparticles, which can lead to significant increase in oil recovery. Although BrightWater is not a PNP, its successful implementation provides guidelines for the design of PNPs and demonstrates that PNPs do have potential for use in EOR.
Figure 4. Schematic and SEM image of BrightWater polymeric nanoparticles. The particles expand at elevated temperatures, diverting flow to low permeability regions (Reproduced with permission from Ref.  from Society of Petroleum Engineers). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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