Hydrophobic SiO2 NPs (with a concentration of 0.3 wt% dispersed in ethanol) in the wettability alteration of reservoir rock has been firstly observed by Ogolo et al. in 2012 . After a year, Li et al. confirmed the high efficiency of hydrophilic SiO2 NPs in both IFT reduction and wettability alteration of sandstones in the presence of 30,000 ppm NaCl solution with 0.05 wt% NPs, showing an optimum concentration . Furthermore, Roustaei et el. investigated the potential of hydrophilic SiO2 NPs in the presence of 50,000 ppm NaCl solution in EOR of carbonate rocks and they concluded that at an optimum concentration of 0.4 wt%, maximum wettability alteration is obtained. They also claimed that NPs result in increasing IFT when the wettability is altered toward water-wetness and this IFT increment contributes in the oil recovery improvement. Their assertion is based on the concept of spontaneous imbibition in carbonate reservoirs .
Recently, the effect of temperature and size of SiO2 NPs has been analyzed by Al-Anssari et al. in 2017 and they concluded that the temperature has a positive effect on SiO2 NPs ability to alter wettability of oil-wet carbonate rocks; however, the effect of NPs size is negligible .
ZrO2: Karimi et al. in 2012 investigated the process of spontaneous imbibition of 5wt% ZrO2 nanofluid into an oil-wet carbonate reservoir. They claimed that adsorption of hydrophilic synthesized ZrO2 NPs and the mixture of nonionic surfactants onto the rock surface leads to the formation of a nano-textured surface which can modify reservoir rock wettability toward more water-wetness better than NPs alone and consequently more oil is recovered .
Al2O3: Ogolo et al. in 2012 claimed that Al2O3 NPs (dispersing in distilled water or brine with a salinity of 30,000 ppm) perform well in EOR due to the oil viscosity reduction . However, a year later, Giraldo et al. observed that Al2O3 can also alter the wettability of sandstones from severe oil-wet to severe water-wet when synthesized by an anionic surfactant. Among a wide range of nanofluid concentration between 0.01 to 1wt%, the efficiency of surfactant to wettability modification is improved just at a relatively low NPs concentration (equal or lower than 0.05 wt%). Besides, they asserted that imbibition experiments are a good evaluation tool for analyzing the performance of NPs . The ability of Al2O3 NPs for IFT reduction is expressed by Joonaki et al. in 2014 and they observed that the interfacial tension between phases began to reduce when propanol is used as a dispersing agent for Al2O3 NPs .
TiO2: Ehtesabi et al. in 2013 investigated the application of synthesized TiO2 NPs through core-flooding experiments in sandstone rocks and they confirmed their application in EOR in terms of wettability alteration and oil viscosity reduction. They observe that TiO2 NPs do not affect the oil viscosity but in a low concentration of 0.01wt% could alter significantly the wettability of rock surface . Application of SiO2, Al2O3 and TiO2 NPs in oil-wet carbonate reservoirs have been analyzed by Esfandyari et al. in 2014 under various temperatures (26, 40, 50, 60 ℃). The maximum and minimum reduction in contact angles were achieved by SiO2 and Al2O3 NPs, respectively at all temperatures. Besides, they observed a reduction in oil viscosity at higher temperatures of 50 and 60 ℃ by Al2O3 and TiO2 NPs and the oil recovery by Al2O3. TiO2 was also more significant at all temperatures comparing to SiO2 NPs . The potential of these three hydrophilic NPs has been also investigated by Hendraningrat et al. in different wettability sandstone reservoirs. They confirmed the mechanism of wettability alteration as the main mechanism of these NPs although IFT reduction is also observed. As a result of their experiments, TiO2 NPs performed well in all types of wettability systems among other NPs .
Stability of Nanoparticles
One of the main challenges of nanofluids which limits their application especially in long period is their stability. Forming an agglomeration may restrict the flow of NPs through micron-sized pore spaces and may lead to pore spaces plugging and permeability reduction . Zeta potential of nanofluid has a direct relationship with the suspension stability in a way that the suspension with higher zeta potential (even positive or negative) shows higher stability. The water chemistry such as pH, ionic strength (salinity) and its various component usually influence the stability of nanofluids . Accordingly, it is expected that adjusting the pH value of system, reducing its salinity or adding a stabilizer to the nano-suspension will provide a higher stability for system.
Adjusting pH value: The pH value of a nanofluid solution influence their surface charge and consequently their stability. According to Huang et al. there exist an optimal pH value by which a maximum nanofluid stability is obtained. They have conducted a series of experiments on Al2O3 and CuO nanofluids in which the pH value of the solution is adjusted by addition of HCl or NaOH to obtain an acidic or basic suspension. They observed that Al2O3 and CuO nano-suspension obtain the highest stability at an optimum pH value of 7.5- 8.9 and 7.5-9.5, respectively .
Effect of salinity: Esfandyari et al. in 2014 measured the zeta potential of the nanofluid of Al2O3, TiO2 and SiO2 in various dispersion media of deionized water, NaCl solution (3و000 ppm) and synthetic brine (25,000 ppm). As a result of their experiments, the zeta potential values of SiO2, Al2O3 and TiO2 in deionized water are -38.5, 31.1 and -19.1 mV, respectively indicating the highest to lowest stable nanofluids. However, the zeta potential values of SiO2, Al2O3 and TiO2 nanofluids have been decreased to -32.3, 27.4 and -15.8 in NaCl solution and to -22.4, 21.6 and -9.9 in the synthetic brine respectively, all of which indicate the detrimental effect of salinity on the stability of nanofluids .
Adding stabilizer: Hendraningrat et al. in 2015 analyzed the stability of nanofluid of Al2O3, TiO2 and SiO2 NPs. Analysis of nanofluids visual stability show that Al2O3 NPs precipitated at initial stages (approximately after 3 hs) and TiO2 NPs precipitated slightly later. However, SiO2 NPs exhibit a better stability of 24-48 hrs. In order to get longer stability of Al2O3 and TiO2 NPs, they proposed addition of polyvinylpyrrolidone (PVP) as a stabilizer to alter NPs surface conductivities, resulting in 96 and 48 h stability for Al2O3 and TiO2 nanofluids, respectively . The influence of pH value, existence of divalent cations presented in water and surfactant concentration as a stabilizer on TiO2 nanofluid stability have been analyzed by Loosli et al. in 2017. They observed that when the pH values are altered from pH=3 (addition of HCl) to pH=11 (addition of NaOH) the zeta potential continuously decreased from +36 mV to -40 mV. At a pH range of 5-7.2, the absolute zeta potential values are minimum indicating the range of instability. Zeta potential is equal to zero at a pH value of 6.1, indicating the most instable situation. Besides, they also concluded that the presence of divalent cations (Ca2+ and Mg2+) leads to the more agglomeration of NPs due to the cation bridging. In contrast, they claimed that adsorption of negatively charged sodium dodecyl sulfate, as an anionic surfactant, onto the positively charged surface of TiO2 NPs make the nano-suspension more stable .
In order to prevent chemical degradation of commercial PAM during EOR in high salinity and high temperature conditions, Tamsilian et al. in 2016 have synthesized a protected polyacrylamide nanoparticles (PPNs) in which a hydrophobic polystyrene (PSt) sell is created by one-pot two-step inverse emulsion surface polymerization method. The existence of hydrophobic PSt shell protects the active PAM chains from degradation in harsh reservoir conditions. Besides, the efficiency of PPNs as a viscosity modifier is increased by their time-dependent releasement; hence, it will remain active in a wide range of approximately 30 days. Finally, a comparison between the performance of PAM and PPN introduces PPNs as a good candidate for EOR processes, since a higher viscosity is obtained by the usage of PPN instead of PAM, although al lower amount of it is required for achieving same recovery . Shaban et al. in 2016 have synthesized a new cauliflower-like amphiphilic copolymer by aerosol-photopolymerization method. The advantage of this method over usual methods such as emulsion polymerization is that no surfactants are required. They concluded that their new method of synthesis has a great ability to produce novel copolymers with large amount of hydrophobic and hydrophilic monomers.
Hence, it has a long applicability to viscosity enhancement during the process of polymer-flooding .
Another new type of acrylamide-based thermoassociative copolymers (TAP), applicable in high salinity and high temperature reservoirs, have been considered by the current research community, called NanoChemical Group, through the process of a copolymerization mechanism of free-radical of acrylamide monomer and thermosensitive macromonomers. In a saline reservoir, the viscosity of a solution is augmented if the temperature is above a critical value called associating temperature. Meanwhile, it is expected that addition of a type of stabilizer which is highly resistant in high salinity conditions could provide a desired stability for nanofluids at high salinity conditions. Considering this in mind, the novel hydrophobically thermoassociating copolymer (HTAP) consisting of polyacrylamide and polystyrene, prepared by Tamsilian et al. in 2016, which has been recently used as EOR agent is recommended to stabilize NPs to compensate for the detrimental effect of high salinity.