Nanotechnology has found its way to petroleum engineering, it is well-accepted path in the oil and gas industry to recover more oil trapped in the reservoir. But the addition of nanoparticles to a liquid can result in the simplest flow becoming complex. To understand the working mechanism, there is a need to study the flow behaviour of these particles. This review highlights the mechanism affecting the flow of nanoparticles in porous media as it relates to enhanced oil recovery. The discussion focuses on chemical-enhanced oil recovery, a review on laboratory experiment on wettability alteration, effect of interfacial tension and the stability of emulsion and foam is discussed. The flow behaviour of nanoparticles in porous media was discussed laying emphasis on the physical aspect of the flow, the microscopic rheological behaviour and the adsorption of the nanoparticles. It was observed that nanofluids exhibit Newtonian behaviour at low shear rate and non-Newtonian behaviour at high shear rate. Gravitational and capillary forces are responsible for the shift in wettability from oil-wet to water-wet. The dominant mechanisms of foam flow process were lamellae division and bubble to multiple bubble lamellae division. In a water-wet system, the dominant mechanism of flow process and residual oil mobilization are lamellae division and emulsification, respectively. Whereas in an oil-wet system, the generation of pre-spinning continuous gas foam was the dominant mechanism. The literature review on oil displacement test and field trials indicates that nanoparticles can recover additional oil. The challenges encountered have opened new frontier for research and are highlighted herein. Keywords Nanoparticles · Porous media · Adsorption · Stability · Mechanisms · Enhanced oil recovery Introduction The major chemical used in the industry to achieve this are polymer, surfactant and alkaline. Surfactant lowers the inter- As most of the oil fields in the world are approaching matu - facial tension between the oil and water, alter the wettability ration, more emphasis is placed on enhanced oil recovery of the rock, generate emulsion and stabilize foam. Polymers (EOR) methods, because two thirds of the original oil in on the other hand increases the viscosity, which reduces the place (OOIP) is left unproduced. Since the porosity and per- mobility ratio, and therefore improves the sweep efficiency, meability of most reservoirs varies, early breakthrough result whereas alkali increase pH, generate in situ surfactants and in by-passed crude. EOR process can improve the recovery reduce adsorption of anionic surfactants . The combi- efficiency; chemical-enhanced oil recovery (CEOR) can nation of all the chemicals alkaline, surfactant and poly- recover about 37% OOIP. Oil can be trapped microscopi- mer (ASP) is also in use but due to high cost of chemicals cally in the reservoir by capillary forces and oil can also be and fall in crude oil price, petroleum engineers has sought by passed macroscopically by water flood. CEOR therefore, for ways to remedy this situation. With the introduction of is aimed at finding chemical that could be added to water to nanotechnology in EOR, it is a way to solve the numerous reduce the capillary forces and increase the sweep efficiency. problem plaguing the oil and gas industry. Nanotechnology is the use of nanoparticles ranging from 1 to 100 nm size in the study of combination science, medi- * Radzuan Junin cal, engineering and technology. At present, nanotechnology firstname.lastname@example.org has been used in medicine, electronics, electrical, space, sci- Department of Petroleum Engineering, Faculty of Chemical ence and engineering . It has enjoyed wide range of use and Energy Engineering, Universiti Teknologi Malaysia, because of its large surface area which makes it easier to UTM Skudai, 81310 Johor Bahru, Malaysia Vol.:(0123456789) 1 3 50 International Nano Letters (2018) 8:49–77 interact with solvent molecules when added to make sus- pension, optical transparency (copper nanoparticle), elec- trical conductivity (silicon nanoparticle), chemical catalyst (platinum nanoparticle), colour change (gold nanoparticle), thermal properties like heat transfer, cooling, insulation, and property of mechanical strength like ultra-high strength of material [3, 4]. Nanotechnology has also found its way to petroleum engineering, it is a well-accepted path in the oil and gas industry to recover more oil trapped in the reservoir, it has Fig. 1 Caption application of nanoparticles in EOR recorded success in reservoir characterization, drilling and well-completion jobs . In EOR, nanoparticle is still in the laboratory stage where its efficiency is being studied. nanoparticles include; sucrose, maltose, carbohydrate, chi- And few field trials have been reported [6 ]. Different labora- tosan, Arabic gum and biomolecules while the synthetic is tory studies [7, 8] and pilot field application have reported mostly derived from petroleum products .  that nanofluids can recover oil trapped in the reservoir. The addition of nanoparticles to a liquid, can result in the Therefore, nanoparticle can change the wettability of the simplest flow becoming complex, to understand the work - rock surface, reduce interfacial tension between oil and ing mechanism, there is need to study the flow behaviour of water interface, and lower the chemical adsorption into the these particles. There are a lot of reviews on nanoparticles in reservoir rock surface . oil and gas industry from Kong and Ohadi  to Sun et al. The application of nanoparticles in EOR process can be . The work of Hadi et al.  was on polymer-coated summarized into three major approaches; nanofluids, nano- nanoparticles, and Sharma et al.  discussed a little on catalyst and nanoemulsion . Nanofluids are usually base the effect of surfactant on nanoparticles, but they are more fluid with nanoparticles size less than 100 nm in colloidal general in the discussion, making the work of on this subject suspension . The base fluid in this case can be water, oil to be scattered in the literature. or gas. Nanofluid in EOR are usually used to improve water This review, therefore, highlights the mechanism affecting flooding and the mechanism responsible include; IFT reduction, the flow of nanoparticles in porous media as it relates to EOR. disjoining pressure, wettability reduction, viscosity increase, The discussion focuses on CEOR, a critical review on labora- pore channel plugging and asphaltene precipitation. Nanoparti- tory experiment on wettability alteration, effect of interfacial cles can also be used to stabilize emulsion, it can overcome the tension and the stability of emulsion and foam is discussed. challenges encountered by other stabilizers such as surfactants The mechanisms affecting the flow behaviour of nanoparticles . The high viscosity of nanoemulsion can control mobility in porous media was discussed laying emphasis on the physi- ratio during flooding. It is therefore, very useful in heavy oil cal aspect of the flow, the microscopic rheological behaviour recovery . And can penetrate through pore throats without and the adsorption of the nanoparticle. A compilation of the much retention. Whereas nanocatalyst are used as catalyst in experimental studies on nanoparticles flooding process as it steam injection of heavy oil reservoir . The advantages of affects incremental oil recovery is presented and two field using nanocatalyst to conventional catalyst include; upgrading trials are included. This review will provide update on exist- of heavy oil reservoir by converting bitumen to lighter products. ing literature on this subject and the recent development in Nanocatalyst such as iron and nickel can also breakdown car- CEOR. The challenges encountered have opened new frontier bon–sulphur bonds within asphaltenes, increase saturates and for research and are all highlighted herein. aromatics in the heavy oil . The summary of the application of nanoparticles in EOR is shown in Fig. 1. Two characteristics of nanoparticles that make them Nanotechnology in CEOR very attractive to the oil and gas industry are their large surface area and the ability to manipulate their behaviour Wettability alteration and structure. Therefore, nanotechnology involves the tai- loring of a material at the atomic level to attain a unique Wettability can be defined as the relationship between the property, which can then be manipulated at will for a certain solid surface to the solid–fluid and fluid–fluid interaction application. The different types of nanoparticles are; metal . Spreading of liquids and wetting of surface influences nanoparticles (gold, silver, lead and iron), organic poly- oil recovery mechanism. Surface agents can change the wet- mer nanoparticles and the inorganic polymer nanoparticles tability of rock surface; wettability affects capillary pressure (silica, tin, germanium phosphorus and sulfur), the organic and relative permeability curve. Nanoparticles have been polymers can be natural or synthetic. The natural polymer proven to have great impacts on wettability alteration. The 1 3 International Nano Letters (2018) 8:49–77 51 use of nanoparticles can form a two-dimensional layer in the correlation between the degree of wetting and the area under presence of a three-phase contact region formed between the the capillary pressure curve, it is consistent with the Amott oily soil and the solid substrate . It plays an important method but better in near neutral area. role in disjoining the structural pressure (force normal to the Wettability alteration by nanoparticles is due to their interface). The wettability of any solid surface can be deter- adsorption on the rock surface, which forms a water-wet mined using spontaneous imbibition, contact angle measure- layer on it. Adsorption of a very active and energetic mate- ment, zeta potential measurement and surface imaging test. rial on a solid surface can significantly alter the surface The changing of the wettability of the reservoir rock from energy and wettability of the system . Adsorption hap- oil-wet to water-wet can ease extraction of oil and improve pens when the balance between the capillary force and vis- oil recovery . A surface is said to be water-wet if the water cous forces changes. The viscous force is decreased, whereas contact angle is < 90° and oil-wet if the water contact angle the capillary force increases. Therefore, the viscous force is > 90° IFT. needed to overcome capillary forces are reduced . The large surface area of nanoparticles can lead to an increase Mechanism of wettability alteration by nanoparticles in the proportion of atoms on the surface of the particles, which results in an increase in surface energy with the abil- The surface and interfacial energies are used to determine if ity to alter the wettability of the rock . Mohammed and a surface is water or oil wet, the contact angle formed is as a Babadagli  described wettability alteration from oil- result of the force balance between the spreading coefficient wet to mix-wet and then to strongly water-wet condition by (S) of water on a solid in contact with both oil and water. analysing the balance between capillary and gravitational Nanoparticles increase S by removing the oily soils from forces. The shift in wettability from strong oil-wet to water- solid surface using the mechanism called ‘Rollup’. In labo- wet is because of two mechanisms. Firstly, from strong ratory experimentation, the contact angle, the Amott test, oil-wet to mix-wet and secondly from mixed-wet to strong the centrifuge method, the core displacement test is used water-wet . The main factor in the first mechanism is to determine wettability. The contact angle is used to deter- gravitational forces, which change the balance of the wetting mine the wettability of a three-phase system, it is used to forces. And displaces the weight of the fluid from the bulk determine formation wettability. The imaging method gives to the interface. Defined as capillary forces, by reducing the a clear picture of the wetting mechanism in oil–water–rock capillary forces, which leads to wettability alteration from system as shown in Fig. 2. The centrifuge method uses the strong oil-wet to mixed-wet . Also, the component for Fig. 2 Contact angle on a rock, a oil-air–rock before treatment, b oil–air–rock after treatment with silica, c water–air–rock before treatment, d water–air– rock after treatment with silica nanoparticles  1 3 52 International Nano Letters (2018) 8:49–77 oil/brine/rock such as oil/brine component, mineral surface, to increase recovery by 15.38% when treated with water- and system history condition may also control the wetting wetting wettability control agent (IWWCA) and increasing tendency . the oil-wetting wettability control agent (IOWCA) improves Giraldo  observed that in a fluid containing nanopar - recovery (60–80%) in water injection rate. Onyekonwu and ticles or micelles immersed in oil droplet on a surface, they Ogolo  studied the wettability alteration of three dif- were two contact lines instead of one that is usually observed ferent polysilicon nanoparticles to enhance oil recovery. in a traditional mechanism. This inner and outer spreading The nanoparticles used were lipophobic and hydrophobic lines, therefore, account for faster spreading of a nanopar- (LHPN), hydrophobic and lipophobic (HLPN) and neutrally ticle solution on a surface for higher concentration and vis- wet (NWPN). They reported that HLPN and NWPN are cosity compared to traditional mechanism. Which might good agent for enhanced oil recovery with a total recovery have led to increase in relative permeability of oil due to between 42.95 and 53.38% for LHPN, 70.62–86.92% for the wettability alteration by nanoparticles. Oil recovery can NWPN and 70.0–93.13% for HLPN. increase by 20% when high concentration of nanoparticles Wan Sulaiman et al.  used silica nanoparticle in car- is injected. They suggested 2–3% concentration as increase bonate rock to alter the wettability of the rock, and to test the in concentration can result to decrease in permeability . efficiency of silica nanoparticle in enhancing oil recovery in This is usually associated with the nanoparticle size, salt high salinity. They reported a 65.5% recovery of the original concentration and bulk volume [27, 28]. oil in place. Nazari et al.  in their work on wettability Moustafa et al.  used a combination of Magnesium/ alteration of carbonate rocks, ZrO , titanium dioxide (TiO ), 2 2 Aluminium layered double hydroxide, their results show magnesium oxide (MgO), aluminium dioxide (Al O ), 2 3 −1 that 4.0 g L concentration decreases the brine phase cerium oxide (CeO ) and carbon nanotube (CNT), where contact angle in the presence of oil from 66° to 60°. Hen- all used for contact angle measurement while calcium car- draningrat and Torsaeter  using Aluminium, Silicium bonate (CaCO ) and silicon dioxide (SiO ) where used for 3 2 and Titanium oxide nanoparticles on Berea sandstone core, core flooding. The result showed 8–9% additional recovery. reported a 5–7% increase in recovery by Titanium oxide, Maghzi et al.  monitored the wettability alteration using this shows that the wettability was altered as Titanium made silica nanoparticles during water flooding of heavy oil, they the quartz plate more water-wet. They concluded that oil experimented with micromodel and reported a 26% incre- recovery increases as nanoparticles size decreases and the mental oil recovery using silica nanoparticles. Experimental contact angle alteration decreases towards more water-wet results by some researcher are summarised in Table 1. quartz. Karimi et al.  demonstrated that zircon oxide nanoparticles can alter the wettability of carbonate rock Challenges of nanoparticles in altering wettability reservoir, the SEM and XRD analysis verified that nanopar - ticles can significantly change the wettability of reservoir Some factors could mitigate against the success of wetta- rock. The influence of nanoparticles on wetting behaviour bility alteration using nanoparticles, such factors include; of fractured limestone was also demonstrated by Nwidee concentration of the nanoparticles, nature of the reservoir, et al. . They investigated the influence of zirconium(iv) hydrophobicity of the nanoparticles, type of nanoparticles oxide (ZrO ) and nickel(ii) oxide (NiO). The contact angle and nature of the oil. measurement demonstrated that application of nanofluid in Hydrophilic polysilicon makes a water-wet formation oil-wet or intermediate-wet formation can increase oil recov- more water-wet, and this might lead to poor recovery [7, ery by wettability alteration as the nanoparticles adsorb on 38]. Therefore, hydrophilic polysilicon nanoparticles should the surface of the calcite crystals and promote oil displace- be restricted to oil-wet formation while, the hydrophobic ment. Time, concentration and salinity where responsible polysilicon should be restricted to water-wet formation. for the decrease in the water contact angle. The work of The nature of the oil also has a significant role to play in Mohammadi et al.  shows the success of gamma alumina wettability alteration, the selection of nanoparticles should nanoparticles in enhancing oil recovery in carbonate rock depend on the type of oil. Roustaei et al.  reported that when they observed a maximum change in contact angle silicon oxide is more effective in the wettability alteration and 11.25% increase in recovery. Ju and Fan  used lipo- of light oil. Whereas Huibers et al.  recommend the use phobic and hydrophobic polysilicon nanoparticles to alter of silica nanoparticles when the contact angle in light crude the wettability of sandstone from oil-wet to water-wet. TEM was mostly affected by the addition of 0.001 wt% nanopar - analysis revealed that nanoparticles where attached to the ticle, which altered the wettability of sandstone. While the pore walls. Again, the work of Li and Torsaeter  was contact angle of light crude plateaux, the heavy oil continued an improved study on wettability alteration and they con- to increase with increase in nanoparticle concentration, indi- firmed earlier reports. Ju et al.  using a physical model, cating that a maximum contact angle in heavy oil was not mathematical model, and a numerical simulator were able achieved. Nanoparticles disrupts the viscoelastic network 1 3 International Nano Letters (2018) 8:49–77 53 Table 1 Summary of the effects of nanoparticles in enhanced oil recovery References Nanoparticle used Recovery (%) Porous MEDIA Behaviour/finding Ju et al.  Polysilicon 20 Due to wettability alteration, recovery increased by 20% Ju and Fan  Polysilicon Sandstone Changed the wettability from oil-wet to water-wet. TEM revealed that nanoparticles where attached to the pore wall Onyekonwu and Ogolo  Polysilicon 42–93 Sandstone Altered the wettability of LHPN, HLPN and NWPN Villamizer et al.  SWCNT Good stability at different pH and salinity Ju et al.  IWWCA and IOWCA 15 and 60–80 Using a physical model, mathemati- cal model and numerical simulator Karimi et al.  Zircon oxide Carbonate rock SEM and XRD verified change of wettability Maghzi et al.  Silica 26 Micromodel Heavy oil Roustaei et al.  Polysilicon 28.5–32.2 Reduction in IFT from 26.3 to 1.75 −1 mN m Baez et al.  Amphiphobic and CNT Reduction in IFT Hendraningrat et al.  Silica 4.5 Reduction in IFT, solid phase more wet Nguyen et al.  Silica 17 Stabilized with CO foam and more stable than SDS Singh and Mohanty  Silica 10 Stabilized with Bioterage AS-40 Hendraningrat and Torsaeter  Aluminium, silicon and TiO 5–7 Berea sandstone Contact angle alteration decreases towards more water-wet quartz Moustafa et al.  Magnesium and aluminium Decrease the face contact angle from 66° to 60° Wan Sulaiman et al.  Silica 65.5% Carbonate rock Silica altered the wettability at high salinity Nazari et al.  ZrO, TiO , MgO, Al O ,CeO , 8–9 Carbonate rock CaCO and SiO where used for the 2 2 2 3 2 3 2 CNT flooding experiment Ragab and Hannora  Alumina 62–81 Reduction in IFT Kim and Krishnamoorti  Poly (oligo (ethylene oxide) Reduced the IFT from 50 to −1 Monomethyl ether methacrylate 20 mN m Griffith et al.  Silica 82 Sandstone core Stabilized with pentane water Kim et al.  Hydrophilic silica Sandpack Generated strong and stable emulsion with surfactant formed by asphaltenes aggregates in the presence of resins better performance than SiO and Al O nanofluids [32, 45]. 2 2 3 which causes viscosity reduction in heavy oil . There- Through microscopic imaging and theoretical calculations, fore, ZrO nanoparticles was reported by Wei and Babadagli the wettability alteration of ZrO is due to its deposition on 2 2  to have a potential to be used in the recovery of heavy rock surface which is governed by the partition coefficient oil (Tables 2, 3). of the nanomaterial in water and oil phase. The nature of reservoir also plays an important role in Hydration of nanoparticles is another major challenge determining the type of nanoparticles to be used. As all car- encountered, ions within the clay material interact with the bonate reservoirs are suggested to be oil-wet . There- water leading to hydration and expansion of the d-space fore, the right nanoparticle and optimum concentration of of the clay material . To combat this problem, the use the nanoparticle is required. A l O, SiO and F e O nano- of swelling inhibitors should be employed. In formulating 2 3 2 2 3 particles are effective for altering wettability of sandstone nanoparticles, the size control is very important for the suc- rocks . Whereas ZrO and NiO nanoparticles are effec- cess of EOR operations. The right size is needed to flow tive for Limestone formations . ZrO was considered a through the porous media, as large size will block the pore- better wettability modifier than NiO, and it also exhibited a throat of the media leading to formation damage. Also, 1 3 54 International Nano Letters (2018) 8:49–77 Table 2 Summary of the Flow Behaviour of Nanoparticles References Nanoparticles used Flow behaviour Findings Tseng and Lin  BaTio Newtonian to Dilatant With distilled water and NH PA as the sur- 3 4 factant, when NH PA was added the suspen- sion deviated from linear to dilatant Tseng and Lin  BaTio Pseudoplastic to dilatant In ethanol isopropanol with KD and PS-2 as the dispersant, change the Maghzi et al.  Silica Pseudoplastic A good match with power-law fluid Tseng and Wu  Al O Shear thinning A transition from shear thinning to shear 2 3 −1 thickening as the shear rate exceed 100 s , for double distilled water all suspension showed shear thinning behaviour Tseng and Chen  Nickel Shear thinning Only with nickel shear thinning of all suspen- sion, with -terpineol exhibited shear thinning for particle suspension Prasher et al. Alumina, Al O NewtonianAlumina/PG; Al O/water; Al O /EG 2 3 2 3 2 3 Tseng and Tzeng  ITO BINGHAM TO SHEAR THINNING In deionized water, with NH PA surfactant, viscosity reduced by 99% at low shear rate Bingham, change to shear thinning at high shear rate Yang et al.  NWCNT/poly-olefin oil Shear thinning and Newtonian Dispersed in polyisobutane succinimide, suspension with low and high concentration exhibited shear thinning while at intermediate was Newtonian Hong et al.  Fe O Newtonian and shear thinning Newtonian at low concentration, shear thinning 3 4 at high concentration I deionized water Phuoc and Massoudi  Fe O Newtonian and shear thinning In deionized water with PVP and PEO sur- 2 3 factant, PEO and PVP exhibited Newtonian behaviour at low conc. and shear thinning at high concentration −1 Tamjid and Guenther  Silver Non-Newtonian Pseudoplastic at shear rate range of 1–200 s with silver/DEG Phuoc et al.  NWCNT Newtonian and shear thinning With deionized water in chitosan as the disper- sant, at low concentration shows Newtonian while at high conc. Shear thinning Esmaeilzadel et al.  ZrO Adsorption Addition of ZrO to SDS surfactant increased 2 2 the adsorption onto fluid/fluid interface rather than solid–liquid interface Abdelhalin et al.  Gold Newtonian Gold in water viscosity decreased with a rise in temperature. Resiga et al.  Magnetite Newtonian Magnetite in transformer oil Duan et al.  Graphite Shear-thinning Graphite in deionized water enhancement of nanoparticle held for 3-days was higher than freshly prepared one Moghaddam et al.  Graphene Shear thinning/Newtonian Shear thinning for at all temperature and low shear rate. At high shear rate it was Newto- nian, then thinning with increase in concentra- tion Moatter and Cagincara  Fe O Shear thinning Fe O in PEG all suspension showed shear- 3 4 3 4 thinning Zaballa et al.  Alumina Adsorption Alumina was used at reservoir condition with Mirador-formation plug cores, production increased by 100,000bbl Bayat et al.  Al O, TiO Retention Decline in recovery as a result of clay in the 2 3 2 porous media, position of clay at the pore- throat caused trapping Cheraghain  Fumed-silica Pseudoplastic Viscosity increased with weight of nanoparticle 1 3 International Nano Letters (2018) 8:49–77 55 Table 3 List of Studies on Oil Displacement Using Nanoparticles References Nanoparticles used Oil recovery (%) Rock type/porous media Remark Onyekonwu and Ogolo  LHPN, NWPN, HLPN 42.9–93.13 Sandstone Total recovery Ogolo et al.  Al O , MgO, Fe O , NiO, 18.3–30 Sandstone Silicon was treated with silane 2 3 2 3 ZrO , SnO, silicon Maghzi et al.  Silica 26 Micromodel Incremental recovery Mohammadi et al.  γ-Al O 11.25 Carbonate rock Change in contact angle 2 3 Maghzi et al.  Silica 10 Micromodel With polyacrylamide solution Joonaki and Ghanaatian  Fe O, Al O , silica 88.6–95.3 OOIP Sandpack Silica was treated with silane 2 3 2 3 Charaghian  Sodium bentonite nanoclay 66.6 OOIP Micromodel Size of nanoclay is 50 nm Charaghian  Nanoclay 5.8 Sandpack Copolymerized Manan et al.  SiO , CuO, TiO 5–14 Sandpack Used as a stabilizer in CO 2 2 2 Nazari et al.  CaCO, SiO 8–9 Carbonate rock Incremental recovery 3 2 Moradi et al.  Silicon 72–79 Carbonate rock OOIP Sharma et al.  Silicon 60 cumulative Berea sandstone Used as a stabilizer for surfactant AND polymer El-Diasty  Silicon 35–50 OOIP Sandstone During breakthrough point Chen et al.  MWNTs and CB 41.7% Sandstone Cumulative recovery Singh and Mohanty  Aluminium coated silica 70–75 Berea sandstone Cumulative recovery Alomair et al.  Al O , NiO, SiO, TiO 1–6 Berea sandstone Incremental recovery 2 3 2 2 Ragab and Hannora  Silicon and aluminium 7 Berea sandstone Incremental recovery Bayat and Junin  Al O, TiO, SiO 48.7–52.6 OOIP Limestone 47.3% OOIP for water flooding 2 3 2 2 Sulaiman et al.  Silica 65.5 Carbonate rock OOIP Cheraghian  Titanium oxide 4 Sandstone Increased viscosity of polymer Cheraghian  Silica 8.3 Sandstone Increased viscosity of polymer Jafernezhad et al.  SnO 61 OOIP Carbonate core plug Nanoparticle-altered wettability and reduced IFT Azarshin et al.  Silica 18 Iranian reservoir core Modified Silica surface was more effective Zallaghi et al.  Silica 11.7 Sandstone core Youssif et al.  Silica 13.28 Sandstone Low concentration decreased perme- ability Nwidee et al.  ZrO ; NiO Limestone Altered wettability and reduced IFT nanoparticle retention and entrapment must be prevented to Mechanism of IFT reduction the barest minimum for a successful wettability alteration to be achieved. IFT is used to determine nanofluid movement in porous media, it is important to determine the IFT between the oil Interfacial tension (IFT) reduction and fluid in EOR technique. The pendant drop and the spin- ning drop methods are widely used to measure IFT. The the- The main mechanism is the migration and arrangement of ory behind the mechanism of operation in nanoparticles IFT the oil–water interface, which is dependent on the hydro- tension is still under debate, but to get a good insight into phobic and hydrophilic properties of the nanoparticles. The the working mechanism between the nanoparticles, rock and purpose of surfactant flooding is to increase the capillary fluid. The Poisson–Boltzmann and Derjaguin–Landau–Ver - number by reducing IFT between the oil and water [1, 47]. wey–Overbeek (DLVO) theory of approximation are used. Nanofluids of 70–150 nm dissolved in aqueous solution of Be that as it may, to understand the forces in play, the par- surfactants can result in effective oil displacement of 35% ticle size, the force of attraction and DLVO theory must be compared to using only surfactants in a homogeneous reser- considered. voir, and 17% in a heterogeneous reservoir at a temperature of 25 °C. The increase in recovery because of lowering of Particle size The morphology of particles as well as the area IFT occurs when the fluid changes characteristics from New - to volume ratio for the particle molecule to interact with tonian to non-Newtonian state . fluid molecules is an important aspect that governs surface 1 3 56 International Nano Letters (2018) 8:49–77 tension in a complex fluid. Near-spherical morphologies has a minor increment in the interfacial energy compared to hexagonal pillar and flake-like morphologies. Therefore, surface tension of nanofluid also shows slight increment with size . Nanoparticles have high surface to volume ratio with high contact area. These enable a high diffusion rate, mass transfer which can change the properties of fluid . Nanoparticles can be measured in microscale that is, they have the tendency to spread homogenously in porous media, reaching corners that originally were not touched by conventional methods. The polarity of fluid can also affect surface tension of nanoparticles according to the electric double layer (EDL) formed at the particle–fluid interface. The particles at the interface will experience a weaker Fig. 3 Schematic representation of nanoparticles at fluid air interface repulsive force from the particles in the bulk. There is a ten-  dency of desorption to the bulk from the interface leading to no appreciable change in surface tension. This might be Poisson–Boltzmann equation is used to predict nanopar- because of high viscosity of the fluids. Particles Brownian motion might have been partially hindered and the particles ticle flow behaviour in porous media at charged interface , which is expressed as; finds it difficult to adsorb from the bulk to the interface and vice versa. 2Zen se The governing mechanism is a multiphase zone compris- ∇ = sinh , (1) BT ing of the interface of the solid nanoparticles, the suspended base fluid and the coexisting interface. The surface energy whereas ψ is the electric potential, z is the axis perpendicu- associated with each of these interfaces depends on the char- lar to the surface, e is the charge of the electron, n is the acteristics of the nanoparticles suspended, the base fluid of particle concentration, is the permittivity, h is the height the suspension and the surrounding fluid region . The of nanochannel, s is the cross section, K is the Boltzmann bulk surface tension of the resulting suspension is deter- constant, T is the temperature. mined by the collective interaction of the vector additions Most colloidal particles are surrounded by EDL. It can of the forces at the microscopic and the summation of such be assumed that nanoparticles suspended in the base fluid interaction. The hydrophilic or hydrophobic nature of the in the form of colloidal particles are surrounded by EDL. particles is the driving force that determines the affinity of The force of attraction and repulsion are exerted on the the particles towards one of the phases . The particles nanoparticles . The force of attraction is governed by affinity towards the surface is a strong function of the equi- van der Waals forces, which induces coalescence of the librium interface contact angle, which favours the particles particles if the distance between the particles exceeds the partial wetting behaviour. The mechanical agitation of the energy barrier. Whereas the force of repulsion is governed particles at the fluid–air interface is shown in Fig. 3. The by Coulombs force, which prevents coalescence of the par- particle must satisfy the minimum energy criteria to be posi- ticles . The distance of the EDL can determine the tioned at the interface with height (h) protruding out from amount of Coulomb force acting on the nanoparticles. As the fluid interface as shown in Fig. 3. Creating a localized sufficient Coulomb force is needed to act on the nanopar - contact angle (θ), which is the point of stable equilibrium ticles to attain a stable nanofluid . made by the particles at nanoscale three-phase contact point . Whereas the particle radius is (r) and the radius of the DLVO theory DVLO theory states that the stability of interface area of the fluid occupied by the solid particle is two particles in close proximity is dependent on the total (b). energy of their interaction. It considers the forces of attrac- tion and repulsion between the nanoparticles, rock and Electrical layer As the reservoir is said to have a charged fluid such as van der Waals attraction, EDL, born repul- surface, so also is the rock surface which is considered to sion, acid–base interactions and hydrodynamic forces have a constant net charge, positive forces are arranged at . Based on DLVO modelling, the basic assumption is the rock surface while negative forces in the fluid inter - that the particles are spherical, which does not hold for face, ignoring the gravitational forces working on the nan- all manufactured nanoparticles as some are rod-like, tri- oparticles for pronouncement of the charged forces, this angular, polygonal. The force of attraction between these region of opposite charge is referred to as the EDL. The 1 3 International Nano Letters (2018) 8:49–77 57 particles differs and there is need to reduce the electro- understand the relationship between the IFT and solubili- static repulsion between the nanoparticle building block sation. Factors such as concentration of surfactant, salin- . Therefore, the small size and large surface area of ity, temperature, divalent ions, and formation can affect nanoparticles conflicts with the fundamental assumptions IFT [1, 60]. of DLVO theory leading to series of challenges. When particles reach a small size, it surfaces curvature becomes Effect of formation The high adsorption rate of anionic too substantial enough to assume it is flat, which becomes surfactant in carbonate reservoir coupled with the high a problem when considering its aggregation with respect cost of cationic surfactant has made the application of sur- to DLVO theory. This is because as the size of the parti- factant flooding to be limited to sandstone reservoirs and cle decreases, high number of the atom exist at the sur- only few applications in carbonate. But Bayat and Junin face which will affect the electronic structure, surface and  reported that nanoparticles of Al O, TiO and SiO 2 3 2 2 reactivity charge . The chemical composition of most were able to recovery 52.6, 50.9 and 48.7% OOIP, respec- material can also pose a problem to this theory, as some tively in limestone porous media. These high recoveries ferromagnetic materials like iron and magnetite can cause were attributed to the nanoparticles mobility. Also, the magnetic attraction without a magnetic field which can magnitude of the EDL repulsion in comparison with the lead to rapid aggregation. Therefore, the shape and size London-van der Waals attraction between the formation of nanoparticles can control nanoparticle aggregation and was greatly diminished when nanoparticles propagated should be taken into consideration when manufacturing through the porous media . Godinez and Darnault  nanoparticles. believed that deposition process is a key retention mecha- Roustaei et al.  measured the IFT and wettability nism of nanoparticles. This because, as the solution pH of of polysilicon nanoparticles, using hydrophilic and hydro- nanoparticles approach point of zero charge, the mobility phobic polysilicon (HLP) and naturally wet polysilicon and transport of the nanoparticles will be limited. This (NWP). They reported a reduction in the IFT from 26.3 to is due to the reduction of electrostatic interaction forces, −1 1.75 mN m and, oil recovery increased to 32.2 and 28.5% leading to the increase in the deposition rate. Whereas Ju −1 with 4 g L of HLP and NWP, respectively. Hendraningrat and Fan  reported that adsorption of nanoparticles at et al.  also reported that hydrophilic silica nanoparticles pore walls and pore throat blocking is high close to the can reduce IFT between water and oil phase and make the inlet of the porous media. The control of particle size dur- solid phase more wet, the nano fluid increase the oil recovery ing injection and reaction times of nanoparticles inside the about 4.5% compared to brine flooding. porous media is very important. This is because the size Baez et al.  used a novel amphiphobic nanoparticle- of the particles could reduce the pressure through perme- based functionalized CNT and they were very effective in ability reduction. Also, particles size could impact on dis- reducing IFT. persion ability, adsorption affinity and catalytic activity of Ragab and Hannora  using alumina in flooding exper - nanoparticles inside the medium . iment had a recovery in the range of 62–81%, the reason was due to the reduction in IFT of the nanoparticles. Effect of salinity and divalent ions Surfactant tends to Kim and Krishnamoorti  studied the behaviour of precipitate at high salinity and for most surfactants the water soluble poly oligo (ethylene oxide) monomethyl ether optimum salinity is not high and as such, divalent may be methacrylate. The synthesized nanoparticles reduced the associated with it, thus affecting surfactant performance −1 interfacial tension from 50 to 20 mN m at a concentration . Wan Sulaiman et al.  reported the use of hydro- of 1–100 ppm. philic silica nanoparticles in oil-wet limestone at differ - ent nanofluid concentration with different formation brine Comparative study on the effect of surfactant concentration. A high recovery of up to 65.4% OOIP by and nanoparticles on IFT reduction lowering of the IFT was reported. Figure 4 shows the oil recovery at different concentration of brine with nanofluid. The use of surfactant to reduce IFT tension has its own Bayat and Junin  concluded that the mobility of nan- limitations, in this section, an attempt is made to compare oparticles through porous media strongly depend on the these limitations with the solutions nanoparticles can pro- stability, porous media surface charge and roughness. This vide vis-a-vis their effects. The mechanism of surfactant implies that nanoparticles with the same surface charge as and nanoparticles is to reduce IFT, in surfactant flooding, the porous media are more stable against deposition and this is closely related to solubilisation of oil and water. can easily be transport through the porous media. Whereas On like nanoparticles which uses the Poisson–Boltzmann those with opposite surface charge will lead to noticeable equation and DLVO theory, surfactant uses the Healy adsorption on the porous media. and Reed correlation equation and the Huh equation to 1 3 58 International Nano Letters (2018) 8:49–77 Challenges of nanoparticles in reducing IFT At lower surfactant concentration, addition of nanoparticle reduces IFT but, at higher concentration, IFT increases when nanoparticles are added. This can be attributed to the elec- trostatic repulsive interaction between the nanoparticles and the surfactant that promoted the diffusion of the surfactant towards the interface . Nanoparticles can act as a car- riers of surfactant molecules towards the interface, but at high concentration the nanoparticles attract the surfactant molecules which can lead to aggregation of the surfactant molecules . One of the major challenges of nanomate- rial design is the control of colloidal stability of the particles Fig. 4 Oil recovery at different concentration of brine with nanofluid to prevent aggregation and damaging interaction with the  surrounding, as tiny particles tends to aggregate and form bigger particle cluster. This is due to the surface energy as a result of high surface energy of the nanoparticles which might lead to adsorption of other particles or molecules on the surface. This might change the physical and chemical properties of the nanoparticles making it less effective in reducing IFT. The hydrating layer surrounding each nano- particles act as a repulsive barrier which prevents the nano- particles from attaching to each other due to the attractive van de Waals forces. Therefore, at lower linker concentration both spherical and rod-like nanoparticles tend to form linear chains because of the need to reduce the electrostatic repul- sion between the building blocks of the nanoparticles. When the concentration of the linkers is increased the attachment is no longer linear . Interfacial interaction between two faces in a hybrid solution is the most decisive factor affect- ing the properties of the resulting material. Therefore, the dispersion of the nanoparticles is of great importance to the properties of the nanoparticles as such surface modification Fig. 5 IFT of oil/aqueous phase at different temperatures and at ambi- with active functional groups can optimize the efficiency of ent temperature  the process . Nanoemulsions in EOR Effect of reservoir temperature Surfactant also precipi- tate at high temperature, the median temperature for the Nanoemulsion have drop length-scale less than 100 nm, they surveyed surfactant project is 25.3 °C . Ranka et al. can retain their morphology with the change in oil volume  stabilized nanoparticles at high-temperature reser- fraction. The ease of nanoemulsion preparation stability and voir condition. They reported the use of hydrophilic silica increased bioavailability are the main features of their for- and hydrophobic polystyrene nanoparticles to achieve a mulation which have attracted researchers . They are long-term colloidal stability up to twice the stability limit basically three types of nanoemulsion; oil in water emulsion previously reported in open literature. Bayat et al.  where, oil droplets are dispersed in the continuous aqueous demonstrated that using Al O, TiO and SiO nanofluid phase. Water in oil emulsion where, water droplets are dis- 2 3 2 2 can reduce IFT by 33, 37 and 42%, respectively compared persed in the continuous aqueous oil phase. And bi-contin- to brine. The reduced trend was observed for all tempera- uous nanoemulsion where, micro domains of oil and water tures as shown in Fig. 5. This shows that nanoparticles intersperse within the system. The characteristics of nanoe- have low affinity to be adsorbed compared to surfactants. mulsion that makes them attractive for EOR application The low adsorption rate can lead to increased oil recovery. includes; their lack of shear-thickening and sedimentation The mechanism in play is the decrease in capillary forces, problem. This is because of their small size. Nanoemulsion by the deformation of the trapped oil droplets. have high dispersibility compared to microemulsion. Their 1 3 International Nano Letters (2018) 8:49–77 59 small size droplet can prevent flocculation. And they are pores . It also helps to entrain oil into the mobile aque- easily stabilized against Brownian collision with a polymeric ous phase which will lead to a better sweep efficiency. Stud- surfactant that produces steric repulsion . ies have also shown that emulsion can increase viscosity and decrease mobility ratio, which will in turn lead to increased Application of nanoemulsion in EOR recovery . Surfactants are mostly used to stabilize emul- sion but under harsh reservoir condition, surfactant emul- Nanoemulsion are suited for large-scale field applications, sion has its limitations and its high mobility ratio cannot be this is because they can penetrate through pore throats with- controlled. Nanoparticles can be used to stabilize emulsion out retention . This attribute of nanoemulsion, has had a and is more advantageous than surfactants. Droplet images huge impact on EOR mechanism as residual oil is recovered of emulsion made with 0.1 wt% cetyl trimethylammonium from the reservoir. Small drop size of nanoemulsion which bromide (CTAB) and various nanoparticle concentration is is usually smaller than the pore throat in gravel-pack and shown in Fig. 6. reservoir rock can result in good injectivity and penetration Nanoparticles have significant effect on the displacement without filtration . This can prevent the issue of gravity- dynamics of wetting and non-wetting phases, in situ emul- driven separation due to the density difference of the two sion takes place when octane displaces brine containing phases. Binks et al.  also reported that nanoemulsion nanoparticles. Nanoparticle stabilized emulsion can also can withstand harsh reservoir condition of high pressure, stabilize oil front, but the physics behind the non-wetting temperature, shear and salinity. Therefore, nanoemulsion can and wetting phase imbibition is different for nanoparticles remain stable in the reservoir at these prevailing conditions. dispersion displacement process. The coated nanoparticles could accumulate at the displacement front at the oil–water Eec ff t of nanoparticles on stability of emulsion interfacial area because of the lipophilic nature of the nano- particles. Nanoparticle can seriously affect the phase behav - Emulsion when generated in situ or injected have the capac- iour of oil–water system, nanoparticle-induced toluene-in- ity to divert flow to the by-passed oil by blocking swept out water and crude oil-in-water emulsions are stable at elevated Fig. 6 Droplet images of emulsion made with 0.1 wt% CTAB and various nanoparticle concentration  1 3 60 International Nano Letters (2018) 8:49–77 temperature. This was achieved by the short-chain polyeth- concentration. The type of nanoparticle and the concentra- ylene oxide polymer with silane end group which coated tion of the nanoparticles are the two influential parameters over the nanoparticles. But other hydrocarbon like hexane, on the stability of foam and the nanoparticle suspension. decane and mineral oil are not able to form stable emulsions. Which can be made to have a better stability thereby, pro- The concentration of salt and concentration nanoparticles longing the generated lifetime of the foam . SiO and has opposite effects on the stability of emulsion. Viscous Al O nanoparticles were reported by Bayat et al.  to 2 3 emulsion is more stable and are very important for oil recov- have high foam stability based on the foam half-life time, ery . and a direct relationship between the nanoparticle stabil- Villamizer et al.  studied the stability of silica nano- ity against deposition in aqueous phase and foam stability. hybrid single wall carbon tube (SWCNT) in EOR. They Therefore, the stability of the nanoparticles against deposi- reported that the hybrid shows good stability at different pH tion in the aqueous phase before it is utilized for fabrication and salinity. Low concentration of nanoparticles can stabi- of foam was also an important factor in stabilizing C O foam lize emulsion for months which has a potential application with nanoparticles. This was observed when the nanoparti- in EOR. cles affected the morphology and size of the foam bubbles Pei et al.  showed that nanoparticles-surfactant sta- as the shape changed from polyhedral to spherical and the bilized emulsion was able to recover heavy oil by mak- size of the bubbles becomes smaller and uniform . But ing emulsion thick and achieved desired mobility that can in the presence of oil, foam stability depends mainly on the improve the sweep efficiency. In their experiment using a viscosity and density of the oil . The addition of nano- micro-model as the porous media, they reported a recovery particles will increase the stability due to aggregation of the of 40% of OOIP. nanoparticles at the thin Lamella of the foam (Fig. 7), which Griffith et al.  in their work on Boise sandstone cores prevented spreading of the oil at the gas–liquid interface. with silica nanoparticles-stabilized pentane in water (natural The modification of silica-sodium dodecyl sulfate (SDS) gas liquid-emulsion) as the injecting fluid, while light oil mixture resulted in optimum foam stability, whereas the was the residual oil, the result showed up to 82% residual slower liquid drainage from the foam did not result in high oil recovery. foam stability. Nanoparticles when used to stabilize foam, Kim et al.  studied the synergy’s benefit of employ - can withstand high temperature reservoir condition, at low ing nanoparticles in emulsion for improved mobility con- concentration, nanoparticles can be used to stabilize foam, trol especially under high salinity control using hydrophilic but at high salinity, the stability of foam decreases . silica nanoparticles and surfactant in oil-in-brine emulsion This was observed by Yekeen et al.  when they noticed formation. They successfully generated a strong and stable that the foam stability decreased in the presence of salt until emulsion with a combination of either cationic or non-ionic the transition salt concentration was reached. Beyond the surfactant with nanoparticles compared to surfactant and transition salt concentration, the foam stability will increase nanoparticles themselves alone. with the increasing salt concentrations . The dominant The combination of two or more nanoparticles can be mechanisms of the foam flow process were lamellae division employed to stabilize emulsion. McClements and Jafari and bubble to multiple bubble lamellae division as shown in  reported that the combination of two different emulsi- Fig. 7. These mechanisms dominated the residual oil mobili- fier may lead to the formation of emulsion with different zation and displacement by the foam and it was found to be a droplet size and has a better stability compared to when used directly proportional to the displacement and emulsification separately. Using mixture emulsie fi r during homogenization of the oil as shown in Fig. 8. In a water-wet system (Fig. 8), process can reduce the size of the droplet produced. This is the dominant mechanism of the flow process and residual because one of the emulsifier may adsorb quickly and reduce oil mobilization are lamellae division and emulsification oil, the IFT. But may be less effective in stabilizing the droplet respectively, but in the oil-wet system, the generation of pore against coalescence. Whereas the other emulsifier may be spinning continuous gas foam was the dominant mechanism less effective at reducing IFT but, maybe very effective in governing the foam flow process and residual oil mobiliza- impeding droplet coalescence. tion . In the generation of S iO -SDS and A l O -SDS 2 2 3 foam, the pore level mechanism controlling the behaviour Foam stability using nanoparticles of the nanoparticle-surfactant foam in the porous media are similar which led to improved foam dynamic stability in The change in height of form generated with time and the water-wet and oil-wet porous media . time taken by the foam to reach half of its original life is Generally, the performance of foam was enhanced with a measure of its stability. The rate of foam collapsed can increasing nanoparticle hydrophobicity which increases sta- decrease with increasing surfactant concentration. However, bility but decreases foamability with increasing concentra- foam stability only increases with increase in surfactant tion of the nanoparticles. Therefore, nanoparticles increase 1 3 International Nano Letters (2018) 8:49–77 61 Fig. 7 Occurrence of bubble-to-multiple-bubble lamellae division, a SiO -SDS foam flow in the absence of oil, b SiO -SDS foam flow process 2 2 in the presence of oil, c Al O -SDS foam flow process in the presence of oil  2 3 foam apparent viscosity, improves foam stability by adsorp- the mobility in EOR using nanoparticle stabilized CO in tion and aggregation at the foam lamellae which increases water foam using microfluidic method. Their report shows the film thickness. And dilatational viscoelasticity which that coated silica nanoparticle stabilized CO foam is more prevents liquid drainage and film thinning leading to bulk stable than SDS, which recorded an additional 17% oil after and bubble scale stability . water flooding. Experimental results show that foam can be generated Mo et al.  reported increase in recovery from 64.9 to −1 at a critical shear rate higher than 4000 s . But using 75.8% when pressure was increased from 1200 to 2500 psi dynamic experiment to produce foam by nanoparticle dis- but an increase in temperature reduced the recovery when persion with C O injection in a glass bead column, sta- working with nanosilica-stabilized CO foam. 2 2 −1 ble form was formed at shear rate higher than 1419 s at 1500 psig. Increase in the injection rate increased the shear −1 rate to 3312 s , and increased viscosity from 1.5 to 2.5 Nanoparticle flow behaviour in porous times higher than the normal dispersion without foam . media Yu et al.  applied nanoparticle stabilized C O foam to improve oil recovery in high and low permeability core after Nanoparticles must be able to flow deep into the reservoir water flooding, they reported a 48.7% recovery in a 33 mD to assist in oil displacement, studies have shown that some permeability and 35.8% in a 270 mD permeability. challenges are encountered in the flow of nanoparticles in Singh and Mohanty  studied foam stabilization with porous media [93, 94]. Therefore, the need to understand silica nanoparticles and Bioterage AS-40 surfactant for the mechanism affecting the flow of nanoparticles in porous EOR. They reported that the concentration of nanoparticles media is of importance. The emphasis here will be on the is important for foam stability, which increases the recov- physical aspect of the flow, the microscopic rheological ery by 10% of OOIP. Also, Nguyen et al.  controlled behaviour and the adsorption of the nanoparticles in porous 1 3 62 International Nano Letters (2018) 8:49–77 Fig. 8 Mechanism of residual oil mobilization in water-wet system, a emulsification of oil during SiO -SDS foam flow, b effective emulsifica- tion, c oil emulsification, d inter-bubble trapping and oil trapping at the pore walls  media. The transparent visualization of fluid flow through Nanoparticle adsorption in porous media porous media and the mechanism that takes place during oil recovery by nanoparticle or nanoparticle retention in porous The flow of nanoparticles through porous media exhibit a media can be well understood by microfluidic approach Brownian motion, due to the size of the particles, several using lab-on-a-chip approach, and by macro and micromodel forces like the Van der walls forces attracts potential forces experiments. and control the interaction between the nanoparticles and the porous media walls . Adsorption and desorption takes place depending on the attraction and repulsion force Nanoparticle filtration between the porous media wall and nanoparticles. During the flow of nanoparticles through porous media, mainly This occur when the particles are larger than some of the diffusion, convection and hydrodynamics play major role pores in the porous media, especially when nanoparticles . However, the adsorption of nanoparticles onto rock are co-polymerized or with surfactants. This may also occur surface is influenced by born repulsion and controlled by for non-aggregated nanoparticles when injected in low per- hydrodynamic forces. meability rocks (tight sandstones). Therefore, the size and shape of the nanoparticles are important parameters that can 1. Born repulsion: the adsorption of nanoparticles onto the affect filtration. It is important to note that filtration can be surface is influenced by the born repulsion which occurs initiated by the larger particles, which will cause further because of the coming together of the nanoparticles sur- filtration, which may lead to decrease in size of the pores face and the walls of the pores of the media. after initial filtration . 1 3 International Nano Letters (2018) 8:49–77 63 2. Hydrodynamic force: the hydrodynamic force controls of the channel and thereby reduce permeability by changing the suspension of a flowing liquid, when the nanoparti- the pressure drop during the experimental process. cles flows through porous media, if the hydrodynamic Bayat and Junin  studied the influence of clay parti- forces are low, the particles will be suspended onto pore cles on Al O and T iO nanoparticles transport and reten- 2 3 2 surface and might get adsorbed depending on the surface tion through limestone porous media. They concluded that charge. there was a decline in the recovery with Al O and TiO 2 3 2 nanoparticles because of the presence of clay in the porous Adding nanoparticles into chemical slugs reduce media. The position of the clay particles at the pore-throat adsorption. These happen during the interaction of chem- and the morphology of the clay caused the nanoparticles icals and the rock surface, hydrogen bonding, covalent to be trapped. Therefore, the mobility of Al O and TiO 2 3 2 bonding, hydrophobic bonding and solvation of vari- nanoparticles through porous media is sensitive to clay type ous species. Therefore, it is a necessity to prevent this and concentration. interaction, EOR chemical adsorbs at solid–liquid inter- face by transferring molecules from solution phase to the Rheological flow behaviour of nanofluids solid–liquid interface. This only happens when the mole- cules are favoured in comparison to the bulk phase. Micel- The rheological flow behaviour of fluid is defined based on lization starts to form at higher concentration and form the relationship between the shear stress (τ) and shear rate hemimicelles with one or two layers . The solid sur- (γ), where the shear stress is, the tangential force applied face adsorption starts to increase until a bilayer is formed per unit area, whereas the shear rate is the change of shear on the solid surface. Addition of nanoparticles helps the strain per unit time. Viscosity (µ) therefore, can simply be molecules at the liquid–liquid interface and affect the defined as the resistance to flow of liquid suspension, which interfacial and adsorption behaviour of the process [96, is expressed as the ratio of the shear stress to shear rate. 97]. Even when nanoparticles are of the right size and Fluid behaviour can, therefore be classified as Newtonian stable in solution, adsorption can also impede their trans- and non-Newtonian. For Newtonian fluids, the relationship portation through porous media. The less the nanopar- between the shear stress and rate is linear, that is it remains ticles are adsorbed, on the rocks, the economics of oil constant while, that of the non-Newtonian changes with recovery is improved . Studies have shown that when shear stress and rate. nanoparticles are coated with polymer, it results to stable Non-Newtonian fluids can be classified into four major particles in solution, but it can also lead to high adsorption group; the dilatant where the viscosity of the fluid increases and retardation of nanoparticles when injected into the when shear is applied example: quicksand, corn flour and porous media [99, 100]. The electrostatic repulsion and water. The pseudoplastic in this case the more shear is the hydrophobic and hydrophilic interaction between the applied, the fluid become less viscous example: ketchup, nanoparticles and the rock can reduce adsorption. Nano- polymer. Rheopectic this is similar to dilatant, but the dif- particles with favourable surface charge that matches that ference is when shear is applied, the viscosity increases of the rock may exhibit less adsorption . with time, that is, it is time dependent example: we have Esmaeilzadel et al.  studied the adsorption of anionic, cream, gypsum paste. Thixotropic where shear is applied, cationic and non-ionic surfactant on carbonate rock in the the viscosity decreases with time, which makes it also time- presence of ZrO nanoparticles. They reported that adding dependent examples are: paint, cosmetic, asphalt, glue. We ZrO to SDS surfactant increases the adsorption onto fluid/ can therefore, conclude that non-Newtonian fluid are either fluid interface rather than solid–liquid interface. time-dependent or non-time dependent. Zabala et al.  investigated the adsorption capacity of alumina nanoparticles at reservoir condition with Mirador- Newtonian and non‑Newtonian flow behaviour formation plug cores. They reported a remarkable increase in of nanofluids production 100,000 bbl cumulative production in 4 months, by injecting nanoparticles suspension into a glass model. The rheology flow behaviour of nanofluids can be measured Hendraningrat et al.  investigated nanoparticles reten- by the rheometer [103, 104]. The rheological flow behaviour tion in porous media. From the result of the visualization of nanofluid affects the pressure drop of the nanofluid. And experiment, they concluded thus; that nanoparticles were therefore, gives an idea of the nanofluid structure which can deposited and adsorbed at the surface, which led to reduction be used in predicting the thermal conductivity of the fluid in permeability by blocking the pore throat of the model. . Li and Torsaeter  investigated the performance of Richmond et al.  studied the rheological behaviour nanoparticles in oil recovery, they reported that adsorption of SiO and TiO in deionized, they observed that SiO alone 2 2 2 of nanoparticles in glass micro model may cause plugging displayed a Newtonian behaviour while, SiO /TiO mixture 2 2 1 3 64 International Nano Letters (2018) 8:49–77 exhibited a Bingham plastic behaviour. The addition of and shear rate range between 1 and 1000. They concluded TiO , increased the plastic viscosity compared to pure SiO . that all the suspensions showed Newtonian behaviour. 2 2 TiO nanofluid in distilled water showed shear thinning, the intensity increased with concentration of the nanofluid, the Eec ff ts of surfactants on the rheological flow behaviour viscosity decreases with temperature [103, 106]. But upon of nanofluids −1 exceeding a shear rate of 100 s , it showed a Newtonian behaviour . Alphonse et al.,  and Penkavova et al. Surfactants are used to prepare stable nanofluids, to achieve  on the other hand reported a Newtonian behaviour for a uniform particle structure throughout the suspension. It is TiO with water at low shear rate and changed to shear thin- observed by most studies that the addition of surfactant to −1. ning when the shear rate exceeded 100 s the nanofluid changes the flow behaviour of the nanofluid Tseng and Wu [109, 110] studied the behaviour of Al O [123, 124]. 2 3 with pure water and double distilled water in a shear range of Yang et al.  work on multi-walled carbon nano- −1 1–1000 s . For pure water, there was a transition from shear tube (MWCNT)/poly-olefin oil, dispersed in polyisobutene −1 thinning to shear thickening as the shear rate exceed 100 s , succinimide. They concluded that the suspension with the while for double distilled water, all the suspension showed lowest (0.3%) and highest (8%) dispersant concentration shear thinning behaviour at low shear rate and shear thick- exhibited a shear thinning behaviour while, the intermediate ening as the shear rate exceed the critical value. Whereas (3 wt%), exhibited a Newtonian behaviour. In their separate the work of Prasher et al.  and Anoop et al.  on work on MWCNT/poly-olefin oil but with PIBSI 1000 and alumina/propylene glycol (PG), Al O/water, Al O /ethylene PIBSI 500 as the dispersant, they reported that the suspen- 2 3 2 3 glycol (EG) nanoparticles, CuO/EG, they observed all the sion without dispersant showed slight shear thinning behav- fluids exhibited Newtonian behaviour. iour at low temperature. But the shear thinning increase with The work of Tseng and Chen [113, 114] was on nickel/ increase in temperature and was very strong at 75 °C. The −1 terpineol at shear rate ranges between 1 and 1000 s . All the suspension with PIBSI 1000 showed a mild shear thinning suspension containing nickel nanofluid showed shear thin- behaviour while, that of PIBSI 500 was very strong . ning behaviour over the entire shear rate. Whereas nickel/- Phuoc et al.  in their work on MWCNT/deionized terpineol exhibited shear thinning behaviour for particle water with chitosan as the dispersant, reported that suspen- concentration. Silver/diethylene glycol (DEG) nanofluid sion with low CNT and chitosan concentration, behaved as exhibited a non-Newtonian (pseudoplastic) flow behaviour Newtonian Fluid. While at high concentration, it exhibited −1 at a shear rate range between 1 and 200 s and the viscos- shear thinning behaviour. Upon adding 0.1–0.2 wt% of chi- ity increased with increase in the concentration of the fluid tosan in water increased the viscosity, while adding 0.5 wt% . While, nanofluid of gold/water, the fluid exhibited decreased the viscosity and changed the flow behaviour to Newtonian behaviour, larger-size (50 nm) nanofluid showed non-Newtonian. higher viscosity compared to smaller size (10–20 nm) nano- Tseng and Lin  investigated the rheological flow fluid. Viscosity decreased with a rise in temperature . behaviour of BaTio /ethanol-isopropanol with anionic and Duan et al.  studied the flow behaviour of graphite cationic polymeric dispersant (KD and PS-2). They observed nanofluid in deionized water, at a shear rate range between 1 that the addition of the dispersant changes the flow behav - −1 and 100 s . The suspension behaviour exhibited shear thin- iour from pseudoplastic to dilatant as the shear rate passed −1 ning behaviour, viscosity increased with increase in concen- 800 s . The viscosity changed to the minimum value when tration of the fluid. They also reported that the enhancement the dispersant KD-6 was added. Similarly, Tseng and Li of the nanofluid held for 3-days was higher than that of the  used B aTio /distilled water with ammonium poly- freshly prepared nanofluid. The work of Moghaddam et al. acrylate surfactant (NH PA), upon adding NH PA, the sus- 4 4  was on graphene nanou fl id in glycerol, at the low shear pension was close to Newtonian at the low shear rate, while rate, the nanofluid showed shear thinning behaviour for all it deviated from linear to dilatant flow at high shear rate. temperature but at high shear rate, the nanofluid behaved as When indium tin oxide (ITO)/deionized water was used with Newtonian fluid and the shear thinning behaviour increased NH PA, the viscosity reduced by about 99% as compared with increase in concentration. to the original suspension. At low shear rate the suspension Fe O nanofluid in deionized water show Newtonian behaved like a Bingham fluid and changed to shear thinning 3 4 behaviour at low concentration and exhibited shear thinning when the shear rate exceeds a critical level . behaviour at higher concentration [119, 120]. But for F e O Phuoc and Massoudi et al.  studied the rheological 3 4 in polyethylene glycol (PEG), all the suspension showed flow behaviour of Fe O /deionized nanofluid with polyvi- 2 3 shear thinning behaviour . nylpyrrolidone (PVP) and poly-ethylene oxide (PEO) sur- Resiga et al.  investigated the flow behaviour of mag - factant. They reported that, the PEO and PVP exhibited a netite/transformer oil nanofluid with size range of 6–7 nm Newtonian behaviour at low concentration (0.2 wt%) and 1 3 International Nano Letters (2018) 8:49–77 65 exhibited shear thinning at higher concentration of the nanoparticles increases the distortion of flow lines and have nanoparticle. effects on the rheological properties such as viscosity and normal stress [37, 129]. Eec ff t of polymer on the rheological flow behaviour At equal concentration, well-dispersed nanofluid exhibit of nanofluids different rheological behaviour compared to agglomer- ated counterparts. The complications therefore, invalidate In polymer flooding activity, the essence of adding poly - any assumptions governing macroscopic homogeneity for mer to brine is to increase the viscosity for better sweep the application of complex equation over the homogenous efficiency [4 ]. Studies have shown that adding nanofluids phase. To account for additional time derivatives, spatial to polymer solution enhances the viscosity of the solution as well as particle–particle and particle–matrix molecular . Adding nanofluids to polymer increases the network interaction, factors such as particle size, porous media such and chain of the polymer, which in turn increases the viscos- as pH, polarity and inherent functionality also dictates the ity of the nanofluid. Therefore, nanofluid induced polymer flow in porous media by regulating aggregation and floc- has higher viscosity than polymer solution alone. Figure 9 culation. The degree of slip at the wall of the porous media is the oil recovery performance of injected polymer and and the amount of phase separation is influenced by the plate nanoparticle-induced polymer fluid. roughness or vane geometry. Viscosity as a function of con- Nanofluids are used to improve the mechanical, electrical centration has been modelled by several authors [130–135]: and barrier properties of polymers, there are used as thicken- = − 1 = ∕ − 1 = 2.5∞, (2) sp r s ing agent for low viscosity Newtonian fluid. It is therefore, desirable to know the effect of these nanoparticles to the = 1 + 2.5∞+ 6.2∞ , (3) rheology of the nanofluid and the approach needed to model the rheology of the system. The rheology also provides a means to determine the degree of exfoliation of the nanofluid =(1 − k�) , (4) in a polymer melt, the particles are therefore, sensitive to structure, shape and particle size . The addition of particles to a flowing liquid with com- �  plementary disturbance of the flow lines, can result in the (5) simplest flow becoming complex. The flow pattern changes 1 − �� s m where there is increase and spatial variation of the shear whereas Ø is the packing volume fraction, k is the constant rate in the continuous phase and transient behaviour of the liquid element. The Van der Waals forces between particles of integration and η is the intrinsic viscosity. Maghzi et al.  investigated the effect of silica nano- encourages agglomeration and influence the flow proper - ties from the increase in flow phase value. Aggregation of fluid on the rheological behaviour of polyacrylamide to enhanced oil production. They concluded that preliminary study showed that Nano solution showed pseudoplastic behaviour and had a good match with the power law model. But the rheological behaviour of the polyacrylamide showed a slight deviation from the power law model at medium shear rate and the deviation increased at higher shear rate. But for both fluids, there was minimum deviation. Addition of nanofluid improved the pseudoplasticity and increased viscosity for all the test. Cheraghian and Khalilinezhad  studied the effect of nanoclay on the rheological behaviour of polymeric solu- tion. They reported that there is a lower and upper limit for effective viscosity of the polymer solution in porous media. The ultimate recovery increased by 5% compared to polymer flooding alone. Cheraghian  applied nano-fumed silica to increase the viscosity of polymer in heavy oil flooding. They con- cluded that, the viscosity of the nano suspension increased with nanofluid weight fraction. More nanoparticles had Fig. 9 Oil recovery performance of injected polymer and nanoparti- direct effect on the fluid shear stress and the ultimate cle-induced polymer fluid  1 3 66 International Nano Letters (2018) 8:49–77 recovery by nanosilica increased by 8.3% compared to can be treated at a mesoscopic level similar to that used in polymer flooding alone. Similarly, the effect of TiO nano- lattice gas and lattice Boltzmann simulation. It also showed fluid on heavy oil recovery during polymer flooding was that complex physics such as multiphase flow, realistic equa- investigated. They concluded that for polymer flooding to tion of state, heat transfer and curing can be included in a be effective, the concentration of the nanofluid should be rigorous manner. Furthermore, complex geometry can be above a threshold value, the threshold value for titanium handled in a simple manner and the extension from two- dioxide was 2.3 wt%. The ultimate oil recovery by nanofluid dimensional flow is straight forward. In the absence of gravi- flooding increased by 3.9% compared to polymer flooding tational force, El-Amin et al.  used a highly non-linear alone . parabolic partial differential equation to numerically solve an efficient algorithm. They developed a mathematical model to describe the magnetic nanoparticles-water suspension that Mathematical modelling of nanoparticle flow imbibes into water–oil two phase flow in a porous media behaviour under magnetic field effect. The saturation of nanoparticles water suspension increases, while the nanoparticle concen- The flow behaviour of nanoparticles in porous media can be tration decreases slightly under the effect of the magnetic govern by the following assumptions; the porous media is field as the deposited nanoparticles concentration increases. heterogenous, the flow is one-dimensional under isothermal This was demonstrated by a set of numerical exercise of condition, the rock and fluid are incompressible, the oil and hypothetical cases to show how an external magnetic field water are governed by the Darcy’s law and as such gravita- can influence the transport of nanoparticles in a two-phase tional force is neglected. Oil and water are Newtonian fluids system in a porous media . The water–nanoparticles therefore, the viscosity and density are kept constant, the suspension was treated as a miscible mixture, whereas it nanoparticles are discretized into n size intervals. is immiscible with the oil phase. They concluded that the The continuity equation of oil (o) and water (w) are gov- magnetic source location has a significant influence on the erned by the following equations: physical variables of the model. Based on the flow direction and the location of the magnet, the magnetic field can assist k p l l �s − = o; l = o, w, (6) or oppose the flow of this two-phase system. The concentra- l x x tion of the nanoparticles is observed to decrease slightly as a result of slight increase in deposition of nanoparticles. The a + bS P = P − P = , (7) magnet can assist the flow of the ferrofluid suspension when C o w 1 + cS placed next to the inflow/outflow boundary. The concentra- whereas x is the distance from the inlet of the porous media, tion of the nanoparticles seems to increase under the effect l is the line, Ø is the porosity, s is the saturation, µ is viscos- of magnetic field as the concentration of the nanoparticles l l ity, p is pressure of the phase l and k (= krl) is the effective deposited decreases . l l permeability of phase l. Whereas Eq. (7) is the expression for capillary force, where a, b, c are empirical parameters Modification of nanoparticles and S is water saturation. Ju and Fan  used a mathematical model to describe The surface chemistry of a material determines its filtration, nanoparticles transport in a two-phase flow. The water adsorption and the rheological behaviour of the material in phase permeabilities increased from 1.6 to 2.1 of the origi- porous media. Therefore, proper surface modification can nal values, but there was a decrease in absolute perme- control particle properties which can lead to proper emul- abilities because of the adsorption of the nanoparticles at sification, reduced particle retention, wettability alteration the pore throat of the porous media. The hybrid computa- and stabilization of foam. A well-designed surface modi- tional approach that combines the lattice Boltzmann model fied nanoparticle can change particle hydrophobicity and for binary fluid with Brownian dynamic model for nano- thus alleviate particle retention on the rock surface . As particles was used to capture the interaction among fluids, careful control of emulsification and demulsification leads to nanoparticles and pore wall. Ma et al.  demonstrated delivery of nanoparticles to targeted areas in the formation. that nanoparticles can alter the IFT between two fluids and The interfacial interaction between two phases present contact angle at the pore walls which affects the dynamics in a hybrid solution is one of the decisive factors affect- of the capillary fillings. But Cleary et al.  confirmed ing properties of nanoparticles, a good dispersion can be the Darcy’s law for drift velocities in a saturated medium. achieved by surface modification of the nanoparticles Whereas a non-linear behaviour was observed for higher val- with active functional groups to enhance the compatibil- ues using the smoothed particle hydrodynamics (SPH) when ity between the nanoparticles with surfactant or polymer they demonstrated that the flow through a porous structure thereby, optimizing the efficiency of the process. 1 3 International Nano Letters (2018) 8:49–77 67 The surface modification of nanoparticles can be real- The advantages of grafting polymerization include; ized by chemical or physical methods. (a) Grafting of polymers can improve the interfacial inter- 1. Chemical methods: the use of chemical methods involve action between the grafting polymer on the nanoparti- modification with either modifier agents or by grafting cles and the matrix polymer. This can be achieved by polymers. The most commonly used chemical modify- molecular entanglement of the polymer to the nanopar- ing agents are the silane coupling agents which possess ticles. hydrolysable and organofunctional ends. The general (b) Grafted polymerization can also increase the hydropho- structure of the coupling agents can be represented as bicity of nanoparticles. Which can be useful as fillers RSiX , where the X represents the hydrolysable groups, and for matrix miscibility. which are typically chloro, ethoxy, or methoxy groups. (c) It provides flexible structural properties between the Whereas the R is an organic group of different func - nanocomposites. This can be achieved by changing tionalities chosen to meet the requirements of the poly- the species of the grafting monomers and the grafting mer. The functional group X reacts with the hydroxyl conditions. This is because, die ff rent grafting polymers groups on the surface of the nanoparticles, while the might have different interfacial characteristics. alkyl chain reacts with the polymer matrix. In this way, the hydrophilic surface of nanoparticles is converted Guo et al.  functionalized SiO nanoparticle with into hydrophobic . Grafting of polymer is also an a silane compound, 3-methacryloxypropyltrimethoxysilane effective method used to increases the hydrophobicity of (MPS) (Fig. 10) and found that the grafting ratio of MPS on particles. Surface modification by grafting is achieved the surface of nanosilica increased with MPS content. through two main approaches; Sabzi et al.  carried out surface modification of TiO nanoparticles with amino propyltrimethoxysilane (APS) and investigated its effect on polyurethane composite coating. (i) ‘Grafting to’ method: this is the covalent attach- Iijima et al.  tuned the stability of TiO nanoparticles ment of end-functionalized polymers to the sur- in various solvents by mixed silane alkoxides and obtained a face of the nanoparticles . well dispersed TiO in acidic aqueous solution, while Ukaji (ii) ‘Grafting from’ method: its the in situ monomer et al.  used 3-aminopropyltriethoxysilane (APTES) and polymerization with monomer growth of poly- n-propyltriethoxysilane (PTES) to suppress the photo-cata- mer chain from an immobilized initiator . lytic activity of fine TiO particles as inorganic ultraviolet filter. All the researchers observed an improvement of the Fig. 10 Modification of a nanoparticle with MPS (silane coupling agent)  1 3 68 International Nano Letters (2018) 8:49–77 properties of nanoparticles used after the surface modifica-nano-sized SiO with oleic acid (OA) and the surface-mod- tion process. ified silica nanoparticles (SiO –OA) were dispersed in poly (amic acid). They observed that the surface modification of 2. Physical method: the surface modification of nanopar - the nanoparticles caused an enhanced dynamic mechanical ticles based on physical interaction is usually imple- properties and thermal stability of the polymer. mented by adsorption of surfactants or macromolecules Behzadi and Mohammadi et al.  modified silica nan- onto the surface of the nanoparticles. The principle of oparticles with polyethylene glycol as a hydrophobic agent surfactant treatment is the preferential adsorption of a and propyl chain as the hydrophilic agent. The oil water polar group of a surfactant to the surface of the nano- IFT was decreased by 50% and it modified the oil wetted particles by electrostatic interaction. The surfactant used surface from strongly oil wet to water-wet. The increase in for surface modification of nanoparticles are the cationic nanoparticle concentration increased the surface activity as and anionic surfactants . The hydrophilic head of the functional nanoparticles can greatly improve the perfor- the surfactant reduces the interaction between the nano- mance of the biochemical analysis. Which can accelerate particles within agglomerates by reducing the physical the signal transduction, enhance signal intensity and enable interaction while, the hydrophobic tail easily incorpo- the radiance of the signal due to the unique properties of the rates them into the polymer matrix (Fig. 11). Thereby, nanoparticles. The pre-level investigation experiment with improving their ec ffi iency [ 143]. For instance, silica was the surface modified silica nanoparticles reveals that hydro- treated with CTAB to improve the chemical interaction philic and environmental friendly silica nanoparticles can between the SiO and the polymer . Nanoslica was also modify micro model wettability. also modified with oleic acid, the oleic acid was bonded The surface chemistry does not only affect the quality to the silica surface with a single hydrogen bond . of the nanoparticles in terms of stability, mono-dispersity and biocompatibility but can also prove that the functional − + Adsorption of polymer can also provide surface hydro- groups of –COO , –NH , –CHO or the charges on the phobicity of silica nanoparticles. This was demonstrated nanoparticles can be exploited for ligand exchange. Yang by Reculusa et al.  when they modified the surface of et al.  modified ALOOH nanoparticles with partial silica nanoparticles by adsorption of an oxyethylene based hydrophobic, positive and slightly negative charged. Small macromonomers. Hydrogen bond with the silanol functions aggregate adsorbs to the surface and form compatibility net- present on the surface of the silica was formed. This was work in the foam film resulting in a stable foam. The surface possible due to the hydrophilic nature of the monomers due chemistry is therefore, a vital tool for surface medication to the presence of ethylene oxide group. On the other hand, as it dictates the sensitivity and mediation of the specific the methacrylate group which contains a polymerizable nanoparticles assay, which is vital to the orientation of the group for the syrene reaction might also be responsible. functional ligand on the nanoparticles . Taber et al. Lai et al.  modified SiO nanoparticles with stearic  determined the best nanohybrid that can be used as a acid to improve their dispersion and the adhesion between pickering emulsion for EOR. They prepared different carbon the filler and polymer matrix. They reported that the modi- structure (single-walled carbon nanotube, SWCNT and mul- fied nano-SiO viewed under scanning electron microscope tiwalled carbon nanotube, MWCNT) nanohybrid with SiO 2 2 was seen to disperse uniformly in poly (ether ether ketone) nanoparticles with different weight-percent using sol–gel than the unmodified counterpart. Tang et al.  modified method. The results showed that nanofluid could signifi- cantly change the wettability of the carbonate rock from oil to water wet and decreases the IFT. Pickering emulsion have a good stability at 0.1, 1% salinity, at moderate and high temperature (25 and 90 °C), neutral and alkaline (7, 10) pH, which is suitable for oil reservoir condition. Therefore, 70% NWCNT/SiO nanohybrid pickering emulsion can be used for EOR. Characterization methods for evaluating the influence of nanoparticles in EOR Nanoparticles can be characterised based on their surface morphology, particle size and surface charge. The surface morphology, particle size and shape are determined by Fig. 11 Commonly used surfactants for functionalisation of nanopar- the electron microscopy technique. Whereas the physical ticles  1 3 International Nano Letters (2018) 8:49–77 69 stability and dispersibility of the nanoparticles are affected The efficiency of different nanoparticles by the surface charge of the nanoparticles . during oil displacement test (a) Surface morphology: the electron microscopy tech- The interest of nanotechnology in the oil and gas indus- nique is usually used to determine the surface mor- try is increasing by the day, silica nanoparticles are the phology with a direct visualization of the nanoparti- most widely tested and has shown good EOR application. cles. The scanning electron microscopy (SEM) and the But recently, studies have been carried out on aluminium transmission electron microscope (TEM) are used for oxide, titanium oxide, iron oxide, and magnesium oxide, this purpose. The SEM can provide the morphology when combined they have yielded better result. In the near and size analysis of the nanoparticles. But its limita- future, the oil and gas industry will benefit tremendously tion is insufficient information on the size distribution from nanotechnology. and the true population average . The difficulty Adding nanoparticles to fluid greatly increases the in studying the structure of nanoparticles is due to mobility of trapped oil, reduces interfacial tension, their small size, which can hinder the use of traditional changes the properties of fluid, wettability alteration . methods in measuring their physical properties. TEM Although, the exact mechanism for oil displacement using technique can provide imaging, diffraction and spectro- nanoparticles is still not clear [164, 165], nanotechnology scopic information of the nanoparticles. The advantage has been seen as the alternative to recover the remaining of TEM is that, it can provide the shape, size, defect, oil trapped in the reservoir [166, 167]. surface structure, crystals, electronic state and compo- Maghzi et al.  monitored the wettability alteration sition in nanometre-size region of thin film, nanoparti- using silica nanoparticles during water flooding of heavy cles and nanomaterial system . oil, they experimented with micromodel and reported a (b) Particle size: particle size is a physical property that 26% incremental oil recovery using silica nanoparticles. gives the basic information of the nanoparticles. Maghzi et al.  study using silica nanoparticles with It determines the distribution and retention of the polyacrylamide solution, with glass micromodel had a 10% nanoparticles in the target area . Dynamic light additional oil recovery. scattering (DLS) is used to determine the size of the Joonaki and Ghanaatian  in their study used iron nanoparticles. It measures the Brownian motion of the oxide, aluminium oxide, and silicon oxide treated by nanoparticles in suspension and relates it to velocity silane, using sand pack as the porous media, reported that (Known as the translational diffusion coefficient) to the the ultimate recovery was 92.5% with iron oxide, 88.6% size of the nanoparticle. According to the Stoke’s–Ein- with aluminium, and 95.3% with silica oxide. stein equation  DLS is fast and can provides a sim- Charaghian  also using a glass micromodel but ple estimate of the particles size. But the limitation of with sodium bentonite nanoclay with a size of 50 nm was DLS is that, it is poor in analysing multimodal particle able to recover 60.6% of the OOIP. Also, in another of his size distribution . Nanoparticle tracking analy- work using nanoclay and polymer with sandpack as the sis (NTA) is another imaging technique. It can track a porous media to recovery heavy oil, the additional recov- single particle based on the fluorescence microscopy ery this time was 5.8%. atomic image analysis. The size is determined from the Manan et al.  used SiO , aluminium oxide, copper average displacement of the nanoparticle undergoing oxide and titanium dioxide as a stabilizing agent in car- Brownian motion at a time . The advantage of this bon dioxide foam flooding, using sand pack as the porous method is that, it can track a single nanoparticle and media. The additional recovery reported was 14% for alu- provides a high resolution for the sample and aggrega- mina, 11% for silica while, titanium and copper recovered tion. 5% each. (c) Surface charge: the surface charge influences the inter - Nazari et al.  in his work on wettability alteration action of the nanoparticles with the environment. It of carbonate rocks used ZrO, TiO , MgO, Al O, CeO , 2 2 2 3 2 also, determine the intensity and electrostatic inter- CNT, where all used for contact angle measurement while, action with bioactive compound . The stability CaCO and SiO were used for core flooding. The result 3 2 of colloidal material is usually analysed with the zeta showed 8–9% additional recovery. potential of the nanoparticles. It is an indirect measure Moradi et al.  also used silicon dioxide nano- of the surface charge, it is useful in determining the sta- particle on a carbonate porous media and they reported bility of the nanoparticles, to avoid aggregation of the a 72–79% recovery of the original oil in place. Sharma nanoparticles. It can also be used to evaluate surface et al.  also worked on silica nanoparticles but with hydrophobicity and nature of the nanoparticles . Berea sandstone, the nanoparticle was used as a stabilizer 1 3 70 International Nano Letters (2018) 8:49–77 for surfactant and polymer. And the cumulative oil recov- polymer with high viscosity can improve oil recovery, the ery was 60%. The work of El-Diasty  also with sil- displacement test indicate a 4% increase in oil recovery. ica nanoparticle on sandstone media, reported a 35–50% Cheraghian  this time, focused on silica nanoparticle recovery of the OOIP during breakthrough point. Chen effect on the viscosity of polymer and its improvement in et al.  used carbonaceous nanoparticles MWCNT and recovering heavy oil. The result indicated that nanoparti- carbon black (CB) injected with surfactant blend. They cles with polymer at higher viscosity can improve oil recov- reported a fast and high recovery of 42.7% compared to ery, the displacement test for silica nanoparticle was 8.3% surfactant alone. increase in recovery. Singh and Mohanty  used aluminium coated silica, Jafarnezhad et al.  applied SnO nanoparticles to with Berea sandstone as the porous media, the cumulative increase the efficiency of water flooding in heavy oil using oil recovery reported was 70–75%. Alomair et al.  in carbonate core plug. The nanoparticles altered the wettabil- their work also using Berea sandstone as the porous media ity from oil-wet to water-wet and reduced the IFT which but with aluminium oxide and nickel oxide, silicon oxide, led to the increase in the recovery factor from 39 to 61% titanium oxide, observed a 6% incremental oil recovery for at low concentration of the nanoparticles. Azarshin et al. silicon oxide and aluminium oxide, 1–3% for nickel oxide  modified the surface of the silicon nanoparticles to and titanium oxide. Ragab and Hannora  also worked make them more effective for EOR purposes. They found out with silica and aluminium nanoparticles, with Berea sand- that the amine-functionalized silica nanoparticles are signifi- stone as the porous media, they reported a 7% additional cantly more effective than the typical nanoparticles when the recovery. core flood test recovered 18% increase in total recovery com- Bayat and Junin  in their work on nanoparticle trans- pared to the typical nanoparticles. But Zallaghi et al.  port through porous media, using limestone as the porous reported an increase from 4.87 to 11.7% in sandstone core media and Al O, TiO and SiO nanoparticles, reported that when the surface of silica nanoparticles was modified. While 2 3 2 2 a tremendous increase in recovery of 52.6% for aluminium, Youssif et al.  concluded that silicon nanoparticle is 50.9% for titanium and 48.7% for silica as compared to the environmentally compatible with sandstone rocks. Recovery original 47.3% of water flooding. increased by 13.28% with increase in the concentration of Ogolo et al.  using sandstone as the porous media work nanoparticles and that injecting silica at low concentration on eight different nanoparticles namely; aluminium oxide, rate will decrease the permeability impairment. Whereas magnesium oxide, iron oxide, nickel oxide, zinc oxide, zir- high concentration will increase the impairment but, will conium oxide, tin oxide, and silane treated silicon oxide. increase the recovery to a certain extent. They reported that iron oxide gave the highest recovery, Nwidee et al.  investigated the influence of ZrO and while magnesium was the least. The total recovery for the NiO nanoparticles on the wetting preference of fractured oil- nanoparticles is as follows: aluminium 26.7%, magnesium wet limestone formation. Both nanoparticles demonstrated 18.3%, iron 30%, nickel 25%, zinc 29.2%, zirconium 27.5%, similar behavior at the same particle concentration but the Tin 25%, and silane treated silicon 27.5%. ZrO demonstrated a better efficiency by altering oil-wet to Onyekonwu and Ogolo  studied the wettability altera- water-wet condition while, NiO changed oil-wet to interme- tion of three different polysilicon nanoparticles to enhance diate condition. They concluded that ZrO is very efficient oil recovery, the nanoparticles used were lipophobic and in terms of inducing strong water-wettability and has high hydrophobic (LHPN), hydrophobic and lipophobic (HLPN) potential as an EOR agent. and neutrally wet (NWPN). They reported that HLPN and NWPN are good agent for enhanced oil recovery with a total recovery between 42.95 and 53.38% for LHPN, Field application of nanoparticles in EOR 70.62–86.92% for NWPN and 70.0–93.13% for HLPN. Mohammadi et al.  investigated the effect of γ-Al O Nanoparticles was used to improve the mobility and alter 2 3 on the wettability alteration of one Iranian carbonate reser- the wettability of two Columbian heavy oil fields. Two field voir, they observed a maximum change in contact angle and trials were performed in Castilla and Chichimene fields [6 ]. the oil recovery increased by 11.25%. In Castilla 200 and 150 bbl of nanofluid were injected into Wan Sulaiman et al.  they used silica nanoparticle wells CN154 and CN174, respectively. The oil rate increased in carbonate rock to alter the wettability of the rock, and by 270 bpd in CNI54 and 280 bpd in CN174. Whereas in to test the efficiency of silica nanoparticle in enhancing oil Chichimene 86 and 107 bbl of nanofluid were injected into recovery in high salinity, they reported a 65.5% recovery of wells CHSW26 and CH39, respectively. Oil rate increased by the OOIP. 310 bopd in CHSW26 and 87 bopd in CH39. It was observed Cheraghian  focused on the role of TiO nanopar- that the oil viscosity reduced, and the mobility ratio was ticle on the viscosity of polymer, his results showed that increased for both fields. About 98% viscosity was observed 1 3 International Nano Letters (2018) 8:49–77 71 for Chichimene field in the first 9 days whereas 47% reduc- Future direction tion in viscosity was observed for Castilla in the first 30 days . The nanofluid was pumped into the formation just once Nanotechnology in EOR is still at the laboratory and field and the improvement in oil mobility was still the same for trial stage, where there is an attempt to understand the 269 days. Indicating the ec ffi iency and economical nature of mechanism of its adsorption onto rock surface during the nanofluid. The residual concentration of the nanoparticle recovery, because different rock has different affinity to after 269 days was 56 ppm, indicating that the long life of the adsorbed nanoparticles. The mechanism of nanoparticle nanofluid. Also, 11% reduction in BSW shows that the forma- retention and entrapment needs to be understood, they tion has been altered to a strong water wet system. The project are in need for size control and the right concentration of was very economical as the capital invested was recovered nanoparticles for flooding. within 4 months. Considering the number of publication and good result obtained from laboratory, nanoparticles are potential Challenges of nanoparticles applications future candidate for EOR, but this has not reflected on its application in the field. Several challenges still exist that Processing and manufacture need to be addressed, such as the stability of the nano- particles at harsh conditions, to solve these problems, co- There is little or no doubt that nanoparticles have tremendous polymerization, nanoparticle coating and surface modifi- market potentials, but as a replacement for current EOR chem- cation of these nanoparticles should be implemented. icals and in the creation of new market through their outstand- The cost of nanoparticles is another limiting factor ing properties. But the challenge is the processing and manu- hindering field application. There is need to develop tai- facture technology in terms of quantity and commercialization lor-made nanoparticles that are cost-effective from local is the greatest challenge. The dispersion of nanoparticles or the material such as natural starch and cellulose. The choice chemical compatibility of the nanoparticle with the material is of these materials is due to their rigid chemical structure important. The homogenous dispersion of the nanoparticles in which can withstand harsh reservoir condition and they are polymer is very difficult due to the strong tendency of particles the most abundant natural product in nature. to agglomerate [178, 179]. Scale up is needed to produce large quantities of nanomaterials for manufacturing purposes, there is need for characterization method, tools, instrumentation as well as affordable infrastructure, and the education of scientist Conclusions and engineers in academics and industry. Molecular dynamics simulation and theoretical analysis are based on assumption 1. The main aim of this research is to bring to light the that may not be feasible in real life situation . mechanism affecting the flow of nanoparticles in porous media as it relates to EOR. Environmental challenges 2. Nanoparticle in CEOR can alter the wettability of the reservoir rock from oil-wet to water-wet, it can also There are general views that nanoscale particles may have lower the interfacial tension between the oil and water negative health and environmental hazards, since most of interface. Nanoparticles are found to be effective in them are metals or their oxides [181, 182]. Although most of stabilizing of emulsion, foam and can also stabilize oil them are introduced into the body for detecting disease and front. infections, imaging and treatments, there is growing concern 3. Gravitational and capillary forces are responsible for the about their toxicity. The properties of carbon nanoparticles shift in wettability from oil-wet to water-wet. may also lead to health hazard . Because the size of the 4. The flow of nanoparticles in porous media was described nanoparticles is comparable to human cells and large protein laying emphasis on the physical aspect of the flow, the with the result that the regular human immune system may microscopic rheological behaviour and the adsorption not work against them . Nanoparticles can also seep into of the nanoparticles in porous media. under underground water to affect the aquifer, which is a seri- 5. It was observed that nanoparticles exhibit Newtonian ous environmental problem. behaviour at low shear rate and non-Newtonian behav- iour at high shear rate. 6. The dominant mechanism of foam flow process were lamellae division and bubble to multiple lamellae divi- sion. 1 3 72 International Nano Letters (2018) 8:49–77 10. Sun, X., Zhang, Y., Chen, G., Gai, G.: Application of nanoparti- 7. 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Published: Jun 4, 2018
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