Review on application of nanoparticles for EOR purposes:A critical review of the opportunities and challenges

Yousef Kazemzadeh,Sanaz Shojaei,Masoud Riazi,3,*,Mohammad Shari fi

1Department of Petroleum Engineering,Amirkabir University of Technology,Tehran Polytechnic,Tehran,Iran

2Enhanced Oil Recovery(EOR)Research Centre,IOR/EOR Research Institute,Shiraz University,Shiraz,Iran

3Petroleum Engineering Department,School of Chemical and Petroleum Eng.,Shiraz University,Shiraz,Iran

Keywords:Enhance oil recovery Nanofluid injection Nanoparticle Interfacial tension Wettability alteration Pore blockage

A B S T R A C T Nanoparticles have already gained attentions for their countless potential applications in enhanced oil recovery.Nano-sized particles would help to recover trapped oil by several mechanisms including interfacial tension reduction,impulsive emulsion formation and wettability alteration of porous media.The presence of dispersed nanoparticles in injected fluids would enhance the recovery process through their movement towards oilwater interface.This would cause the interfacial tension to be reduced.In this research,the effects of different types of nanoparticles and different nanoparticle concentrations on EOR processes were investigated.Differentflooding experiments were investigated to reveal enhancing oil recovery mechanisms.The results showed that nanoparticles have the ability to reduce the IFT as well as contact angle,making the solid surface to more water wet.As nanoparticle concentration increases more trapped oil was produced mainly due to wettability alteration to water wet and IFT reduction.However,pore blockage was also observed due to adsorption of nanoparticles,a phenomenon which caused the injection pressure to increase.Nonetheless,such higher injection pressure could displace some trapped oil in the small pore channels out of the model.The investigated results gave a clear indication that the EOR potential of nanoparticle fluid is significant.

1.Introduction

In-situ oil production and recovery with primary and secondary methodshaveresultedinweakandunstableconditions,hence,efficient technologies such as nanotechnology for enhanced oil recovery(EOR)of oil reservoirs have recently attracted much attentions[1-7].On average,about one-third of the initial in-situ oil can be produced by primary and secondary methods.The remaining part of oil is trapped in a reservoir rock and left behind due to surface and interface tensions[8-14].This trapped oil can be pulled out by reducing the capillary forces which prevent oil movement through small pores of the reservoir[15-19].

Nanotechnology proposes a unique approach in controlling the oil recovery process[20-23].The nano factor can enhance oil recovery by improving the geomechanism of reservoir which is obtained by improvement in surface tension and the spontaneous modification of oil reservoirs[24-30].The capillary force is the required force for compressing small droplets of hydrocarbons in the pore throat of reservoirs which is reduced by the decline in water-oil interface tension and wettability alteration[31-35].Nanoparticles possess unique properties which can be used to obtain desirable features.These properties are high specific area,special chemical reactivity and having active surfaces[36-40].Itisprovedthatnanoparticlescansignificantlyincreaseoilproduction by means of:

·Improving quality of the injecting fluid

·Viscosity alteration of the injecting fluid

·Density alteration of the injecting fluid

·Diminishing the surface tension

·Improving the emulsion formation

·Improving the conductivity and specific heat

·Improving the interactions between rock and oil

·Wettability alteration

·Altering the heat transfer coefficient

·Diminishing the formation damage by reducing released particles on pore surfaces[41-49].

2.Stability of Nanofluids

Oneofthechallengesinnanofloodingisstabilityofnanofluid.Stability of nanoparticles in different fluids is determined by integrating the attraction and repulsion forces of nanoparticle surfaces.If the repulsion force becomes greater,the suspension gets stable and there will be no nanoparticle agglomeration[50-52].

·Zeta potential and charge density are two parameters that can be manipulated to form a stable suspension of nanoparticles.That is by an increase in zeta potential regardless of its sign(i.e.its absolute value),the stability and suspension of nanoparticles in fluid increase due to an increase in electrostatic repulsion force between the particles.Increasing zeta potential,however,intensifies the ionic power of the solution.The surface charge density of particles increases and results in higher attraction forces between particles and existing ions.Thiswilldiminishoreliminatestability ofnanoparticles.Thestability is therefore provided through the manipulation of the zeta potential and particle charge density.In fact,zeta potential is the representation for salinity,alkalinity,ion type and its concentration,which can be adjusted to form or improve stability in nanoparticles[53].Oneofthewaystoimprovezetapotentialandchargedensityparameters is pH alteration and adjustment.The surface charge load of the existing particles allows them to be stable in acidic,basic or neutralsystems.Iftheparticlesarenegativelycharged(e.g.silicaparticle),stability increases by reducing solution pH.This is due to the electrostatic adsorption of hydronium ions towards silica particles which repulses these particles[53].The effects of salinity,temperature and formation lithology can be summarized as follows:

o Salinity of the nanoparticle containing fluid is one of the important instability factors for different fluids in oil reservoirs.Oppositely charged particles have negative impacts on stability of nanoparticles in the carrier fluid[54].

o Temperature is another important factor in nanofluid stability.In fact,a directrelation between salinity and temperature of thecarrierfluid existed with reactivity and agglomeration rate of the nanoparticles.Thermal stability is a parameter which depends on the fluid salinity and formation lithology[55].

o Formation lithology also adds some complications to nanoparticle stability.Similar to the presence of different ions in the formation water,nanoparticle agglomeration and their attraction towards rock reservoir are highly plausible if the reservoir rock is oppositely charged towards nanoparticles[56].

·Surface coating of nanoparticles by different functional groups to improve their surface properties is another method for enhancing nanofluid stability.Surfacecoating of nanoparticlesnotonlyimproves the zeta potential but also can enhance stability in high-saline systems.For instance,surface of the silica can be altered by Ceylon.These functional groups thus can be used to stabilize nanofluids in high temperature and salinities[57].

By the movement of nanofluids in porous media,various mechanisms contribute in reducing the nanoparticle concentration in nanofluids.The most important factors involved in such phenomenon are the adhesion of nanoparticlestorocksur faces and blockage of different throats of the rock.The throats are blocked by two ways in rocks:

o The solid particles cannot pass through pores if the pore diameters are smaller than their size.According to which nanoparticle diameter is usually smaller than pore diameter,occurrence of such phenomenon is of scarcity.

o Rock pores can be blocked under mechanisms of agglomeration and bridge if the diameter of pores is bigger than nanoparticles.While the nanoparticle containing water moves through the pore throats of rocks,the smaller water droplets pass with higher speed and comfort and hence,nanoparticles accumulate at throats.Diameter of the pores is reduced and eventually blocked due to the placement of nanoparticlesonsurfaces.Agglomerationgenerallydependsonnanoparticle concentration,flow rate and the diameter ratio of droplets to pores[58].

3.Oil Displacement Mechanisms by Nanofluids

Generally,three different mechanisms have been proposed for nanofluids.

3.1.Disjoining pressure

This pressure is the one which in fact resists the adhesion force offluids towards the solid surface to separate the fluid.This actually is the pressure difference between the thin layer of fluid and the bulk offluid.A wedge-like film is formed by nanoparticles at the interface of discontinuous fluid through this mechanism.Such mechanisms are formedduetoenergies includingBrownianmotionandelectrostatic repulsionforcebetweennanoparticles.Greaterrepulsionforcesexistwith smallernanoparticles[59].Moreover,thisforceisintensifiedbyincreasing nanoparticle concentration.Schematic view of this mechanism is illustrated in Fig.1.

Such an arrangement for nanoparticles exerts a greater pressure at the interface of two fluids.The greater forces will be at the interface with the increased number of particles in fluid.

If such energy is applied at the interface,displacement occurs to reach the equilibrium.Like other colloidal systems,magnitude of disjoining pressure is affected by parameters including particle size,temperature,and salinity of carrier fluid and surface properties of the present phases[60].The form of the meniscus height,in the presence and the absence of nanoparticles,is shown in Fig.2.

3.2.Density difference

In the ultra-small throats of pores,nanoparticles agglomerate at the entrance of throats due to the density difference between water and nanoparticles.The agglomeration allows the injecting fluid to flow towards adjacent pores and increases their pressure.This leads the oil in adjacentporestomoveandbeproduced.Pressureisreducedbythedisplacementoftheoilandresultsingradualrestorationofporeblockages.The nanoparticles can be displaced by the carrier fluid once more under this circumstance[59].

3.3.Wettability alteration and interface tension

Floodingwithnanofluidsimproveswettabilityand interfacetension properties of the system.Furthermore,nanoparticles affect the density of the injecting fluid and the reservoir fluid.Silicone nanofluids are againusuallyusedtoobtainsuchmechanism.Asit ismentioned earlier,thesenanoparticlescanpossessdifferentwettability.Alcoholisaproper solvent for neutral and hydrophobic nanoparticles while hydrophilic nanoparticles are properly suspended in aqueous environments.These hydrophilic nanoparticles are mainly used for enhanced oil recovery of oil reservoirs which are mostly oil-wet.The main production mechanism by using these nanoparticles is to alter wettability from oil-wet to neutral or water-wet and vice versa.Wettability alteration is accomplished by adsorbing nanoparticles with proper wettability on the rock surface[61-63].

4.Factors Influencing the Process of Flooding with Nanoparticles

Flooding process with nanoparticles is affected by various factors which requires a sensitivity analysis to effectively determine these factors.These factors can be mentioned as follows.

4.1.Property of nanoparticles

4.1.1.Nanoparticle concentration

Nanoparticle concentrationhas a bilateral influence onnanofluid injection.From one side,an increase in nanoparticle concentration escalates the disjoining pressure and Brownian motion due to the influence of increased repulsion forces,and nanofluid efficiency for altering wettability is consequently enhanced.On the other hand,an increase in the concentration results in a decline in porosity and permeability of the reservoir rock due to the increased rate of nanoparticle deposition on the rock surfaces.

4.1.2.Nanoparticle size

Disjoining pressure is also affected by the size of nanoparticles and theircorresponding chargedensity.As itis mentioned earlier,thesmaller theparticlesizesare,thegreateristherepulsionforceandthusthehigher is the disjoining pressure that exist between them.Particles however agglomerate with higher rates if the particles become ultra-small.

4.1.3.Nanoparticle wettability

The arrangement of nanoparticles at the interface of oil and water is defined by their wettability.Wettability of the reservoir rock also alters by adsorbing these particles on rock surfaces.Wettability alteration in the surface of reservoir rock changes the resistances against fluid flow and thus alters the relative permeability of oil and water in the reservoir.

Fig.2.Effect of disjoining pressure on form of the meniscus height in the wedge region[60].

4.2.Nanofluid system

4.2.1.Salinity

Generally,stability of nanoparticles decreases by increasing the salinityofthesystem.Infact,increaseinsalinityreducesthezetapotential and therefore results in agglomeration of colloidal particles.However,increasingthesalinitybyaddingdifferentionsdoesn'tpreventnanoparticles from moving,it significantly increases the deposition of nanoparticles on rock surfaces.

4.2.2.Temperature

Reservoirtemperatureismuchhigherthanthesurface temperature.Each nanoparticle thus has to maintain its efficiency in the reservoir temperature to be applicable for injection in oil fields.By increasing thetemperature,particlesagglomerateduetothedeclineinzetapotential.In fact,stability of nanoparticles is diminished by increasing the temperature.Nevertheless,according to the results,performance of thenanofluidsinenhancedoilrecoveryhasimprovedinhighertemperatures.This can be due to the decreased surface tension in higher temperatures as the result of decline in interactions of liquid molecules.

4.2.3.Injection rate

Smallwatermoleculesmovefasterthansuspendednanoparticlesby increasing the injection speed and results in agglomeration of nanoparticles which blocks pores and reduces oil production.By increasing the injection rate,nanofluid efficiency is therefore diminished due to particle agglomeration and pore blockages and its resulting permeability decline.

4.3.Reservoir rock

4.3.1.Size of rock particles

In fact,specific area of the porous media is related to the size of the reservoir rock.The specific area of the reservoir rock is decreased by increasing the size of rock particles in which nanoparticle deposition on the rock surfaces also diminishes.

4.3.2.Clay content

The adsorption of nanoparticles is escalated by increasing the clay content.This is primarily due to which available specific area for nanoparticlesisenhancedandmorespotsareavailablefornanoparticletobe deposited on.

5.Literature Review on Using Nanoparticles and Its Efficiency in Enhanced Oil Recovery

5.1.Influence of nanoparticles on wettability alteration

Zhang et al.have compared the performance of nanoparticles made by the Illinois technology institute and the silicone nanoparticles,and proved that nanoparticles which are made by this institute have more effectively performed in increasing the production of the trapped oil and wettability alteration[64].

InanotherstudybyJuandFan[65]thewettabilityalterationofoil-wet sandstones has been evaluated by injecting hydrophilic nanoparticles of silica.ResultsincludingTEMimageshaveconfirmedthewettabilityalteration and the adsorption of nanoparticles on the rock surfaces[65].

Onyekonwu and Ogolo[66]have studied the ability of poly-silicone nanoparticles in wettability alteration and enhanced oil recovery.Solventsincludingwaterandethanolhavebeenusedtosuspendthenanoparticles.Itwasfoundthatinwater-wetcores,oilrecoveryin3000 ppm concentrationcanbeimprovedbyusingneutralandhydrophobicnanoparticles due to mechanisms of wettability alteration and interface tension reduction,while,the oil recovery was diminished by using hydrophilic nanoparticles simply due to enhanced core water-wet property[66].

Karimi et al.[67]also have utilized nanoparticles with zirconium oxide base to investigate the influences on enhanced oil recovery.They have concluded that these nanoparticles are capable of altering carbonate rock wettability from highly oil-wet to highly water-wet.Considerable amount of oil was produced through spontaneous experiments by using these nanofluids[67].

In a study by Giraldo et al.[68],the influence of nanofluids containing aluminum-oxide-base nanoparticles has been evaluated.They have revealed that these nanoparticles can considerably alter the wettability of highly oil-wet sandstones to water-wet.Efficiency of surfactants as wettability alternation additives also has been improved[68].

By conducting contact angle experiments in high temperature and pressure condition,Cao et al.[69]have concluded that the ionic liquids and the utilized nanofluids are efficient and stable for wettability alteration at high temperatures.Fig.3 indicates the percentage of papers on wettability alteration compared to the other chemical EOR processes published between 2001 and 2017.

Fig.3.The percentage of papers published on wettability alteration by nanoparticles compared to the other chemical EOR processes.

5.2.Influence of nanoparticles on reducing interface tension

By using iron nanoparticles,Suleimanov et al.[70]have evaluated theapplicationofthenanofluidsinenhancedoilrecovery.Theyhavereported that systems containing both nanoparticles and surfactants can enhance oil recovery up to 35%.This is while in systems containing only surfactants,oil recovery was reported to be 17%.They have explained the increased oil production by relating it to the reduced interface tension of the system containing nanoparticles[70].

Roustaei et al.[71]have studied the efficiency of poly-silicone nanoparticles by measuring the contact angle and interface tension parameters.Theyhavedetecteda declineintheinterfaceofoilandwater(from about26to2 mN·m?1)andanincreaseintheenhancedoilrecoveryby using the nanoparticles[71].

Efficiency of the suspended solutions containing hydrophilic polysilicone nanoparticles for enhanced oil recovery was investigated by Hendraningrat et al.[72].They have reported a decline in the interface tension of oil and water at the presence of nanoparticles in which solid surfaces get more water-wet.They have eventually observed a 5-percent improvement in the oil recovery at the presence of nanoparticles[72].

Moghadam and Azizian have studied the influence of zinc oxide nanoparticles on the behavior of anionic surfactants at the interface of the two fluids.They have confirmed a significant decline in the interface tensionofthesurfactantandoilatthepresenceofnanoparticles[73].Fig.4 illustrates the percentage of papers on interfacial tension compared to the other chemical EOR processes published between 2001 and 2017.

Fig.4.ThepercentageofpapersoninterfacialtensioncomparedtotheotherchemicalEOR processes published between 2001 and 2017.

5.3.Influence of nanoparticles on emulsion stability

Asumadu-Mensah et al.[74]have studied the impacts of nanoparticles including aluminum,copper and their oxides on emulsion stability.It was found that oil-water separation will be easier by using hydrophilic nanoparticles and the emulsion stability will decrease[74].

In another study,Al Otaibi et al.[75]have revealed that stable emulsion can be formed by carbon dioxide,nanoparticle and water.Concentrationof thenanoparticles and watervolumefraction werereported as the most affecting parameters on the emulsion stability[75].

Influence of nanoparticles on emulsion stability under the reservoir condition was also investigated by Binks and Whitby[76].They have concluded that the surface charge is an essential parameter for stability and pH must be adjusted and controlled to reach stability[76].

Pei et al.[77]have studied the stability of emulsion by adding nanoparticlesinmicromodelexperiments.Theresultsascertainedthatadding nanoparticles can increase the thickness of the emulsion layer and improve the mobility.The heavy oil also gets emulsion by injecting the emulsions which were stabilized with nanoparticles and surfactants.They have claimed that these stabilized emulsions can produce 40%of the in-situ oil by means of EOR processes[77].

Fig.5.The percentage of papers on formation and stability of emulsions compared to the other chemical EOR processes published between 2001 and 2017.

Fig.6.The percentage of papers on formation and stability of foam compared to the other chemical EOR processes published between 2001 and 2017.

In a recent effort by Kim and Krishnamoorti[78],the formed emulsion of oil in saltwater was measured and evaluated by using silica nanoparticlesandfourtypesofsurfactants(cationic,anionic,ion-dipole and nonionic).It was found that these emulsions are highly efficient as well as low cost[78].

Griffith et al.[79]have studied the influence of mineral oil production through the injection of pentane emulsions in water which was stabilized with nanoparticles and surfactants.They have concluded by performing core flooding experiments that up to 82%of the in-situ oil canbeproduced bythis method[79].Fig.5 shows the percentage of papers on formation and stability of emulsions compared to the other chemical EOR processes published between 2001 and 2017.

5.4.Influence of nanoparticles on foam stability

Espinoza et al.[80]have studied the effects of nanoparticles on controllingmobility of superheated carbon-dioxide foams.Based on the results,nanoparticles with 0.05 wt.%.can create stable foams.They reported that salinity has great impacts on the foam stability.They also succeeded to create foams in high temperatures such as 95°C by using nanoparticles[80].

Aminzadeh-goharrizi et al.[81]have studied the stability of carbondioxide foams in water under the effects of surfaced modified nanoparticles.These nanoparticles were claimed to alter the movement pattern and the distribution of carbon-dioxide in environment,also,they have showed the capabilities to reduce carbon-dioxide mobility[81].

Moetal.[82]havealsoconductedexperimentstodeterminethefactors affecting foam stability at the presence of nanoparticles.They have measured the optimum nanoparticle concentration under the experimental conditions to be in the range of 2000 to 3500 μl·L?1[82].

Performanceofthenanoparticle-stabilizedfoamsinenhancedoilrecovery was investigated by Yu et al.[83].They have claimed that such foamscanenhanceoilrecovery incoreswithlowandhighpermeability[83].

Performance of the modified nanoparticles in stabilizing carbon-dioxide foams was also evaluated by Zhang et al.[84].They have confi rmed the high effectiveness of the surface modified nanoparticles of silica in forming stable emulsions of oil in water and carbon-dioxide foams in water.Foams which were produced in this way can be stable in 95°C and up to 10%salinity[84].Fig.6 shows the percentage of papers on formation and stability of foam compared to the other chemical EOR processes published between 2001 and 2017.

5.5.Influence of nanoparticles on enhanced oil recovery

As noted above,nanofluids with different mechanisms contribute to the enhanced oil recovery.Therefore,various aspects of the application of nanoparticles have been discussed in previous studies.The present study addresses changes in wettability,reduction of interfacial tension,emulsion formation,and foam stability and formation.Fig.7 shows the percentage of nanofluid application in each of the mechanisms studied in different years.

Fig.7.The percentage of nanofluid application in each of EOR mechanisms compared with other techniques.

As it is shown in Fig.7,nanofluid studies have paid more focus on wettability than other mechanisms.Fig.7 shows that the application of nanofluids for enhanced oil recovery gains increasing attention compared to other methods.

Researchers have recently succeeded to create water-oil emulsions with metal and metal-oxide nanoparticles and improve oil properties by in-situ utilization of the emulsions.These emulsions contain nanoparticles with catalytic and absorption properties which can bedispersed in the porous structure of the reservoir and the well pipes to provide multiple applications such as removal of the asphaltene sediments through the adsorption process.Some experiments have been conducted on the influence of different parameters and the performance of nanoparticles which are briefly addressed as follows.

Table 1 An overview of studies on the use of nanofluids in enhanced oil recovery from reservoirs

Table 2 An overview of studies on flooding carbonate reservoirs using nanofluids(1mD=10-3μm2)

Chen et al.[85]also have studied on reducing the viscosity of heavy oils by nanoparticles.In this approach,nanoparticles react with the heavy oil by means of steam and act as a catalyst that loosens the bonds of N,S and O with the carbon;hence,components including aromaticsand saturatedhydrocarbonsare produced due to thevariationin the heavy oil structure.It also diminishes the percentage of asphaltene and resins primarily due to the breaks in the C--O bonds of the molecule,C--C bonds in the adjacent hydrocarbon chains and the C--S bonds.Accordingly,in addition to the reduction in heavy parts of the oil,light parts act as solvents and reduce the oil viscosity.Results have emphasized on 80 to 93%reduction in the heavy oil in 200 and 280°C[85].

Nassar et al.[86]have conducted multiple experiments on the influence of nanoparticles and different parameters on the recovery of oil comprising high asphaltene content.The results are briefly mentioned as follows.

Performance of the nanoparticles including oxides of nickel,cobalt and iron can be sorted as:NiO＞Co3O4＞Fe3O4.Here,the performance is defined as the abilities to oxidation and absorption as well as the catalytic ability.Asphalteneisspontaneously adsorbed on thenanoparticle surfaces with an exothermic nature[86].

Ogolo et al.[87]have claimed that some nanoparticles can enhance the oil recovery which this performance is also improved at the presence of ethanol.They also introduced the mechanisms by which nanoparticles improve the recovery process as:wettability alteration of the rock,IFT reduction,reduction in the oil viscosity,reduction of mobility ratio and permeability variations[87].

Haroun et al.[88]have compared the smart injection of water and nanoparticles.They have concluded that the recovery coefficient of a reservoir which is measured to be 46%to 63%by injecting the water,can be enhanced to the range of 57%to 85%by selecting the proper nanoparticle[88].

6.Field Applications of Nanoparticles

The first field experience of nanoparticle injection was performed in the Liauhe field of China in which nanoparticles were injected through the steam.4http://oilprice.com/Energy/Crude-Oil/Chinese-Scientists-Discover-New-Method-to-Increase-Recoverable-Oil-from-Wells.htmlBut in the Ghawar field of Saudi Arabia,about 5 kg of A-Dot nanoparticle was mixed with 255 barrels of sea water in 2012 and injectedtooneoftheobservationwells.Theinjectionprocesswascontinueduntilthenanoparticleshadadvancedabout20 ft(1 ft=0.3048m)in thereservoir.Thewellwassubsequentlyclosedfor3 daysandtheoilproduction was then resumed.The samples indicated that 90%of the nanoparticles had been successfully recovered.5https://www.aramcoexpats.com

Table 1 summarizes studies on the application of nanoparticles in theenhancedoilrecoveryfromreservoirs.Thetableshows theresearch methodology,the reservoir rock materials,and a part of the results.

One of the common uses of nanoparticles is their use in carbonate rockreservoirs.Thefollowingtableshowssomeoftheresultsandapplication of nanoparticles in the flooding of carbonate rocks.Table 2 also shows the permeability,porosity of the rock,the type and size of the nanoparticles along with the main results.

Table3showssomeoftheresultsandapplicationofnanoparticlesin the flooding of sandstone rocks.

Table 3 An overview of studies on flooding sandstone reservoirs using nanofluids

Acknowledgment

Authors would like to sincerely thank Mr.Hosein Doryani and Mr.Hossein Rezvani for their collaboration in this work.