Effects of synthesized nanoparticles and Henna-Tragacanth solutions on oil/water interfacial tension: Nanofluids stability considerations

Pouy Tvkoli,Seyed Rez Shdizdeh,*,Frzn Hyti,Moslem Ftthi

a Department of Petroleum Engineering,Abadan Faculty of Petroleum Engineering,Petroleum University of Technology (PUT),Abadan,Iran

b Department of Chemical Engineering,Abadan Faculty of Petroleum Engineering,Petroleum University of Technology (PUT),Abadan,Iran

ABSTRACT Rising global energy demand has encouraged engineers to create and design new methods to improve oil recovery from reservoirs.In this study,feasibility of using Henna extract as a natural surfactant and synthesized nanoparticles (Titanium dioxide (TiO2),Silicon dioxide (SiO2),Graphene and composite of TiO2-Graphene)for reduction of oil-water interfacial tension has been experimentally investigated.Nanoparticles were synthesized via sol-gel method and XRD,FESEM,EDAX and FTIR tests were conducted to confirm the authenticity of this synthesizing materials.Nano-surfactants were stabled with a natural water-based suspending surfactant called Tragacanth extract,which could be introduced as a practical substitute for industrial nanoparticles' stabilizers in oil industry.After CMC determination of Henna extract surfactant,the optimal concentration of Tragacanth extract surfactant,with the purpose of nano-surfactants’ stabilization,was determined through particle size and zeta potential tests.Results of interfacial tension (IFT)measurements showed that the increase of Henna extract concentration from 0 wt% to 10 wt% reduced IFT between kerosene and water from 37.23 to 15.24 mN/m.Furthermore,adding 1 wt% of synthesized TiO2 nanoparticle to the Henna extract surfactant at its CMC value reduced IFT from 18.43 to 14.57 mN/m.As an impact of this significant reduction in IFT value,oil recovery factor could be improved drastically during EOR operations.Results proved that TiO2 nano-surfactant was as effective as industrial surfactants,which put human's and environment's health at risk and impose heavy economic strain on governments.

Keywords:Tragacanth extract Henna extract Nanoparticles Sol-gel method Graphene Interfacial tension

1.Introduction

Despite the advent of renewable energies in recent years,global demand for fossil fuels and hydrocarbons has not decreased and is rising momentarily.Due to a significant decline in the exploration of new hydrocarbon reservoirs,engineers have always been looking for new methods to improve oil recovery from current oil reservoirs.Arranging these methods should not only increase the oil recovery factor but also should be economically feasible,environmentally friendly,and capable of performing on a field scale.There are some chemical enhanced oil recovery (EOR)approaches which utilized to ameliorate oil recovery from reservoirs.Surfactant flooding is one the most practical chemical EOR methods which increase the oil production through reduction of interfacial tension between injected water and residual oil,altering the rock surface wettability and emulsions formation [1].Determination of critical micelle concentration (CMC)for each surfactant in surfactant flooding processes is of great importance,since formation of surfactants’ micelles in solutions at concentrations above the CMC value will improve their performance in reservoir [2].CMC measurement methods include surface tension determination,UV-Vis spectroscopy,electrical conductivity,luminescence spectroscopy and turbidity which shows break at CMC value after it stays steadily with a further rise in surfactant concentration in the base fluid[3-5].

Despite the significant impact of industrial surfactants on improving oil recovery,using them faces many barriers such as availability,environmental damages,high costs and surfactant lost due to adsorption on the reservoir rock.For this reason,application of natural-based or biodegradable surfactants in petroleum upstream has become one of the interesting topics for many researchers in recent years [1,6-16].Henna extract is a natural dye that has shown many applications in drilling fluids as a bio-based additive in recent studies.For the first time,the extract of Henna was used to inhibit corrosion of steel in HCl acid and aluminum in NaOH [6].Subsequently,the effect of Henna extract on corrosion inhibition of some metals such as zinc,nickel,aluminum,and iron in acidic and alkaline solutions has been investigated in few studies[7-9].Other uses of Henna extract in the upstream oil industry could be mentioned as improving the cement resistance against HCl in oil well acidizing treatments,decreasing the sodium bentonite swelling in aqueous solution,and deflocculating as well as swelling inhibition of clay particles [10-12].Despite the limited research performed on the application of Henna extract in the drilling area,applicability of this surfactant in EOR and its impact on the interfacial tension reduction has not been studied so far.

The use of natural surfactants in EOR was introduced through adding the bio-surfactant extracted from Zizyphus Spina Christi tree leaves,called Saponin,to distilled water.Interfacial tension measurements showed that Saponin as a nonionic surfactant could reduce the IFT between kerosene and water from 48 to 9 dyn/cm [1].Deymeh et al.looked into the viability of implementing Seidlitzia Rosmarinus,a novel natural cationic surfactant,as an agent for chemical enhanced petroleum recovery by measuring interfacial tension between oil and surfactant solution.Kerosene-water interfacial tension was decreased from 32 to 9 mN/m after addition of Seidlitzia Rosmarinus surfactant to water [13].The effect of new plant surfactant which was extracted from Mulberry tree leaves on interfacial tension reduction,wettability alteration and oil recovery for sandstone and carbonate rocks were investigated experimentally by Ravi et al.their results of IFT measurements revealed that interfacial tension between water and kerosene lowered from 42 to 20 dyn/cm after using Mulberry extract [14].Moreover,Shadizadeh and Kharrat introduced new plant surfactant(extract of Matricaria chamomilla)as an oil-water-interfacial-tensionreduction agent,which decreased IFT value from 30.63 to 12.57 mN/m[15].

Nowadays,nanotechnology has been able to address many of the unresolved challenges in different fields such as medicine,food,agriculture,and also industry and provide more efficient and cost-effective solutions.Recently,the use of nanoscale materials has found a special place in the industry,which has not been the exception for the upstream oil industry.The unique feature of nanoparticles is a high surface-to-volume ratio,which means the presence of more atoms on the surface of the particles,leading to enhancement in surface energy [17].Some studies have acknowledged the application of nanoparticles in improving oil recovery through a reduction in oil-water interfacial tension [16,18-20].Emadi et al.using natural nano surfactant composed of nano silica and Cedar extraction surfactant to investigate oilwater interfacial tension changes and oil recovery factor in sandstone core.IFT of the aqueous phase and kerosene reduced from 35 to 11.9 mN/m after increasing Cedar extraction concentration from 0 to 10 wt %.However,increasing the concentration of nano silica had negligible effect on IFT reduction [16].Hosseini et al.investigated the effect of silicon oxide and aluminum (III)oxide nanoparticles on enhancing oil recovery through evaluating wettability alteration and interfacial tension.They used brine with 35,000 ppm NaCl salinity as a base fluid for nanofluids preparation.Their results showed that the increase of both nanofluids concentration from 100 ppm to 10,000 ppm,causes IFT reduction between oil and nanofluids.However,this IFT reduction was relatively low [19].The results of research by Hendraningrat and Torsaeter revealed that metal oxide nanofluids could improve oil recovery by rock wettability alteration and Interfacial tension reduction.They used Polyvinylpyrrolidone (PVP)as a nanoparticles-stabilizer agent with 1 wt% concentration.They found that Al2O3and SiO2nanoparticles with 0.05 wt% concentration could reduce oil-water interfacial tension from 19.2 to 12.8 and 17.5 mN/m,respectively [20].

Despite the significant impact of nanofluids on oil recovery improvement,stabilizing these fluids is one of the most critical challenges facing the oil industry.The tendency of nanofluids to instability in the long-term results in precipitation and agglomeration of nanoparticles on the rock surface which has consequences such as impairment in permeability and porosity of reservoir rock,heterogeneous distribution of particles in various sections of reservoir and loss of nano-suspension unique properties [21].Harsh reservoir states,high salinity and high temperature,is one of the most highlighted factors of nanofluids instability [22].Also,since the volume of water injection in the EOR processes is very high,river or sea water with a salinity of more than 30,000 ppm is used as a base fluid for injecting chemicals into oil reservoirs,which causes intensifying the instability of injected nanofluids[23].Research has shown that the presence of ions in a nanofluid leads to a reduction in the nanoparticles hydrodynamic diameter and thus limits the ability of them to repel each other [24].Zeta potential of particles is a criterion for determining the stability or instability of a colloidal system.Zeta potential defined as the measured point between two distinct particles and the strength of electrostatic repulsion/attraction between them.The larger the zeta potential,(whether negative or positive)indicates stronger electrostatic repulsion between nanoparticles and,as a result,more solution stability [24].Therefore,nanoparticle dispersions’ stability could be improved by changing zeta potential,hydrodynamic diameter,and charge density of their particles[24].These changes can be applied on nanoparticles through modifying their surface,using non-ionic surfactants or polymers and employing ionic surfactants [25-31].Since the stability of nanoparticles is a function of their size,type and concentration in the suspension [21],determining the optimum values of mentioned variables,as well as appropriate stabilization method,are critical and essential factors in order to maximize the effects of nanofluids in EOR process.Therefore,by minimizing the reduction of reservoir rock porosity and permeability,stabilized nanoparticles could be able to recover the most portion of the remained oil in the underground.

Dehghan Monfared et al.studied the effect of silica nanoparticles’concentration and ionic strength on the stability of nanofluids by implementing two different methods including visual observation and optical absorbance analysis utilizing an ultraviolet-visible (UV-Vis)spectrophotometer.Based on their results,the stability of nanofluids decreased with increasing the concentration of nanoparticles and salt due to the reduction of repulsive electrostatic interaction and energy barrier [32].Sarmad Al-Anssari and et al.investigated the effect of pH,salinity and nano-silica concentration on zeta potential values.Their experiments showed that the absolute value of zeta potential increased with decreasing the nano-silica concentration and salinity,and increasing the pH [33].Hendraningrat and Torsaeter evaluated the stability of the brine (NaCl 3 wt%)containing 0.05 wt% of Al2O3,TiO2and SiO2after adding PVP stabilizer using three different methods including direct visual,nanosight and surface conductivity measurement.The results showed that all three untreated nanofluids started to deposit in less than an hour and completely deposited after 3 h.However,after adding 1 wt % of PVP to TiO2and Al2O3nanofluids,their stability improved to 48 and 96 h,respectively [20].Songolzadeh and Moghadasi studied the impact of SDS and CTAB ionic surfactants on the stability of γ-Al2O3and SiO2nanoparticles by measuring the UV adsorption and the zeta potential of nanofluids.Based on their results,SDS could stabilize greatly exceedingly SiO2nanofluids by supercharging their silica nanoparticles and enhancing electrical repulsion between them.On the other hands,CTAB did not have a noticeable impact on alumina nanoparticles stability [31].

Despite extensive studies on the implementation of new technologies in EOR,few types of research have been conducted on the utilization of nanoparticle/natural-surfactant systems as a novel chemical material.Since chemical and physical characteristics are diverse in nanoscale,evaluation of the performance of nanoparticle/naturalsurfactant solution has great importance.Although the loss of surfactant during chemical flooding is one of the major concerns of oil recovery processes,there is a great deal of literature showing that the use of nanoparticles and surfactants simultaneously has much greater impact on increasing the oil recovery factor than when each of them is utilized separately.Suleimanov et al.reported that oil recovery factor increased up to 35% when nanoparticles and surfactants were used simultaneously.While in systems containing only surfactants,the oil recovery factor reached to 17% at best condition [34].In another literature,the results of core flooding test revealed that natural surfactant was able to produce only 53% of OOIP.However,with the addition of nanoparticles to the surfactant,the oil recovery factor increased up to 74% [16].Therefore,the combination of nanoparticles and natural surfactants will be able to recover most of the trapped oil in reservoir,which,even if large portions of the surfactant is lost,will still be economically feasible and operationally efficient.

In this study,the effects of graphene,titanium dioxide (TiO2),and silicon dioxide (SiO2)nanoparticles and also nano-composite of TiO2/Graphene and Henna extract surfactant on oil-water interfacial tension reduction were investigated experimentally.The aforementioned nanoparticles have synthesized by sol-gel techniques.A natural waterbased suspending surfactant (Tragacanth extract)was used to stabilize the nano-surfactants.CMC of Henna extract surfactant was determined by conductivity,turbidity and interfacial tension experiments.In order to stabilize nano-surfactants containing different weights percent and types of nanoparticles,the optimum concentration of Tragacanth surfactant was determined for each of them using particle size and zeta potential measurements.Finally,stabilized nano-surfactants were used for investigating their effect on IFT through interfacial tension measurements.

2.Experimental methodology

Materials,equipment and experimental procedures were presented through this section.In the first step,the natural water-based surfactants were introduced.Then,the process of nanoparticles synthesis was described using the sol-gel method.Following this,the stabilization steps of nano-surfactants was explained and evaluated by zeta potential and particle size measurements.Finally,interfacial tension experiments were explained in details.It should be noted that brine (distilled water containing 100 g/L NaCl)was used in all experiments as an aqueous phase in order to bring the test conditions closer to the actual conditions of EOR process and carbonate reservoirs.Besides,kerosene was used as an oil phase in interfacial tension measurements.

2.1.Henna extract and CMC determination

Lawsonia inermisL,which is widely referred to “Henna” is a small tree or shrub found in abundance in Iran,Algeria,Pakistan,Yemen,India,Afghanistan and Egypt [35].Chaudhary et al.have reported that Henna leaves contain glucose,mucilage,lawsone,fats,gallic acid,resin and traces of alkaloid [36].The Henna extract surfactant that was used in this study obtained from Henna tree leaves and extracted and prepared by Ebnemasouyeh corporation,Tehran,Iran.Gallic acid (3,4,5-trihydroxybenzoic acid,C7H6O5),lawsone (2-hydroxy-1,4 napthaquinone,C10H6O3),tannic acid and dextrose (α- D -Glucose.C6H12O6)are the principal components of Henna extract [9].Henna is commonly used for various purposes,such as treatment,drug,tattoo and hair dye.Table 1 shows the major properties of Henna extract,which were reported by Ebnemasouyeh Corporation,Tehran,Iran.

Songolzadeh and Moghadasi showed that at concentrations of surfactant higher than CMC,due to a phenomenon called osmotic depletion,the nanofluids will become unstable [31].Additionally,in concentrations less than CMC,the efficacy of surfactants is strongly reduced due to lack of micelles formation in the solution.Therefore,accurate CMC determination of Henna extract surfactant in order to maximize its performance has great importance.In this study,conductivity,turbidity and interfacial tension experiments were carried out to conduct CMC measurements.

In order to measure the conductivity,turbidity,and interfacial tension,a Jenway 4510 conductivity meter,Aqua Lytic AL250T-IR turbidimeter and Fars EOR technology VIT6000 pendant drop apparatus were employed,respectively.Concentrations of the Henna surfactant solutions were selected in the range of 0-10 wt %.Fig.1 shows the conductivity,turbidity,and interfacial tension versus Henna concentration under 24 °C and 1 atm.As it can be seen in Fig.1,the value of 5 wt % was considered as CMC.

2.2.Tragacanth extract

Tragacanth is a dried gum exuded from the branches and stems of Astragalus Asiatic species [37].The quality measurement of gum Tragacanth is considered by its ability to modification aqueous media rheology even at low concentrations.It has been used as a suspending agent,emulsifier,stabilizer and thickener in pharmaceutical,cosmetic and food industries [38].Gum Tragacanth is an anionic,branched and heterogeneous carbohydrate that consists of two main fractions: bassorin (insoluble but water-swellable)and tragacanthin (water-soluble)[39].In this study,the extract of Tragacanth was used as a natural water-based suspending surfactant in order to stabilize the nanoparticles dispersed in the base fluid.This surfactant was purchased from Ebnemaouyeh Corporation,Tehran,Iran.Major properties of Tragacanth extract are represented in Table 1,which was reported by Ebnemasouyeh Corporation,Tehran,Iran.

2.3.Nanoparticles synthesis

2.3.1.Materials

Graphene nanosheets was purchased from US Research nano-materials (USA).Titanium (iv)butoxide (TBu,99.9%)and tetra-ethylortho-silica (TEOS,99.9%)was purchased from Merck,Germany,and utilized as the precursor of TiO2and SiO2,respectively.Other chemicals were purchased from Merck Company,Germany.All other materials were in the analytical grade and utilized without further purification.

2.3.2.TiO2 synthesis

The TiO2synthesized using sol-gel-method.In brief,5 mL of TBu was added to 20 mL of absolute-Ethanol (mixture A)and sonicated for 30 min.A solution of absolute-ethanol: deionized-water: Nitric-acid(5 mL:40 mL:1.5 mL)was added to solution mixture dropwise while through mixing on the magnetic stirrer.After 60 min of mixing,the achieved homogeneous solution was heated at 80 °C for 30 min in a hot water bath to evaporate the excess ethanol.Then,the hot solution was allowed to cool down to form the gel.Next,the achieved gel aged for 24 h.Then,the gel was dried at 100 °C for 8 h.At the end,the fine powder of fine dried-gel was calcined at 400 °C for 4 h to achieve the anatase phase of TiO2[40].

Table 1 Properties of Henna and Tragacanth extract.

Fig.1.(a)Conductivity,(b)turbidity and (c)interfacial tension variations with Henna solution concentration.

2.3.3.SiO2 synthesis

The SiO2nanoparticles was synthesized by the sol-gel method.In brief,1.6 mL of TEOS added to 60 mL of absolute-ethanol.Then,9 mL of ammonia solution added to the previous solution and stirred while the gel was formed.Then,the achieved gel was dried in the oven at 60 °C for 12 h.Then,the aggregated dry gel was grounded and annealed for 2 h at 450 °C to achieve SiO2[41,42].

2.3.4.TiO2/graphene synthesis

A facile hydrothermal method was used for the synthesis of TiO2/graphene nanocomposite.A specific amount of graphene (1 wt % of TiO2)was dissolved in mixture of 20 mL of deionized water and 40 mL of absolute ethanol and sonicated for 1 h.Then,1 gr of TiO2added to the prepared mixture and sonicated for an extra 1 h.The achieved suspension was transferred to a 100 mL Teflon-lined sealed hydrothermal autoclave and heated at 120 °C for 3 h to prepare the nano-composite of TiO2/Graphene.Finally,the attained composite was filtered and washed with ethanol and dried at room temperature.

Fig.2.XRD pattern of a)TiO2,b)TiO2/Graphene,c)Graphene,and d)SiO2.

2.3.5.XRD

XRD is a well-known and powerful method for structural analysis of nanocomposites.Fig.2 displayed the XRD patterns of TiO2,TiO2/Graphene,Graphene and SiO2.As can be seen in Fig.2a and 2 b,the characteristic peaks at 2θ = 25.31,37.99,47.51,54.10,54.92,63.04,69.03 and 70.38 corresponded to (1 0 1),(0 0 4),(2 0 0),(1 0 5),(2 1 1),(2 0 4),(1 1 6),and (2 2 0)planes,respectively.These peaks represent the anatase phase of TiO2according to JCPDS (21-1272).These characterization peaks were repeated in the TiO2/Graphene XRD pattern; however,there was not any peak which related to the Graphene content of composite.Graphene has a characteristic peak at 25.1,but it was not clear because of overlap with (1 0 1)plane of TiO2at 25.3.Besides,it could be related to the low intensity of graphene peak because of the low amount of graphene content in the final composite.As can be seen in Fig.2 c,the characteristic peaks at 2θ = 21.03,36.58,39.51,40.33,42.47,and 45.78 correspond to (1 0 0),(1 1 0),(1 0 2),(1 1 1),(2 0 0)and (2 0 1)plane,respectively.These peaks related to SiO2based on JCPDS (85-0335).

2.3.6.FESEM and EDAX

Fig.3a-f shows the FESEM micrograph and EDAX profile of each micrograph.In Fig.3a and 3 c,the TiO2and SiO2nanocomposites appeared with uniform sphere-shaped morphology.In this regard,similar morphologies have been obtained in the literature.Fig.3 b illustrate the FESEM micrograph of graphene containing nano-composite.The flat-plane-like morphology of graphene has been presented in this figure.Moreover,in the EDAX profile of specimens,the characteristic peaks of Ti,Si,O and C was observed.Based on the obtained results listed in Table 2,the theoretical weights percent composition of elements are with good agreement with experimental weight percent composition of each synthesized sample that demonstrates the good purity of samples.

2.3.7.FTIR

Fig.4a-c indicates the FTIR spectra of nano-particles.For TiO2and TiO2/Graphene FTIR patterns,a wide band located within the range of 630-845 cm-1is observed,which corresponds to Ti-O-Ti stretching vibration of the interconnected octahedral [TiO6].This indicates the presence of titania.Furthermore,the broad peaks at 1628 and 3445 cm-1are attributed to -OH and H-O-H vibration bonds,respectively,suggesting the presence of hydroxyl groups and moisture adsorbed on the nano-particles surface.The peaks which located at 469,801,949,1101 and 1637 cm-1are related to Si-O bending vibration,Si-O stretching vibration,Si-OH bending vibration,Si-O-Si asymmetric stretching vibration and Si-OH bending vibration on the surface of SiO2nanoparticles.Besides,the peak located at 3431 cm-1related to H-O-H vibration bond [43].As can be seen in the FTIR diagram of TiO2/Graphene,bonds located at 3445,2725,1723,1628,1395,1246,1099 cm-1are related to H-O-H stretching,COOH vibrations,C＝O vibrations,O-H vibrations,C-OH stretching vibrations,C-O stretching vibration,and C-OH stretching vibration,respectively.The formation of the mentioned carbon-containing bonds in TiO2/Graphene pattern revealed the existence of graphene in the TiO2/Graphene nanocomposite.

2.4.Preparation and stabilization of nano-surfactants

In order to investigate the effect of nanoparticles’ type and concentration on IFT reduction,solutions containing the nanoparticles of TiO2,SiO2and nano-composite of TiO2and Graphene with the 0.05,0.1,0.5 and 1 wt%,and Henna extract (at CMC value)were prepared.Since the mass to volume ratio of graphene nanoparticles are remarkably lower than other nanoparticles and economic considerations,only the 0.05 and 0.1 wt% of this nanoparticle were utilized through this study.

For stabilizing these solutions,15,20,25,30,35,40 and 45 wt% of Tragacanth extract surfactant were added to them.Since magnetic stirring was not sufficient to homogenize these solutions,Hielscher UIP500hd ultrasonic apparatus was used for 10 min with 80% amplitude to prepare uniform and homogenous colloidal suspensions.In order to determine the optimum concentration of Tragacanth extract surfactant and to ensure that nano-surfactant properties did not altered during the experiments,the stability of the prepared nano-surfactants was evaluated by three methods: (1)visual observation,(2)zeta potential analysis and (3)particle size analysis of nano-surfactants.In visual observation method,the nano-surfactants phase behavior was monitored every 15 min in the first 2 h and every 1 h over 24 h by taking photos of the test glasses.In the second and third method,zeta potential and particle size of each nano-surfactants were measured at the first moment and 24 h later by using Malvern nano-ZS zetasizer apparatus.This apparatus measures particle size from less than a nanometer to several microns utilizing dynamic light scattering,and zeta potential utilizing electrophoretic light scattering through radiation of helium-neon laser light and special analysis software.The scattering angle was adjusted to 90° and the temperature was set to 24 °C.The data were reported as the zeta potential and Z-average.It should be noted that particle size and zeta potential cells were completely stable and had not been shaken during 24 h.Finally,for each of the four nanoparticles (TiO2,SiO2,Graphene,and TiO2/Graphene)with 0.05,0.1,0.5,and 1 wt %,the most optimum weight percent of Tragacanth extract surfactant was selected (in terms of nano-surfactants stability and their particle size).

In order to accurately determine the optimum concentration of Tragacanth extract surfactant to maintain the stability of each nanosurfactant during 24 h,a total of 392 zeta potential and particle size tests were conducted at the first moment and 24 h later.Variable factors in performing these tests were the type and weight percent of nanoparticles and weight percent of Tragacanth extract surfactant.For example,results of zeta potential and particle size tests at the first moment and 24 h later for graphene with 0.1 wt % and different weight percent of Tragacanth extract surfactant were shown in Figs.5 and 6,respectively.Results of the other measurements for different types and concentrations of nanoparticles were not represented due to a large number of charts.

Fig.3.FESEM micrograph of a)TiO2,b)TiO2/Graphene,and c)SiO2 and EDAX pattern of d)TiO2,e)TiO2/Graphene and f)SiO2.

As can be seen,nano-surfactants with 30,35 and 40 wt% of Tragacanth extract surfactant have a good stability and acceptable particle size.However,it seems that the optimum concentration of this surfactant for Graphene nano-surfactant preparation is 30 wt %.Regardless of economic considerations,30 wt% of this surfactant,even after 24 h,was more stable than the 35 and 40 wt% and also the particle size had not increased significantly.Therefore,this nano-surfactant with a less than 270 nm particle size,has the ability to move through the carbonate rock pores and microfractures,without significant effect on rock permeability.

By applying a completely similar aforementioned method,the optimum concentration of Tragacanth extract surfactant for preparation and stabilization of other nano-surfactants with their chemical properties was obtained and summarized in Table 3.These optimal concentrations were able to overcome the effect of solutions high salinity(100,000 ppm NaCl)for at least 24 h and prevented nanoparticles from precipitation and agglomeration.It should be noted that the use of natural surfactants almost costs forty times cheaper than other industrialized surfactants; thus,utilizing these surfactants even at higher concentrations could be economical.

Temperature has a significant impact on the stability of nanofluids and there is a direct relationship between the temperature of carrier fluid and agglomeration rate and reactivity of the nanoparticles [44].Increasing the temperature results in agglomeration of particles and instability of nanofluids,due to decrease in zeta potential.However,based on the results,the performance of nanofluids in EOR processes has ameliorated in higher temperatures [45].This can be due to the reduction of interfacial and surface tensions at high temperature resulting from reduced intermolecular reactions in the liquid,and the decrease in oil viscosity resulting from intensification of Brownian motion [46].

Table 2 Comparison of theoretical wt.% composition of elements and experimental wt.% composition of each synthesized sample.

2.5.Interfacial tension measurement

In this study,Fars EOR technology VIT6000 pendant drop apparatus were employed to measuring interfacial tension between solutions and kerosene through taking photos with a high-resolution camera and evaluating them by image processing software.This apparatus calculates fluid-fluid interfacial tension with the pendant drop method based on drop shape and fluids properties,which described by Drelich et al.[47].To apply this method,finding out the densities of different solutions is crucial,and they measured by KEM DA-640 density meter and presented in Table 3.The results of IFT measurements are reported through digitizing the pictures of the suspended droplet by image processing software.The schematic of VIT6000 pendant drop apparatus is shown in Fig.7.It is worth mentioning that all of IFT measurements were conducted at the ambient pressure (14.7 psi)and temperature(22 °C).

Since the accuracy of output results of pendant drop apparatus is intensely dependent on the cleanness of its components,after each test,the device was washed with toluene and acetone and dried completely using the air pump.Moreover,in interfacial tension experiments,kerosene was used as a surrounding phase instead of crude oil due to cleanness,and the darkness of the nano-surfactants,which made it impossible to accurately take a photo and analysis it.

3.Results and discussion

In this section,the viability of Henna extract and nanoparticles(nano-surfactants)as the eco-friendly agents for interfacial tension reduction was investigated experimentally.

Fig.4.FTIR spectra of a)TiO2,b)TiO2/Graphene and c)SiO2.

Fig.5.Zeta potential values for 0.1 wt% of graphene nanoparticle versus different weight percent of Tragacanth extract surfactant in primary condition and after 24 h.

Fig.6.Particle size values for 0.1 wt% of graphene versus different weight percent of Tragacanth extract surfactant in primary condition and after 24 h.

3.1.Oil-Henna extract interfacial tension (without nanoparticles)

As shown in Fig.1 c,Henna extract reduced oil/water interfacial tension from 37.23 mN/m to 15.24 mN/m by increasing concentration from 0 wt% to 10 wt%.The droplet profiles for different concentrations of Henna extract inside the kerosene at the moment of falling are presented in Fig.8.It can be seen that interfacial tension changes a little in concentrations more than CMC value,which proves that the micelles are entirely formed at CMC point.

3.2.Oil-nanosurfactant interfacial tension

Adding various concentrations of nanoparticles to Henna extract surfactant at its CMC value had a marginal effect on oil/water interfacial tension.However,this slight reduction in interfacial tension could produce a significant amount of trapped oil in tight fractures.Fig.9 compares the performance of different concentrations of nanoparticles (nano-surfactants number 1 to 14 of Table 3)on oil/water interfacial tension.What stands out from the graph is that TiO2nanoparticle had the best performance in comparison to the other three nanoparticles in interfacial tension reduction.It is worth mentioning that graphene nanoparticle had the negligible effect on interfacial tension reduction and when it combined with TiO2and formed TiO2/Graphene nanocomposite,had a better effect on its performance to IFT reduction.

Fig.7.The schematic of VIT6000 pendant drop apparatus used for interfacial tension measurements.

3.3.Comparison of Henna interfacial tension reduction with different surfactants

In recent studies,various plant surfactants were used for chemical flooding [13-16,44].These surfactants are environmentally friendly in comparison with industrial surfactants.In order to investigate the effect of each plant surfactants on IFT reduction,the results of interfacial tension measurements are illustrated in Fig.10.As seen,the effect of Henna extract on IFT reduction is similar to other plant surfactants.All of them are in the range of 15 mN/m to 20 mN/m at their CMC point except Seidlitzia Rosmarinus which its IFT is 9.5 mN/m at 8 wt % (CMC point).However,the addition of 1 wt % of TiO2nanoparticle to the Henna extract surfactant resulted in a decrease in IFT number to less than 15 mN/m.

3.4.Mechanism of IFT reduction

For the trapped oil droplets to move through the pore throat,deformation is required.This deformation is facilitated by the reduction of oil/water interfacial tension.Since interfacial tension is a thermodynamic characteristic,the IFT between nano-surfactant and crude oil starts to alter from the early moments of combination.In fact,movement of chemical ingredients available in the nano-surfactant toward the oil/surfactant interface as well as the mass transfer from oil to the nano-surfactant could be the main reasons for this alteration [46].

Fig.8.Pendant drop schematic of different Henna extract solutions in Kerosene as oil phase.

Fig.9.IFT variation with nanoparticles concentration variations.The error bars represents the standard deviation of three different interfacial tension measurements conducted for each nano-surfactant.

Asphaltene adsorption on the surface of the Henna extract surfactant's micelles and nanoparticles could be the principle mechanism for the IFT reduction by nano-surfactants.Rezvani et al.measured the absorbance spectroscopy for nanocomposits and its components in the base fluids and revealed that the responsible mechanism for the reduction of oil/water interfacial tension was asphaltene adsorption on the nanoparticles' surface [46].Therefore,this mechanism reduces the viscosity of oil,decreases the deposition of asphaltene and pore throat blockage,and facilitates the easier movement of oil droplets through the pore throat of porous media,which ultimately leads to an increase in oil recovery [46,49].

4.Conclusions

Since global energy demand is rising rapidly and current EOR techniques are able to improve oil recovery factor up to 60% in the best state,engineers have always been searching for new methods to increase production of trapped oil in reservoirs.These methods should not only increase the oil recovery factor but also should be economically feasible,environmentally friendly,and capable of performing on a field scale.Combination of natural surfactants and nanoparticles as the injection fluid can largely meet these expectations.Implementation of nanofluid flooding for EOR processes faces many challenges and limitations such as stability of nanoparticles in solution,interaction of nanofluids with formation rocks and fluids at the reservoir,lack of specialized equipment for the preparation and injection of nanofluids,and application in harsh reservoir condition.However,by providing practical solutions to the mentioned challenges,significant improvements can be made in enhancing oil recovery from reservoirs using nanofluids.Our experiments proved that natural nano-surfactants might be good substitute for industrial surfactants and polymers.The main conclusions of this study are as follows:

1.Interfacial tension measurements revealed that adding Henna extract to the water,results in a reduction in interfacial tension value between oil and water from 37 mN/m to 15.24 mN/m.

2.TiO2,SiO2and nano-composite of TiO2and Graphene nanoparticles were synthesized with sol-gel method in PUT chemistry laboratory which results of XRD,FESEM,EDAX and FTIR tests confirmed the authenticity of these synthesizing method.

3.For the first time,a natural water-based suspending surfactant called Tragacanth extract was used to stabilize different nanofluids for 24 h.The optimum weight percent of this surfactant was determined through particle size and zeta potential tests.

4.Interfacial tension measurements showed that addition of nanoparticles to Henna extract at its CMC value (nano-surfactant)has the slight impact on interfacial tension value between oil and nanosurfactants,and in the best case adding 1 wt % of TiO2nanoparticle to Henna extract reduced interfacial tension from 18.43 mN/m to 14.57 mN/m.

Fig.10.Interfacial tension of different surfactants in kerosene as oil phase:Seidlitzia Rosmarinus [13],Mulberry [14],Matricaria Chamomilla [15],Cedr and Nanofluid [16],Saponin (Ziziphus Spina-christi)[48],and Henna [this study].The error bars represent the standard deviation of ±5%.

5.Graphene nanoparticles alone had negligible effect on reducing oil/water interfacial tension and negatively affected the performance of TiO2nanoparticles (pure TiO2nanoparticles)when combined with them and formed TiO2/Graphene nanocomposites.Therefore,graphene nanoparticles are not functionally effective and economically feasible for EOR purposes.

6.TiO2nano-surfactant was the second-best natural surfactant in reducing oil/water interfacial tension amongst the other ones,and its performance was as influential as industrial surfactants.