Synergistic effect of hydrophilic nanoparticles and anionic surfactant on the stability and viscoelastic properties of oil in water (o/w) emulations; application for enhanced oil recovery (EOR)

With the rapidly increased global energy demand, great attention has been focused on utilizing nanotechnology and particularly nanofluids in enhanced oil recovery (EOR) to produce more oil from low-productivity oil reservoirs. Nanofluid flooding has introduced as one of the promising methods for enhanced oil recovery using environment-friendly nanoparticles (NPs) to be as an innovative-alternative for chemical methods of EOR. This work investigates the synergistic effects of anionic surfactant and hydrophilic silica nanoparticles on the stability and the mechanical behavior of oil in water (O/W) emulsions for their application in EOR. To achieve this, an extensive series of experiments were conducted at a wide range of temperatures (23 – 70 °C) and ambient pressure to systematically evaluate the stability and the viscoelastic properties of the oil in water (O/W) emulsion with the presence of hydrophilic silica nanoparticles and an anionic surfactant. In this context, the initial oil to water volume ratio was 25:75. Sodium dodecylsulfate (SDS) was used as the anionic No.29(12) 2020 Journal of Petroleum Research & Studies (JPRS) E34 surfactant and n-decane was used as model oil. A wide concentration ranges of NPs (0.01 – 0.2 wt%) and surfactant (0.1 – 0.3 wt%) were used to formulate different emulsions. For stability measurements, a dynamic light scattering and zetasizer were used to measure the particle size distribution and zeta potential respectively. Creaming and phase behaviors were also investigated. The viscoelastic measurements were conducted using Discovery Hybrid Rheometer. Results show that in the presence of surfactant, and NPs mitigates the coalescence of dispersed oil droplets giving high promises in EOR applications. Further, over the tested range of temperatures, the viscosity of O/W emulsion remains stable which indicates thermal stability. Despite studies examining the use of nanoparticle-surfactant combination in sub-surface applications, no reported data is currently available, to the best of our knowledge, about the potential synergistic effect of this combination on the stability and viscoelastic properties of O/W emulsion. This study gives the first insight on nanoparticle-surfactant synergistic effect on oil in water (O/W) emulsion for EOR applications. تاطبثمو ةيونانلا تاميسجلل كرتشملا ريثأتلا صاوخو رارقتسلاا ةلاح ىلع ةينويلاا يحطسلا دشلا ماخلا طفنلا جاتنا نيسحت تايلمعل قيبطت :ءاملا يف تيزلا تابلحتسمل ةجوزللا

NPs -surfactant combination has unique properties better than that of the sole NPs or surfactant formulations. Such combination can synergistically enhance the performance of NPs and surfactant in many applications including EOR [17]. In this context, loss of surfactant molecules by adsorption into rock surface can reduce the amount of free surfactant to adsorb on the oil-water interface and thus increase the interfacial tension [18][19][20].
However, with the presence of NPs, the amount of loss surfactant is significantly decrease since the pore spaces will be totally or partly coated with NPs. Further, NPs can act as a carrier for surfactant via the Brownian motion of NPs into the oil/water interface [21].
Meanwhile, NPs dispersion without surfactant is critically unstable due to the rapid aggregation of NPs once it comes into contact with the liquid phase [15,22,23]. The rapid aggregation of NPs is related to the high surface energy of NPs as a result of the high surface to volume ratio [15]. The addition of surfactant, however, can significantly enhance the stability of NPs in the dispersion via supercharging NPs surface and thus induces the repulsive force between similarly charges NPs [24].
In the last decade, several studies have investigated the mutual effect of NPs and surfactant on the stability and rheological properties of o/w and w/o emulsions. Lan, et al. [21] investigated the formulation of stable o/w emulsion using a combination of silica NPs (200 nm) and cationic surfactant (CTAB). Their results showed that appropriate concentrations of NPs and surfactant molecules can synergistically result in very stable o/w emulsion.
However, the increase in surfactant concentration can dramatically destabilize the emulsion due to the excessive desorption of NPs in the form of large aggregates from the interface.
Mechanistically, the increase in cationic concentration can totally neutralize the surface charge of NPs leading to an accelerated coalescence and aggregation process of NPs [15]. At the same time and using the same types of NPs and surfactant (CTAB), Ravera, et al. [25] investigated the interfacial tension and rheological properties of hexane/water emulsion as well as the behavior of micrometric oil droplets in the emulsion. Without a deep knowledge of the mechanisms, the study showed that the NPs transfer and attachment to droplet interface are mainly governed by their surface properties and the interaction of the surfactant in the emulsion. In this context, the status of NPs at the interface varying from strongly No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS) E38 adsorbed to strongly desorb depending on the concentration of CTAB at the interface. In line with that, Vashisth, et al. [26] showed that adding surfactant molecules which preferentially adsorb at the oil-water interface displaces NPs effortlessly from the interface. Fan, et al. [27] using silica NPs with three different surface chemistry, revealed that the adsorptiondesorption behaviour on NPs at the oil/water interface is entirely controlled by the surface properties of NPs. Typically, the surface chemistry of NPs is a function of surfactant concentration in the nanofluid [28]. Esmaeilzadeh, et al. [29] reported that above the critical water-wet via the structural disjoining pressure (SDP) of NPs. However, the activation of SDP requires in between wedge between the rock surface and oil droplet. Typically, this wedge is not available when the contact angle is higher than 90° which is mostly the case in carbonate reservoir. Thus it is essential to use another surface active material to reduce the contact angle below 90° the NPs can act to synergistically alter the wettability into strongly water-wet [32]. Moreover, the presence of NPs reduces the CMC and improves the efficiency and feasibility of surfactant in EOR projects.

Despite studies investigate the o/w emulsions and the interaction effects of surfactant and
NPs on the stability, interfacial and rheological properties of emulsions, there is a serious lack of information about the correct NPs/surfactant ratios to formulate stable and efficient o/w emulsion for EOR application. This study thus presents the formulation of stable o/w emulsion augmented by silica NPs and SDS surfactant and investigates the optimum concentration of these surface-active agents to achieve a stable emulsion. Also, the range of NPs-SDS concentration that assures a synergistic effect on emulsion properties was investigated.

Materials:
Silica NPs (white powder, 99.5 wt% SiO 2 , Molecular mass of 60.08 g/mol) with initial particle size ranging between 5 -10 nm was supplied by Sigma-Aldrich. These insoluble Homogenizer, Biologics) with the presence of NPs. All laboratory devices used in this study was illustrated and showed in our previous work [38].

Formulation of o/w emulsion:
In this study, o/w emulsion was formulated via dispersing of n-decane in the water phase which holding a suspension of NPs in a surfactant solution. the emulsification ratio was constant at 25:75 oil to water volume ratio [13,14,31]. Initially, brine at the desired concentration was prepared to be used as a water phase in the subsequent steps. Then measurements. In this context, ζ was obtained from the electrophoretic mobility and application of the Smoluchowski-Helmholtz equation [23]. Each experiment was repeated three times.

Rheological Measurements:
Rheological measurements were conducted using Discovery HR-3 hybrid rheometer, which equipped with temperature control. Experimentally, the effect of different parameters including SDS, and NPs concentrations in the emulsion, aging and operational conditions (time, temperature, and share rate) on emulsion rheological properties. Further, viscosity was measured at different rotational speeds (1 -500 rpm). The viscosity curves can be analyzed using mathematical models such as Power low (Ostwald de Waele) and Bingham plastic model [40].

Result and Discussion:
The studies on physicochemical characterization and droplet size distribution of o/w emulsion augmented by anionic surfactant and hydrophilic NPs are presented and discussed in this section.

Stability of o/w emulsion augmented by NPs-surfactant combination:
O/W emulsion stability in the presence of silica NPs/SDS surfactant was first evaluated visually using transparent bottles and then via physicochemical measurements. Visually, all the formulated emulsions were stable against creaming and/or water phase separation along the two weeks of testing. However, a slight water separation in some samples was later noticed in the second week. Monitoring of o/w phase behavior showed that at relatively high surfactant concentration (≥ CMC), the increase in NPs concentration has entirely no influence on the stability of the emulsion. While below CMC, NPs increases induce the stability of the emulsion. Thus, the NPs/SDS synergistic effect on o/w emulsion stability is limited by the concentrations below CMC. This is consistent with the reported data in the literature regarding the mutual effect of NPs-surfactant combination on oil/water interfacial tension [33]. Typically, stable o/w emulsion can be formulated when the oil/water interfacial tension is ultimately low [41]. In this context, the reduction of oil/water interfacial tension depends on the extent of surfactant adsorption on the oil/water interface. At low surfactant

No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS)
E42 concentration (≤ CMC), a limited number of monomers can reach the oil/water interface via monomer diffusion. In this case and due to the Brownian motion. NPs may act as carrier agents to increase surfactant monomers near the interface. Thus, the rate of surfactant adsorption increases leading to a significant reduction on o/w interfacial tension referring to the stable emulsion. This synergistic effect will no longer available at high surfactant concentration (≥ CMC). In this case, the high amount of surfactant monomer near the interface, due to high concentration, subsequently increases the rate of monomers adsorption and thus reduces the interfacial tension without the effect of NPs [41]. Some studies argued that at high surfactant concentration, the significant repulsive forces between similarly charged NPs and surfactant monomer can push the monomer towards the interface and that is also synergistic effect [25]. However, Al-Anssari, et al. [15] demonstrated that surfactant monomer can be attached into similarly charged NPs from the tail group.

Characterization of o/w emulsions:
Sufficient electrostatic repulsive force between o/w emulsion droplets is a key for a stable or thermodynamically stable emulsion. In this context, zeta potential (ζ) of emulsion droplets is a direct scale for such repulsive forces. Physiochemically, ζ of each droplet in the emulsion is the electrostatic potential at the electrical double layer surrounding the colloid particle or droplet in solution [39]. Characteristically, Droplets or NPs with ζ between -10 and +10 mV are considered electrostatically neutral and thus significantly unstable. While droplets with ζ of greater than +30 mV or less than -30 mV are electrostatically stable against collision and coalescence. Table.1 contains more details on ζ ranges and stability. Emulsion with ζ value lower than ± 30 mV tends to aggregate and coalesce due to the weak repulsive force between droplets.
Typically, the ionic surfactant can improve the stability of colloidal systems including o/w emulsion if correctly formulated. Thus, ζ was measured at different SDS concentrations       Results showed that increased temperature reduces the viscosity of the emulsion which is consistent with reported data regarding all type of emulsions including pickling o/w emulsions. Typically, viscosity reduction with increased temperature is mainly related to the effect of the head on the continuous phase of the emulsion. Further, flocculation of the dispersed phase at higher temperatures, mainly due to higher surfactant solubility at a higher temperature, can induce the reduction in viscosity [10,13].
On the other hand, salinity has shown more complicated influences on emulsion viscosity.
Results showed that at relatively low salinity (0 -0.2 wt% NaCl), no significant effect was noticed on the viscosity at all temperatures. Further increase in salinity (0.5 wt% NaCl) can increase the viscosity more significantly. While salinity increasing into higher NaCl concentration reduce the viscosity again to a value lower than that recorded with DI-water.
Specifically, at high salinity, the more salt is added the less viscosity is recorded. The complex behavior of emulsion viscosity at elevated salinity is related to several factors such as the effect of salinity on the CMC and solubility of surfactant molecules as well as the wettability of NPs surfaces which controls the adsorption of these NPs onto the oil/liquid interface [33].

Conclusion:
The stability and rheological property of decane/water oil-in-water emulsion which stabilized by silica nanoparticles (NPs)-SDS anionic surfactant were investigated to assess the efficiency NPs/SDS combination in enhanced oil recovery (EOR) projects. Results from extensive zeta potential (ζ) and droplet size distribution indicate the high potential of this combination as an emulsion stabilizer. Addition of NPs-SDS combination can shift ζ of the emulsion into the stable region (≥ 30 mV) via inducing the negativity of the aqueous phase.
In this context, attachment of anionic surfactant via its tail group into similarly charged hydrophilic silica NPs can supercharge the colloid particles [24]. These supercharged NPs by SDS monomers is a superior emulsion stabilizer [10].