SPE 129885

Nanoparticle-Stabilized Emulsions for Applications in Enhanced Oil Recovery Tiantian Zhang, SPE, Andrew Davidson, Steven L. Bryant, SPE, and Chun Huh, SPE, The University of Texas at Austin

Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the 2010 SPE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, USA, 24–28 April 2010. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Nanoparticle-stabilized emulsions have attracted many researchers’ attention in recent years due to many of their specific characteristics and advantages over conventional emulsions stabilized by surfactants or by colloidal particles. For example, the solid nanoparticles can be irreversibly attached to the oil-water interface and form a rigid nanoparticle monolayer on the droplet surfaces, which induce highly stable emulsions. Those emulsions can withstand harsh conditions. Compared to colloidal particles, nanoparticles are one hundred times smaller, and emulsions stabilized by them can travel a long distance in reservoirs without much retention.

Oil-in-water and water-in-oil emulsions that are stabilized with different surface-coated silica nanoparticles of uniform size have been developed; these emulsions remain stable for several months without coalescence. The wettability of the nanoparticle determines the type of emulsion formed. The phase behavior with respect to the initial water/oil volume ratio (IVR), salinity, nanoparticle concentration and nanoparticle wettability was systematically examined. The emulsions were also characterized by measuring their droplet size and their apparent viscosity. Employing the hard-sphere liquid theory for nano-scale dispersions, the correlation between droplet/droplet interaction forces and droplet/droplet equilibrium separation distances has also been examined.

Introduction Oil/water emulsions stabilized by surfactants are frequently used in the oil industry. Emulsions are also producible with solid particles as stabilizers. These are called “Pickering emulsions”. Such emulsions have many advantages over conventional surfactant-stabilized emulsions, and are widely used in food, pharmacy and cosmetics industry, but are rarely applied for oil recovery purpose. This is because the solid stabilizers they use are colloidal particles, which are in micron size and easily trapped in the rock pores. Thus the long-distance propagation of emulsions made with them is unfeasible in reservoirs.

Nanoparticles have properties potentially useful for certain oil recovery processes, as they are solid and two orders of magnitude smaller than colloidal particles. The nanoparticle stabilized emulsions droplets are small enough to pass typical pores, and flow through the reservoir rock without much retention. They also remain stable despite harsh conditions in reservoirs due to the irreversible adsorption of the nanoparticles on their droplet surface. In addition, the large viscosity of nanoparticle-stabilized emulsions can help to manage the mobility ratio during flooding, which provides a viable method to push highly viscous oil from the subsurface. Therefore, they have significant potential in reservoir engineering applications.

In recent years, nanoparticle-stabilized emulsions have triggered great interest. Active research efforts are on-going in many areas, especially in chemical engineering and materials science. These research efforts led to the detailed characterization of the properties of emulsions solely stabilized by nanoparticles in many aspects, e.g., emulsion type, droplet size, stability, bulk viscosity, and interfacial properties, etc. The influence of experimental conditions such as nanoparticle wettability, particle concentration, their initial location (i.e., dispersed in water or dispersed in oil), salt concentration and pH of the aqueous phase, as well as the oil type, on the emulsion system has also elucidated, and detailed reviews are available (Binks and Lumdson,2000a, 2002b; Binks et al.,2005; Binks and Rodrigues, 2005; Horozov, et al., 2007).

The most commonly used nanoparticles are spherical fumed silica particles with a diameter in the range of several to tens of nanometers. Their wettability is controlled by the coating extent of silanol groups on their surface (Binks, 2002). The nanoparticles can be made hydrophilic with high percentage (over 90%) of silanol groups on the surface, and consequently they form stable oil-in-water (o/w) emulsions. On the other hand, when the silica particles are only coated about 10% on their surface by silanol groups, they are hydrophobic and yield water-in-oil (w/o) emulsions. Furthermore, when the nanoparticles

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are only partially coated with silanol groups (e.g., 70%), they become particles with “intermediate hydrophobicity;” the stable emulsion type they generate depends on the oil polarity, i.e., formation of o/w emulsions are favored with nonpolar oils whereas w/o emulsions are preferred with polar oils. The schematic of nanoparticle stabilized emulsion drops with different contact angles is shown in Fig. 1.

The effect of the initial phase in which intermediate nanoparticles are dispersed on the emulsion they subsequently produce was investigated by Binks and Lumsdon (2000a). They showed that with equal volumes of toluene and water, w/o emulsions were preferred when particles were originally dispersed in toluene, while o/w emulsions were produced when particles were dispersed in water.

Experiments have been carried out to investigate the dependence of emulsion stability and rheological properties on electrolyte concentrations and pH (Horozov et al., 2007). They showed that fumed silica particle-stabilized emulsions were not very sensitive to NaCl concentration at pH of 7. Furthermore, it was also found that emulsion stability increased and the average size of emulsion droplets deceased with decreasing particle size (Hunter et al., 2008; Lopetinsky et al., 2006).

To understand and quantify the conditions for equilibrium and stability of emulsion systems stabilized with solid particles, many theoretical models have been developed (Levine and Bowen, 1991, 1992, 1993; Kralchevsky et al., 2005; Reincke et al., 2006). Every nanoparticle in the emulsion may experience electrostatic repulsions, van der Waals attractions, and capillary attractive forces, etc. (Bresme and Oettel, 2007; Binks and Horozov, 2006; and Fernandez-Toledano et al., 2006). The nanoparticle monolayer on a droplet surface can reach equilibrium when and only when the interactions between the nanoparticles are balanced. From the free energy considerations, the equilibrium density of the spheres at the interface was defined. Some qualitative comparisons of the model prediction with the measured data are discussed in some reviews (Reincke et al., 2006; Paunov et al., 2002).

Kralchevsky et al. (2005) also investigated the nanoparticle-stabilized emulsion stability by defining the preferred interfacial bending moment and deducing the emulsion droplet size as a function of the interfacial density of the adsorbed particles. This investigation of emulsion stability accounted for the interactions between the emulsion droplets. The well-established DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, together with the viscous resistance to the thinning of the liquid film between the droplets, explained the emulsion stability reasonably well. Stability mechanisms other than the above conventional approach have also been proposed (Friberg, 2005; Lopetinsky et al., 2006).

As mentioned above, nanoparticle-stabilized emulsions can avoid the deficiencies caused by the large size of Pickering emulsion droplets with colloidal particles. In addition these nanoparticles can be made with required characteristics so that the formation and destabilization of emulsions as well as their rheological properties can be precisely controlled. These attributes of nanoparticle-stabilized emulsions offer exciting opportunities for the upstream oil industry. Examples include water in oil emulsions with synthesized nanoparticles for heavy oil upgrading (Thompson et al., 2008), and external control of the rheology and mobility of the drilling fluids and the flooding fluids during enhanced oil recovery processes (Melle et al., 2005).

Another potential application is using nanoparticles to generate double or multiple emulsions with a compact interfacial layer of nanoparticles on the surface. Such armored droplets can be extremely stable and can be employed within drilling fluids or as carriers of special-purpose chemicals deep into reservoir formations.

The overall goal of our research is to determine technical viability of field applications of nanoparticle-stabilized emulsions. It is achieved by carrying out experiments and calculations with the following objectives:

• Study the stability and phase behavior of various nanoparticle-stabilized emulsions. • Investigate the dependence of emulsion phase behavior, inner structure and rheology on different experimental

conditions. • Gain fundamental insight into the emulsion interfacial properties and understand the rheology of those emulsion

systems. In the next Section, the nanoparticles and fluids used to form emulsions are described. In Section III, the phase behaviors

of o/w emulsions stabilized by hydrophilic silica nanoparticles and of w/o emulsions with hydrophobic nanoparticles are studied. Section IV describes the droplet size distributions of two types of emulsions. In Section V, the nanoparticle mass balance and the inner structure of o/w emulsion systems are investigated theoretically. In section VI, the rheological properties of two types of emulsions are provided, followed by the conclusions.

Experimental Set-up and Materials Materials. Two kinds of silica nano spheres (5-nm diameters) with different surface coatings (~ 2.5 nm thick, so the size of coated particles is about 10 nm) were received from 3M Co., St. Paul, MN. The hydrophilic nanoparticles were coated with polyethylene glycol (chains with about 7 EG units). These surface-modified silica spheres were dispersed in de-ionized water, and were provided to us as 23.04 wt% dispersion. The hydrophobic silica nanoparticles, however, were provided as white, dry powder. These particles were modified differently on the surface to be hydrophobic with a contact angle greater than 90° at the oil-water interface.

The water used to make emulsions was de-ionized water. Regent grade decane was used as the oil phase. The salt was reagent grade sodium chloride (NaCl).

Experiments were conducted at room temperature.

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Procedure to Make Emulsions. To make decane-in-water emulsions with hydrophilic nanoparticles, brine containing different nanoparticle loadings (0.05, 0.1, 0.5, 1 and 5 % by weight) with different salinities (0, 0.1%, 1%, and 10% by weight NaCl) were prepared by mixing the received dispersions with de-ionized water and NaCl. Certain volumes of decane and the water (containing nanoparticles) were placed in a vial. The mixture had a total volume of 4 ml, and was vigorously agitated for 2 minutes by a sonification gun.

Before making water-in-decane emulsions with hydrophobic nanoparticles, the nanoparticle powder was first dispersed in decane to make nanoparticle-in-decane dispersions with different nanoparticle concentrations. A sonifier was used to induce particle suspension. The nanoparticle concentration in the dispersion varied as 0.05, 0.1, 0.5, 1 and 5 by wt%. NaCl was added to de-ionized water to prepare brines with different salinities (0, 0.1%, 1% and 10%). Then the brine and the decane loaded with nanoparticles were added together into a vial. The mixture had a total volume of 4 ml, and was agitated for 2 minutes using a sonification gun.

Following sonification, the generated emulsion along with any excess phase was transferred into a 5 ml graduated glass pipette. Argon gas was used to flush out the air from the top void space before the pipette was sealed. The mixture was allowed to settle at room temperature for a few days for stability observation and phase volume fraction measurement.

There are five groups of emulsion samples in our study. Each group was prepared with a range of variables, as shown in Table 1. When preparing emulsion samples in each group, one or two variables were constant while the others were varying. All samples in one group were prepared simultaneously. Emulsion properties were compared afterwards among the samples to evaluate the effect of different operation conditions independently.

Emulsion Type Determination. Nanoparticles with different wettabilities may stabilize different types of emulsions (Binks, 2002). As shown in Fig. 1, the nanoparticles having a contact angle at the oil-water interface smaller than 90° are hydrophilic, and stabilize oil-in-water (o/w) emulsions, while the nanoparticles having a contact angle greater than 90° are hydrophobic, and yield water-in-oil (w/o) emulsions. The emulsion type produced with intermediate nanoparticles, which have a contact angle around 90°, is determined by other factors, e.g., salinity, oil type, and the initial phase volume ratio, etc.

After the emulsions were sonificated and reached equilibrium, two drops of the same kind of generated emulsion were added to a bulk water phase and a bulk decane phase, separately, to verify the emulsion type. If the emulsion drop disperses in oil and keeps intact as a drop in water, it is a w/o emulsion. Conversely, if the drop disperses in water and keeps intact in oil, it is an o/w emulsion.

Emulsion Phase Volume Measurements. The emulsion stability was determined by monitoring the post-sonification emulsion state with time. Due to the density difference between oil and water, a separation of the excess phase always occurred after sonification. Oil-in-water emulsion drops always cream to the top, while water-in-oil drops always sediment to the bottom. Either would form a bulk emulsion phase in the pipette, as shown in Fig. 2. In an o/w emulsion system, the creaming of oil drops caused an upward movement of the interface between the bulk emulsion phase and excess water phase. Whereas, in a w/o emulsion system, the sedimentation of water drops caused the downward movement of the interface between the bulk emulsion phase and the excess oil. The movement stopped once the system reached equilibrium. Measurements of the volumes of the separated phases (e.g., bulk emulsion phase, excess water or oil phase) after sonification were carried out once a day by recording the location of every interface (e.g., decane-emulsion interface, water-emulsion interface, etc.) in the graduated pipettes until no more changes were observed.

Emulsion Droplet Size Measurements. The emulsion droplet size measurement equipment consists of one Nikon Digital Sight DS-Fi1 camera attached to one Nikon Labophot-Pol microscope. The microscope apparatus is connected to a PC running Nikon NIS-Elements D3.00 Build 449 imaging software. The emulsion microscope images were analyzed for droplet size distribution via ImageJ.

Nanoparticle Concentration Measurements. The nanoparticle concentration in the excess aqueous phase was determined indirectly by measuring the refractive index of the liquid using a refractometer. With a constant salinity, the nanoparticle concentration in the nanoparticle dispersion increases almost linearly with its refractive index. Therefore, the nanoparticle concentration can be calculated as long as the refractive index was measured and their linear relationship was calibrated.

Emulsion Viscosity Measurements. The equilibrated emulsion viscosity was measured across a range of shear rates by using the TA Instruments’ Advanced Rheometric Expansion System (ARES) LS-1 rheometer.

Phase Behavior Study Table 1 summarizes the ranges of parameters for emulsions were prepared. This section reports the phase behavior observed for each group of emulsions.

Decane-in-Water Emulsions with Hydrophilic Nanoparticles. The goal of this component of research was to create a set of decane-in-water emulsions with different nanoparticle concentrations, salinities and initial water/decane volume ratios (IVR), thereby mapping their phase behavior.

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The emulsions in Group 1 of Table 1 were prepared with 2 ml of water and 2 ml of decane. The nanoparticle concentration in water varied from 0.05 wt% to 5 wt%, and the salinity varied from 0 to 10 wt%, independently. When the nanoparticle concentration in water was 0.5 wt% or greater, little decane was in excess after sonification. The emulsified oil fraction at equilibrium was calculated through volume measurement and the value is shown in Table 2. Well over half of the decane was emulsified for nearly all the samples. The fraction increased gradually with increasing nanoparticle concentration, and reached a plateau close to 1 when the nanoparticle concentration was over 0.5 wt%. The fraction also changed with salinity. With a fixed nanoparticle concentration of 0.05 wt%, the fraction decreased as the salinity increased. At larger nanoparticle concentrations, the fraction fluctuated slightly when the salinity was changed.

The dispersed phase (i.e., decane) volume fraction φ within the bulk emulsion phase was calculated and tabulated in Table 3. Researchers investigated the critical value for φ in a monodisperse emulsion system (Mason, 1999; Princen et al., 1980; Brujić et al., 2003): 0.64 is the largest value of φ for randomly packed spherical droplets, and with the most compact arrangement of spherical droplets (rhombohedra packing), φ could be as high as 0.74. If the droplets were deformed under compression, the φ fraction could be even larger in a condensed emulsion.

All values in Table 3 are over 0.5, which means that the decane droplets are densely packed, some with deformation. When the nanoparticle concentration is below 0.5 wt%, the oil volume fraction is over 0.74. The high values indicate droplet deformation as well as wide size-distribution of droplets (Fig. 3). When the nanoparticle concentration increased to 0.5wt% or larger, φ dropped to around 0.7; the emulsion was still close-packed, but the droplet deformation was not significant. It appears that, with the lower nanoparticle concentration, the adsorbed nanoparticle layer at the oil/water interface is not compact, and the droplet surface can be easily deformed. With the higher nanoparticle concentration, on the other hand, a compact nanoparticle layer is formed at the droplet surface whose deformation then becomes much more difficult. Also note that, for a fixed nanoparticle concentration (between 0.5 and 5 wt%), higher salinity yielded lower oil volume fraction in the emulsion phase. This is because more salt ions attached onto the nanoparticle surface with higher salinity, and they increased the electrostatic repulsion as well as the separation between the droplets in the condensed emulsion phase (see Table 12).

The droplets deformation in a compressed emulsion system can be caused by many reasons (Brujić et al., 2003). The most probable one for our samples is the creaming of oil droplets under gravity, which induced a condensed emulsion phase with high volume fraction of the dispersed phase.

Emulsion samples in Group 2 had a constant nanoparticle concentration of 0.05 wt% in water. Emulsion samples in Group 3 were prepared with a constant nanoparticle concentration of 5 wt% in water. The initial volume ratio (IVR) of water to oil for the emulsion samples in both groups varied from 9:1 to 1:1, and the salinity changed from 0 to 10 wt%, independently. Tables 4 and 5 give the emulsified decane fraction for the samples in Group 2 and in Group 3, respectively.

In Group 2, more than 90% of the decane was stabilized as droplets in the bulk emulsion phase with an IVR larger than 1. Otherwise, only about half of the decane was stabilized as emulsion drops. More oil was emulsified when the IVR increased. In Group 3, all the decane was emulsified. Neither the varied IVR nor the increased salinity made any difference.

Water-in-decane Emulsions with Hydrophobic Nanoparticles. The goal of this component of research was to create a set of water-in-decane emulsions with different particle concentrations, salinities, and initial decane volume fractions in the mixture, thereby mapping their phase behavior.

The emulsions in Group 4 were prepared with equal volumes of decane (with hydrophobic nanoparticle loading) and brine (or de-ionized water). The nanoparticle concentration in the decane varied from 0.05 to 5 by wt%, and the salinity of water changed as 0, 0.1, 1, and 10 wt%, independently.

The emulsified water fractions with hydrophobic nanoparticles in the samples are shown in Table 6. The phase behavior suggests a threshold nanoparticle concentration exists. When the nanoparticle concentration was 0.5 wt% or higher, no excess water existed, no matter what the salinity was. Otherwise, less than 10 % water was emulsified. With a nanoparticle concentration of 0.05 wt% or 0.1 wt%, the emulsified water fraction increased initially when the salinity increased from 0 to 1%, then dropped when the salinity additionally increased.

The dispersed phase (i.e., water) volume fraction within the w/o emulsion phase for Group 4 is tabulated in Table 7. The fraction increased when the nanoparticle concentration increased from 0.05 to 0.5 wt%, then it decreased when the particle concentration kept increasing. When the nanoparticle loading is below 0.5 wt%, most of the water fractions are less than 0.5, and those emulsions were sparsely dispersed with water drops. This phenomenon is different from the result for the o/w emulsion with low nanoparticle concentration (see Table 3), where the dispersed phase volume fraction was very high. The hydrophilic and hydrophobic nanoparticles behaved differently in stabilizing emulsions at small nanoparticle concentrations.

Within w/o emulsions prepared with at least 0.5 wt% hydrophobic nanoparticles, the water volume fraction in the emulsion phase is large, suggesting dense packing of droplets. Moreover, when the nanoparticle concentration was fixed, the water volume fraction did not change much with varying salinity. It can be explained by the fact that the salt ions stay only inside the dispersed phase in the emulsion. Thus they cannot significantly affect the electrostatic repulsion between the droplets .

Comparing the data in Table 2 and in Table 6, as well as the data in Table 3 and in Table 7 reveals different phase behaviors of o/w and of w/o emulsions. Stabilized by different nanoparticles, o/w and w/o emulsions have different dispersed phase and different continuous phase. Secondly, the salinity has different effect in the two types of emulsions. In o/w

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emulsions, salt exists in the continuous phase, which affects the electrostatic repulsion and the equilibrium distance between the droplets; while in w/o emulsions, salt exists in the dispersed phase and has no such effect.

Group 5 is composed of emulsions prepared with varied initial volume fraction of decane from 0.1 to 0.5 but a constant nanoparticle concentration of 1 wt% in decane. For all the previous emulsions samples, phase separation always occurred after sonification, and a condensed emulsion phase with the excess phase emerged when the system reached equilibrium. Neighboring phases were distinguished by clear interfaces. However, a new appearance was observed in the emulsion which was prepared with an initial decane volume fraction between 0.2 and 0.4. Those samples did not experience remarkable phase separation. At equilibrium, they showed opaque and cloudy emulsion flocculation throughout the pipette, as shown in Fig. 2 (right). The new appearance suggests something unusual happened in those samples, e.g., the coexistence of two types of emulsions (o/w and w/o) in one pipette, or the occurrence of multiple emulsions.

Emulsion Droplet Size Analysis Decane-in-Water Emulsions with Hydrophilic Nanoparticles. Duplications of emulsion samples in Group 1 were made and the microscopic images of them (shown in Fig. 3) were taken 5 days after sonification. The emulsion droplets are spherical with diameters between several microns to hundreds of microns. Overall, smaller droplets were generated with higher nanoparticle concentration; especially when the nanoparticle concentration increased from 0.1 wt% to 0.5 wt%, the droplet sizes shrink dramatically.

When the nanoparticle concentration was less than 0.5 wt% in the aqueous phase, the emulsion droplets exhibited a dual-mode diameter distribution, with one mode around ten microns and the other around one hundred microns in the presence of salt. In addition, the increased salinity widened the droplet size range by making the larger drops even larger, as shown in Fig. 4. However, mono-dispersed droplets within a diameter range of 2~10 µm were observed within emulsions with a nanoparticle concentration over 0.5 wt%. Only in the emulsions with a nanoparticle concentration of 5 wt%, it was observed that higher salinity yielded smaller emulsion droplets overall, as shown in Fig. 5.

The arithmetic mean of all the measured droplet radii in every microscope image was calculated and tabulated in Table 8. The table mostly verified the observations in the microscope images (Fig. 3) regarding the nanoparticle concentration and the salinity effects on the emulsion droplet size.

Figure 6 provides the droplet images for o/w emulsions in Group 2, where the IVR and salinity varied but the nanoparticle concentration remained constant at 0.05 wt%. Smaller emulsion droplets were generated with higher IVR when the salinity was fixed or when the IVR was fixed and the salinity was lower. The droplet diameter distributions of the emulsions with IVR of 9, and IVR of 3 with no salt and with 10 % salt are shown in Figs. 7 and 8, respectively. Larger IVR always induced smaller emulsion droplets with a narrower size range. Most of the droplets had diameters around 2 microns. Keeping the other parameters fixed, adding salt made emulsion droplets larger as well as their size range broader. A dual-mode distribution of droplet sizes was exhibited clearly in the emulsion with 10% salt. The two modes were closer to each other in the presence of more nanoparticles.

The emulsion prepared with 0.05 wt% exhibited a distinct increase in droplet size as well as a shape transformation one month after sonification. The spherical emulsion drops became oval-like shapes, as shown in Fig. 9. However, this change was not observed in emulsions with a nanoparticle concentration higher than 0.5 wt%. The non-spherical droplets proved to be less stable than the spherical droplets. When a small force was applied to the emulsion, the non-spherical droplets broke quickly, while the others just slid or bounced around.

Water-in-decane Emulsions with Hydrophobic Nanoparticles. Duplicates of w/o emulsion samples in Group 4 were made for droplet size analysis. Figure 10 provides their microscopic images with different nanoparticle concentrations and different salinities. Overall, those water-in-decane emulsion droplets were smaller than the decane-in-water ones. Within w/o emulsions, the water was dispersed as the inner phase, while decane was the continuous phase. When the nanoparticle concentration increased from 0.1 wt% to 5 wt%, the emulsion droplet diameters decreased gradually from tens of microns to a couple of microns. In addition, the emulsion droplet size distribution attained a narrower range with more nanoparticles when the salinity was fixed; or with higher salinity when the nanoparticle concentration was fixed, as shown in Figs. 11 and 12.

Some differences in size distribution compared to o/w emulsions were observed in w/o emulsions with a low nanoparticle concentration (~0.1 wt %). First, the bimodal size distribution was not evident, as shown in Fig. 11, compared to Fig. 4. Secondly, the droplet size distribution scale changed oppositely when the salinity increased..

When prepared with a nanoparticle concentration of 5 wt%, similar to o/w emulsions, the w/o emulsion droplets had a normal size distribution, and higher salinity induced smaller emulsion droplets overall. The arithmetic mean of the counted droplet radii in each of the images in Fig. 10 is shown in Table 9.

Inner Structure of Decane-in-Water Emulsion with Hydrophilic Nanoparticles Emulsion Droplet Surface Coverage. For the o/w emulsions in Group 1, using the measured emulsion droplet radii r in the microscope images, the arithmetic means of r2 and r3 were calculated. Assuming that the droplet size distribution obtained from microscope analysis applies to the whole emulsion phase, and using the measured decane volume Vd in the bulk emulsion, the total surface area of all the emulsion droplets can be estimated by . Then the ratio of

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emulsion total surface area to its total volume Ve, which came from volume measurement, was calculated and is shown in Table 10. The plot of the ratio versus nanoparticle concentration is shown in Fig. 13.

The surface area to volume ratio increased steadily with the nanoparticle concentration, which means more nanoparticles stabilized more oil-water interface per unit volume of emulsion. With 5 wt% nanoparticles, increasing salinity increased the area/volume ratio for the bulk emulsion. When the nanoparticle concentration was lower than 1 wt%, adding salt depressed the ratio to some extent.

When preparing emulsion samples, the initial amount of nanoparticles in the mixture was known. For the Group 1 samples, the nanoparticle amount left in the excess water after sonification was determined by measuring the refractive index of the dispersion. The difference between initial and final masses of nanoparticles in the aqueous phase is the quantity of nanoparticles attached to the droplet surfaces in the bulk emulsion. Due to the limited precision of the refractometer reading, final concentrations could be obtained reliably only in dispersions with large nanoparticle loading (at least 0.5 wt %).

Knowing the size of each nanoparticle, the total surface area occupied by nanoparticles at droplet surfaces Sp was calculated from the nanoparticle mass. With the total emulsion droplet surface area calculated before, we calculated the nominal fraction of droplet surface covered by nanoparticles (Sp/Se) and tabulated it in Table 11.

The droplet surfaces were more densely covered for emulsions with higher salinity. This effect occurred because electrolyte ions in the water weakened the electrostatic repulsion between the nanoparticles, and therefore brought the particles closer to each other on the droplet surfaces. The ratio Sp/Se approached 0.85 for all the emulsions with high salinity (over 1%), which is close to the packing coefficient for a 2-dimensional hexagonal close-packed pattern (~0.91).

In addition, in emulsions with a smaller nanoparticle concentration, the droplet surface coverage increased with salinity faster than in emulsions with higher nanoparticle concentration. This is because besides staying in the water phase and neutralizing the repulsion between the nanoparticles at the oil-water interface, the electrolyte ions might also attach to the nanoparticle surface first to enhance the electrolyte repulsion between them. In emulsions with a higher nanoparticle concentration, more ions were consumed on the nanoparticle surfaces, leaving fewer ions in the aqueous phase and larger repulsive force between the nanoparticles at the oil-water interface. Therefore, more salt was needed to counteract the nanoparticle repulsion and induce a close-packed pattern for all the droplets in those emulsions.

Modeling of Emulsion Phase Separation with Hard-Sphere Liquid Theory. The investigation of phase separation for surfactant-stabilized microemulsions was carried out based on the hard-sphere liquid theory, which considers the electrostatic and steric repulsions and the van der Waals attraction between colloids (Huh, 1983). Employing a similar approach, the nanoparticle-stabilized emulsion phase separation was also investigated and analyzed.

Following the same calculation process employed by Huh (1983), substituting the parameter values from our experiments, the minimum gap between neighboring emulsion droplets in the emulsion phase was found by solving the equilibrium condition equations for phase separation. The same van der Waals attraction was assumed between the emulsion droplets and a Hamaker constant of 2.49×10-12 erg was chosen for computation. Table 12 presents the predicted average distance between droplets at equilibrium in the bulk emulsion phase for each sample in Group 1.

The distances between the emulsion droplets at equilibrium were primarily determined by the balance between van der Waals attraction and electrostatic double layer repulsion. With the same attraction, a smaller distance between the droplets suggests weaker electrical double layer repulsion. Therefore, the values in Table 12 indicate that higher salinity generated smaller repulsive force between droplets in the emulsion with a nanoparticle concentration smaller than 0.5 wt%. In contrast, for nanoparticle concentrations of 0.5 wt % or more, increasing salinity increased the separation between droplets.

The contrary results at small and large nanoparticle concentrations suggest that the salt concentration in the mixture affected the inner structure of the emulsion phase in varied ways. The electrolyte ions might attach onto the nanoparticles at the emulsion droplet surfaces and enhance the repulsion between the drops. The electrolyte ions in the water films between the oil drops, on the other hand, could weaken the repulsion between the oil drops. Evidently the effect of salinity on the inner structure of emulsions is coupled to the nanoparticle concentration. An expansion of this preliminary model of the emulsion inner structure to account for molecular interactions is indicated.

Rheological Property of Emulsions Decane-in-Water Emulsions with Hydrophilic Nanoparticles. The apparent viscosities of oil-in-water emulsions in Group 1 were measured for a shear rate range of 0.01~100 s−1. Figure 14 presents the plot of bulk emulsion apparent viscosity versus shear rate for varied nanoparticle concentrations without salt. The emulsions were shear-thinning across the entire shear rate range. At a smaller nanoparticle concentration, the emulsion viscosity was larger at very low shear rate (~0.01 s−1), dropped faster as the shear rate increased, and finally reached a smaller value at very high shear rate (~100 s−1). The salinity effect on the o/w bulk emulsion apparent viscosity is shown by Fig. 15, with a constant nanoparticle concentration of 5 wt%.

The emulsion apparent viscosity might correspond to the inner structure of the emulsion, e.g., the emulsion droplet sizes, the droplet surface coverage with particles, and the average distance between the drops. Some consistencies between their dependences on the nanoparticle concentration and on the salinity were found. For example, with 5 wt% nanoparticles, the emulsion prepared with 0.1% salinity and the emulsion without salt shared similar inner structures and exhibited almost the same rheological property. Two other o/w emulsion samples, which had 1% salinity and 10% salinity individually, have

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similar inner structures and resemble each other in rheology. Nevertheless, detailed modeling of the emulsion interfacial property is necessary to explain its relation to the emulsion inner structure and for better understanding of the emulsion system, which is not included in this paper.

Water-in-decane Emulsions with Hydrophobic Nanoparticles. The surface plot of w/o bulk emulsion (Group 4) apparent viscosity versus shear rate and nanoparticle concentration in decane is shown in Fig. 16. The emulsions were shear-thinning in a shear rate range of 0.01~100 s−1. Their viscosity did not change significantly when the nanoparticle concentration increased from 0.5 wt% to 5 wt%, which is different from the o/w emulsions. At a shear rate of about 0.01 s−1, the emulsions exhibited very high viscosity, up to 107 cp, which is even higher than that of o/w emulsions. The dependence of w/o emulsion viscosity on salinity is shown in Fig. 17 for a nanoparticle concentration in decane of 5 wt%. The emulsion viscosity decreased with a factor between 10 and 1000 as the salinity increased, and the factor became smaller as the shear rate increased. The dependence of w/o emulsion apparent viscosity on salinity is opposite to that in o/w emulsions, which indicates that different mechanisms occur in o/w emulsions and in w/o emulsions.

Conclusions Very stable emulsions stabilized by 5-nm-diameter silica nanoparticles with different surface coatings were prepared and shown to remain stable for several months. The hydrophilic nanoparticles yield oil-in-water emulsions, while hydrophobic nanoparticles produce water-in-oil emulsions.

The dependence of emulsion properties, such as its phase behavior, emulsion internal structure and rheology on the nanoparticle concentration, salinity, and the initial volume ratio has been studied and analyzed at ambient conditions. The experiments revealed that very stable emulsions could be made with silica nanoparticles when the particle concentration was 0.5 wt% or higher. For stable emulsions, a higher volume fraction of oil than water within the bulk emulsion phase was produced for the o/w emulsion with hydrophilic nanoparticles, while a lower volume fraction of oil to water within the bulk emulsion phase was produced for the w/o emulsion with hydrophobic nanoparticles. For both kinds of emulsions, with increasing nanoparticle concentration, more of the dispersed phase was emulsified, the dispersed phase volume fraction in the emulsion increased, and the average droplet diameter decreased. The o/w emulsions displayed an increasing apparent viscosity with increasing salinity, while the w/o emulsion viscosity decreased with increasing salinity. The emulsion rheology is strongly shear-thinning, and emulsions had very high apparent viscosities at very low shear rates. The rheological characteristics have potential to facilitate the conformance control during oil recovery.

Acknowledgement Jimmie Baran of 3M kindly provided the nanoparticle samples. This work was supported in part by the Advanced Energy Consortium, by the Miscible Gas Flooding Joint Industry Project at The University of Texas at Austin, and by the Donors of the American Chemical Society Petroleum Research Fund.

References Binks, B. P., “Particles as Surfactants—Similarities and Differences”, Current Opinion in Colloids Interface Sci., 7, 21-41 (2002). Binks, B. P., and Horozov, T. S., “Colloidal Particles at Liquid Interfaces: An Introduction”, in Colloidal Particles at Liquid Interfaces,

Chapt. 1, Binks, B. P., and Horozov, T. S., eds., 1-74, Cambridge Univ. Press (2006). Binks, B. P., and Lumdson, S. O., “Effect of Oil Type and Aqueous Phase Composition On Oil-Water Mixtures Containing Particles of

Intermediate Hydrophobicity”, Phys. Chem. Chem. Phys., 2, 2959-2967 (2000a). Binks, B. P., and Lumsdon, S. O., “Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions”, Langmuir,

16, 8622-8631 (2000b). Binks, B. P., Philip, J., and Rodrigues, J. A., “Inversion of Silica-Stabilized Emulsions Induced by Particle Concentration”, Langmuir, 21,

3296-3302 (2005) Binks, B. P., and Rodrigues, J. A., “Inversion of Emulsions Stabilized Solely by Ionizable Nanoparticles”, Angew. Chem., 117, 445-448

(2005). Bresme, F., and Oettel, M., “Nanoparticles at Fluid Interfaces”, J. of Physics-Condensed Matter, 19 (41), (2007). Brujić, J., Edwards, S. F., Hopkinson, I., and Makse, H. A., “Measuring the Distribution of Interdroplet Forces in a Compressed Emulsion

System”, Physica A, 327, 201-212 (2003). Fernandez-Toledano, J. C., Moncho-Jorda, A., Martinez-Lopez, F., and Hidalgo-Alvarez, R., “Theory for Interactions between Particles in

Monolayers”, in Colloidal Particles at Liquid Interfaces, Binks, B. P., and Horozov, T. S., eds., 303-310, Cambridge Univ. Press (2006).

Friberg, S. E., “Emulsion Stabilization by Solid Particles—A Two-Layer Approach: Spherical Particles”, J. Dispersion Sci. Tech., 26, 647-654 (2005).

Horozov, T. S., Binks, B. P., and Gottschalk-Gaudig, T., “Effect of Electrolyte in Silicone Oil-in-Water Emulsions Stabilized by Fumed Silica Particles”, Phys. Chem. Chem. Phys., 9, 6389-6404 (2007).

Huh, C., “Equilibrium of a Microemulsion that Coexists with Oil or Brine”, Soc. Petrol. Eng. J., 829-847, Oct. 1983. Hunter, T. N., Pugh, R. J., Franks, G. V., and Jamenson, G. J., “The Role of Particles in Stabilizing Foams and Emulsions”, Adv. Colloid

Interface Sci., 137, 57-81 (2008). Kralchevsky, P. A., Ivanov, I. B., Ananthapadmanabhan, K. P., and Lips, A., “On the Thermodynamics of Particle-Stabilized Emulsions:

Curvature Effects and Catastrophic Phase Inversion”, Langmuir, 21, 50-63 (2005).

8 SPE 129885

Levine, S., and Bowen, B. D., “Capillary Interaction of Spherical Particles Adsorbed on the Surface of an Oil/Water Droplet Stabilized by the Particles. Part I”, Colloids & Surf., 59, 377-386 (1991).

Levine, S., and Bowen, B. D., “Capillary Interaction of Spherical Particles Adsorbed on the Surface of an Oil/Water Droplet Stabilized by the Particles. Part II”, Colloids & Surf., 65, 273-286 (1992).

Levine, S., and Bowen, B. D., “Capillary Interaction of Spherical Particles Adsorbed on the Surface of an Oil/Water Droplet Stabilized by the Particles. Part III”, Colloids & Surf. A: Physicochem. Eng. Aspects, 70, 33-45 (1993).

Lopetinsky, R. J.. G., Masliyah, J. H., and Xu, Z., “Solids-Stabilized Emulsions: A Review”, in Colloidal Particles at Liquid Interfaces, Chapt. 6, Binks, B. P., and Horozov, T. S., eds., 186-224, Cambridge Univ. Press (2006).

Mason, T. G., “New Fundamental Concepts in Emulsion Rheology”, Current Opinion in Colloids Interface Sci., 4, 231-238 (1999) Melle, S., Lask, M., and Fuller, G. G., “Pickering Emulsions with Controllable Stability”, Langmuir, 21, 2158-2162 (2005). Paunov, V. N., Binks, B. P., and Ashby, N. P., “Adsorption of Charged Colloid Particles to Charged Liquid Surfaces”, Langmuir, 18,

6946-6955 (2002). Princen, H. M., Aronson, M. P., and Moser, J. C., “Highly Concentrated Emulsions. II. Real Systems. The Effect of Film Thickness and

Contact Angle on the Volume Fraction in Creaming Emulsions”, J. Colloid & Interface Sci., 1, 246-270 (May, 1980) Reincke, F., Kegel, W. K., Zhang, H., Nolte, M., Wang, D., Vanmaekelbergh, D., and Mohwald, H., “Understanding the self-assembly of

charged nanoparticles at the water/oil interface”, Phys. Chem. Chem. Phys., 8, 3828-3835 (2006). Thompson, J., Vasquez, A., Hill, J. M., and Pereira-Almao, P., “The Synthesis and Evaluation of Up-scalable Molybdenum based Ultra

Dispersed Catalysts: Effect of Temperature on Particle Size”, Catal. Lett., 123, 16-23 (2008).

Table 1— Overview of Emulsion Groups with Different Variables Variables

Group Nanoparticle type Initial Volume Ratio (IVR) of water to oil Nanoparticle concentration in dispersion (wt %) Salinity (wt % NaCl)1 Hydrophilic 1 0.05, 0.1, 0.5, 1, 5 0, 0.1, 1, 10 2 Hydrophilic 9, 7, 5, 3, 1 0.05 0, 0.1, 1, 10 3 Hydrophilic 9, 7, 5, 3, 1 5 0, 0.1, 1, 10 4 Hydrophobic 1 0.05, 0.1, 0.5, 1, 5 0, 0.1, 1, 10 5 Hydrophobic 9, 8/2, 7/3, 6/4, 1 1 0, 0.1, 1, 10

Table 2—Fraction of oil emulsified by hydrophilic nanoparticles (Group 1) Nanoparticle concentration wt%

Emulsified decane fraction 0.05 0.1 0.5 1 5 0 0.76 0.95 0.95 0.97 0.98

0.1 0.67 0.89 0.99 0.96 0.99 1 0.62 0.94 0.96 0.96 1.00

Salinity (%) 10 0.47 0.90 0.99 0.99 1.00

Table 3—Decane volume fraction within the o/w emulsion phase (Group 1) Nanoparticle concentration wt% Decane volume fraction

within emulsion phase 0.05 0.1 0.5 1 5 0 0.80 0.76 0.72 0.72 0.71

0.1 0.75 0.75 0.72 0.70 0.70 1 0.80 0.76 0.67 0.70 0.60

Sainity (%) 10 0.80 0.78 0.68 0.68 0.55

Table 4—Emulsified decane fraction by 0.05 wt% hydrophilic nanoparticles (Group 2) Salinity %

Emulsified decane fraction 0 0.1 1 10 9 0.97 0.97 0.95 0.97 7 1.00 0.98 0.96 0.98 5 0.95 0.98 0.91 0.98 3 0.91 0.87 0.93 NA*

Initial volume ratio of water to oil

1 0.73 0.56 0.52 NA*

* Emulsions broke (were not stable).

SPE 129885 9

Table 5—Emulsified decane fraction by 5 wt% hydrophilic nanoparticles (Group 3) Salinity %

Emulsified decane fraction 0 0.1 1 10 9 1.00 1.00 1.00 1.00 7 1.00 1.00 1.00 1.00 5 1.00 1.00 1.00 1.00 3 0.99 1.00 1.00 1.00

Initial volume ratio of water to oil

1 0.98 0.99 0.99 0.98

Table 6—Fraction of water emulsified by hydrophobic nanoparticles (Group 4)

Nanoparticle concentration wt% Emulsified water fraction 0.05 0.1 0.5 1 5

0 0.02 0.03 1.00 1.00 1.00 0.1 0.03 0.09 1.00 1.00 1.00 1 0.04 0.17 1.00 1.00 1.00

Salinity (%) 10 0.01 0.10 1.00 1.00 1.00

Table 7—Water volume fraction within w/o emulsions (Group 4) Nanoparticle concentration wt%

Water volume fraction 0.05 0.1 0.5 1 5 0 0.19 0.22 0.69 0.6 0.56

0.1 0.32 0.44 0.64 0.61 0.54 1 0.33 0.64 0.71 0.62 NA*

Salinity (%) 10 0.13 0.5 0.71 0.58 NA*

* No data were available because the emulsion was too viscous to be transferred into a pipette for volume measurement.

Table 8—Arithmetic mean of the measured o/w emulsion droplet radii (Group 1) Salinity /%

Average droplet radius (µm) 0 0.1 1 10 0.05 13.1 16 4.02 4.94 0.1 4.92 7.5 28.2 33.1 0.5 3.75 6.54 6.77 7.02 1 3.92 3.92 2.87 4.6

Nanoparticle concentration (wt %)

5 1.79 1.89 1.29 1.05

Table 9—Arithmetic mean of the measured w/o emulsion droplet radii (Group 4) Salinity /%

Average droplet radius (µm) 0 0.1 1 10 0.1 1.89 3.25 3.58 2.37 0.5 3.25 3.06 1.79 2.84 1 2.83 2.57 1.96 2.98

Nanoparticle concentration (wt %)

5 1.75 0.59 1.14 0.5 Table 10—Ratio of emulsion total surface area to its total volume (Group 1)

Salinity /% Se/Ve (1/µm) 0 0.1 1 10

0.05 0.08 0.03 0.04 0.03 0.1 0.14 0.03 0.04 0.03 0.5 0.23 0.13 0.13 0.12 1 0.31 0.22 0.32 0.23

Nanoparticle concentration (wt %)

5 0.48 0.53 0.64 0.67

10 SPE 129885

Table 11—Ratio of area occupied by nanoparticles attached to the emulsion droplet surfaces to surface area of all the drops* (Group 1)

Salinity /% Sp/Se 0 0.1 1 10

0.5 0 0.81 0.79 0.75 1 0.17 0.68 0.84 0.81

Initial nanoparticle concentration (wt %)

5 0.32 0.38 0.86 0.85 * Values for emulsions with nanoparticle concentration below 0.5 wt% are inaccessible due to measurement limitation of the refractometer. Table 12—The average minimum distance between neighboring o/w emulsion drops (Group 1)

Nanoparticle concentration wt% Minimum distance (µm) 0.05 0.1 0.5 1 5

0 2.E-04 2.E-03 1.E-02 1.E-02 9.E-03 0.1 2.E-04 1.E-02 2.E-02 2.E-02 1.E-02 1 1.E-04 8.E-03 7.E-02 2.E-02 5.E-02

Salinity (%) 10 8.E-05 3.E-03 6.E-02 4.E-02 5.E-02

Fig. 1—Contact angle on particle surface and its relation with emulsion structure.

Fig. 2—Pictures of emulsions made with 1 wt% nanoparticles and no salt. Left: o/w emulsion with hydrophilic nanoparticles (from Group 1 of Table 1); middle: w/o emulsion with hydrophobic nanoparticle (from Group 4); right: emulsion with hydrophobic nanoparticle and initial decane volume fraction of 0.4 (from Group 5). The white part is bulk emulsion, and the clear part is excess water (bottom) or excess decane (top).

SPE 129885 11

Fig. 3—Droplet images for decane-in-water emulsions (Group 1) made with varied nanoparticle concentration (NPC) (each row from top to bottom: 0.05, 0.1, 0.5, 1, and 5 by wt %) and salinity (each column from left to right: 0, 0.1, 1, and 10 by wt %). Initial volume ratio: water/decane = 1.

12 SPE 129885

0

5

10

15

20

0 10 20 30 40 50 60 70 80 90Diameter (µm)

Cou

nt

0123456

0 10 20 30 40 50 60 70 80 90Diameter (µm)

Cou

nt

Fig. 4—O/W emulsion droplet diameter distribution of two samples (from Group 1 of Table 1) with 0.1 wt% nanoparticles. The salinity is 0 for the upper plot and 10% for the lower.

0100200300400500600700

0 1 2 3 4 5 6 7 8 9Diameter (µm)

Cou

nt

0

100

200

300

400

0 1 2 3 4 5 6 7 8 9Diameter (µm)

Cou

nt

Fig. 5—O/W emulsion droplet diameter distribution of two samples (from Group 1) with 5 wt% nanoparticles. The salinity is 0 for the top and 10% for the bottom.

SPE 129885 13

Fig. 6—Droplet images for decane-in-water emulsions (Group 2) made with varied salinity (each row from top to bottom: 0, 0.1, 1, 10) and initial volume ratio of water to decane (each column from left to right: 9, 7, 5, 3, 1). Nanoparticle concentration = 0.05 wt%.

0

100

200

300

400

0 2.6 5.2 7.8 10.4 13 15.6 18.2 20.8 23.4Diameter (µm)

Cou

nt

0

50

100

150

200

250

0 2.6 5.2 7.8 10.4 13 15.6 18.2 20.8 23.4Diameter (µm)

Cou

nt

Fig. 7—O/W emulsion (from Group 2) droplet diameter distribution with nanoparticle concentration of 0.05wt% and IVR of 9. The salinity is 0 for the top and 10% for the bottom.

14 SPE 129885

0

200

400

600

800

0 4.5 9 13.5 18 22.5 27 31.5 36 40.5Diameter (µm)

Cou

nt

020406080

100120

0 4.5 9 13.5 18 22.5 27 31.5 36 40.5Diameter (µm)

Cou

nt

Fig. 8—O/W emulsion (from Group 2) droplet diameter distribution with nanoparticle concentration of 0.05wt% and IVR of 3. The salinity is 0 for the top and 10% for the bottom.

Fig. 9—Image of o/w emulsion (from Group 1) droplets with 0.05 wt% nanoparticles and 10% salinity one month after sonification, showing droplet shape transformation. The scale bar is 200 μm.

SPE 129885 15

Fig. 10—Water-in-decane emulsion (Group 4) droplet images with varying nanoparticle concentration (NPC) in decane (0.1, 0.5, 1, and 5 by wt% for each row from top to bottom) and salinity (0, 0.1%, 1% and 10% for each column from left to right). Initial volume ratio: water/decane = 1.

0100200300400500600

0 3.4 6.8 10.2 13.6 17 20.4 23.8 27.2 30.6Diameter (µm)

Cou

nt

020406080

100120

0 3.4 6.8 10.2 13.6 17 20.4 23.8 27.2 30.6Diameter (µm)

Cou

nt

Fig. 11—W/O emulsion droplet diameter distribution of two samples (from Group 4) with 0.1 wt% nanoparticles. The salinity is 0 for the top and 10% for the bottom.

16 SPE 129885

050

100150200250300350

0 1.2 2.4 3.6 4.8 6 7.2 8.4 9.6 10.8Diameter (µm)

Cou

nt

0

100

200

300

400

0 1.2 2.4 3.6 4.8 6 7.2 8.4 9.6 10.8Diameter (µm)

Cou

nt

Fig. 12—W/O emulsion droplet diameter distribution of two samples (from Group 4) with 5 wt% nanoparticles. The salinity is 0 for the top and 10% for the bottom.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.01 0.1 1 10

Nanoparticle concentration (wt%)

Se/V

e (1

/mic

ron)

0% S

0.1% S

1% S

10% S

Fig. 13—Total droplet surface area over total emulsion volume vs. nanoparticle concentration and salinity for samples in Group 1.

SPE 129885 17

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

0.01 0.1 1 10 100 1000Shear Rate (s-1)

Visc

osity

(cP

)

0.05;00.1;00.5;01;05;0

Fig. 14—Plot of o/w emulsion (a subset from Group 1) viscosity vs. shear rate with varied nanoparticle concentration (0.05, 0.1, 0.5, 1 and 5 by wt %) and no salt. Legend key is (nanoparticle concentration); (salinity).

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

0.01 0.1 1 10 100 1000Shear Rate (s-1)

Visc

osity

(cP

)

5;05;0.15;15;10

Fig. 15—Plot of o/w emulsion (a subset from Group 1) viscosity vs. shear rate with different salinities (0, 0.1%, 1% and 10%) but the same nanoparticle concentration (5 wt%). Legend key is (nanoparticle concentration); (salinity).

18 SPE 129885

10.5

510

-2

100

102

102

104

106

108

NPC in oil phase /wt%Shear rate /s-1

Bul

k em

ulsi

on v

isco

sity

/cP

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

x 106

Fig. 16—Surface plot of w/o emulsion (from Group 4) viscosity vs. shear rate and nanoparticle concentration (NPC) in decane, no salt.

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

0.01 0.1 1 10 100 1000

Shear Rate (s-1)

Visc

osity

(cP)

5;05;0.15;15;10

Fig. 17—W/O emulsion viscosity (from Group 4) vs. shear rate with different salinities (0, 0.1%, 1% and 10%). Nanoparticle concentration in decane is 5 wt%. Legend key is (nanoparticle concentration);(salinity).