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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 through typical pores and thus can 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 this research, 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 used as 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.

                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. The rest of the procedure was analogous to the decane-in-water experiments.

Experiments were conducted at room temperature.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 nanoparticle concentration in the excess aqueous phase was determined indirectly by measuring the refractive index of the liquid using a refractometer. 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.

 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. 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. Mass-balance considerations for the o/w emulsions showed that the nanoparticles formed an almost complete monolayer on the surface of the oil droplets at brine salinities of 1 wt% NaCl and larger. This imparts extreme stability to the emulsions; samples have lasted for months at room temperature. The interfacial concentration of particles was independent of the concentration of particles in the aqueous dispersion.  At small salinities, the interfacial concentration of particles was as little as 17% of a monolayer.

The o/w emulsions displayed an increasing apparent viscosity with increasing salinity, while the w/o emulsion viscosity decreased with increasing salinity. For both types of emulsion the rheology is strongly shear-thinning. Emulsions had very high apparent viscosities (ten thousand to one million centipoise) at low shear rates (0.01 1/s) but moderate viscosities (ten to one hundred centiposise) at large shear rates (100 1/s).

The rheological characteristics of these emulsions have potential to facilitate the conformance control during oil recovery. For example, a common problem is that water injected to displace oil toward production wells flows preferentially through high-permeability, already-swept portions of the reservoir. A nanoparticle-stabilized emulsion prepared at the surface could pumped into the reservoir easily, as it will have low viscosity at the high shear rate within the wellbore. The emulsion would enter the same high-permeability, already swept flow path taken by previously injected water. When flow is stopped after the emulsion is injected, the viscosity of the emulsion increases dramatically because the shear rate is zero. Thus when water injection resumes, the water will no longer be able to enter the already-swept region. In this way, the water is forced to displace oil from previously unswept regions.