Nanoparticle-Stabilized Emulsions for Applications in Enhanced Oil Recovery

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