Foams and Emulsions Stabilized With Nanoparticles for Potential Conformance Control Applications

Abstract

Abstract While emulsions stabilized by colloidal solid particles have been widely used for industrial and consumer applications, their use for enhanced oil recovery purposes has been very limited. This is because the colloidal solids generally cannot be transported long distances within oil reservoirs, e.g. from injection well to production well. Nanoparticles are two orders of magnitude smaller than colloids and thus can migrate through the pore throats in sedimentary rocks. Emulsions stabilized with nanoparticles can withstand the high-temperature reservoir conditions for extended periods. This can substantially expand the range of reservoirs to which EOR can be applied. Finally, nanoparticles can carry additional functionalities such as super-paramagnetism and reaction catalysis. The former could enable transport to be controlled by application of magnetic field. The latter could enable in situ reduction of oil viscosity. We have employed aqueous suspensions of surface-modified silica nanoparticles (5- & 20-nm diam.) in a set of laboratory experiments. We report the phase behavior of nanoparticle-stabilized oil/water emulsions and the transport of these emulsions in porous media. Very stable oil/water emulsions were generated, with average droplet size between 2 and 4 microns, at ambient and at elevated temperature. The emulsion stability was not strongly dependent on nanoparticle concentration or on salinity. The transport in glass-bead packs (ca. 20 Darcy) of the silica-stabilized oil/water emulsions showed a sharp emulsion-bank front, with no visible loss of their integrity, and high apparent viscosity (30 cp). Permeability to the aqueous phase post-flush was significantly reduced. It was not possible to determine whether emulsion droplets were retained in pores during emulsion injection, or whether the post-flush fingered through the emulsion and failed to displace all of it. An on-going modeling effort to characterize the equilibrium and stability of the emulsions suggests formation of relatively compact interfacial layer of nanoparticles at the droplet surface. Introduction Oil/water emulsions are frequently used as special-purpose drilling fluids (Ali et al., 2004), as mobility control agents to improve displacement efficiency of viscous oil (Schmidt et al., 1984; Khambharatana et al., 1998), and for other applications in the upstream oil industry (Schramm, 1992). Emulsions in practice are generally stabilized with surfactants, but emulsions can also be formed using colloidal solids as stabilizers. While such "Pickering emulsions" are widely used in mining, food and cosmetics industry, they are rarely used for oil recovery purposes. This is because the colloidal solids are easily trapped in reservoir rock by straining and deep-bed filtration, and the long-distance propagation of such emulsions in reservoir is therefore generally not feasible. The advantages and disadvantages of Pickering emulsions are clearly illustrated in the work of Bragg (1999) and Bragg and Varadaraj (2003). They carried out a comprehensive laboratory study to develop solid-stabilized oil-in-water and water-in-oil emulsions that can be effectively employed to displace viscous oil from high-permeability reservoir rocks. They also showed that oil-in-water emulsions can enable the transport of a viscous oil through pipelines at smaller pressure drops. Two main advantages of using such emulsions are: (i) very low-cost solids such as clays can be used as the stabilizer, and the reservoir oil can serve as either the dispersed phase or the external phase; and (ii) by adjusting the solids concentration and/or the water pH, a high apparent viscosity for the displacing fluid can be attained for effective mobility control. While potentially very low cost, the process has some weaknesses. First, because of the straining/filtration tendency of the solids, the process is applicable to only very high-permeability reservoirs. Second, the solids employed, e.g., clay, have a wide size and shape variations, so that it is difficult to control the quality and texture of the emulsions (see below).