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Abstract
Viscous C/W foams were generated without the use of polymers or surfactants by shearing CO2 and an aqueous phase containing partially hydrophobic silica nanoparticles in a beadpack filled with 180µm glass beads. Silica particles with 50% SiOH coverage were chosen because they have a hydrophilicity that falls between the 42% SiOH optimum foaming ability for A/W foams (Binks and Horozov 2005) and the 67% SiOH which gave maximum O/W emulsion stability (Binks and Lumsdon 2000). These 50% SiOH silica nanoparticles were found to be interfacially active for CO2-water systems, and stabilized the desired curvature of C/W foams. When the HCB of the nanoparticles is tuned to give contact angles less than 90°, the particles reside primarily in the water phase and C/W foams can be formed. Formation of C/W emulsions stabilized solely with nanoparticles is desirable because it does not require solvation of surfactant tails or polymer chains by CO2. Interfacially active nanoparticles can adsorb at the CO2 water interface without the need for solvation in CO2.

Properly designed nanoparticles generated foams that were more stable than foams generated with polymer-coated nanoparticles or with the nonionic surfactant Tergitol™ 15-S-20 alone. Macroscopic observations showed foams generated solely with 50% SiOH nanoparticles stayed bright white and opaque over 23 hours, while foams generated with PEG-coated silica particles or with surfactant alone resolved nearly completely. Foams generated solely with Tergitol™ 15-S-20 were unstable because surfactant molecules dynamically enter and leave the interface and thus do not provide long-term stabilization. Foams generated with PEG-coated silica particles, though initially very viscous, showed poor long-term stability because of the small particle size and poor solvation of PEG chains in CO2. The larger 50% SiOH nanoparticles strongly adsorbed at the CO2-water interface and provided a barrier around the CO2 bubbles, resulting in very stable foams.

Introduction
Although enhanced oil recovery (EOR) with CO2 is practiced domestically on large scale, the potential for advancement is enormous upon improvement of the volumetric sweep efficiency of the process. CO2 has a low density and low viscosity, which lead to gravity segregation and preferential flow through high permeability regions of a reservoir. The formation of CO2 in water (C/W) foams increases the effective viscosity of injected CO2 and thereby may provide mobility control. For simplicity, the term “foam” is used most often in this study to refer to high internal phase emulsions of carbon dioxide in water. A key challenge in employing CO2 foams is to maintain long-term stability of foam to achieve high sweep efficiency for the duration of the flooding process. Typically foams are stabilized by surfactants, relying on continuous regeneration of lamellae in the small pores of the rock to maintain the mobility control (Rossen 1996). In some cases, a significant fraction of the surfactant can become unavailable for foam stabilization due to adsorption on rock surfaces and to degradation, especially at high temperatures.