Surfactant-stabilized CO2 foams have been used for mobility control for CO2 flooding; however, stabilization of foams with nanoparticles has some important advantages for EOR applications. Bringing the nanoparticles to the bubble interface requires a large adsorption energy, making the resulting foam very stable. Nanoparticles being solid, the nanoparticle-stabilized foams have potential to withstand the high-temperature reservoir conditions for extended periods. With their very small size, nanoparticles (and foam bubbles stabilized by them) can be transported without straining in pore throats in the reservoir rock.

Very stable supercritical CO2-in-water foams were generated with 5-nm silica nanoparticles whose surface was treated with short-chain polyethylene-glycol. The foams were generated by co-injecting CO2 and an aqueous dispersion of the nanoparticles through a glass-beads pack, at mixture flow rates that correspond to shear rates of ~1300 s-1. The domain of foam stability and the normalized mixture viscosity have been measured for a range of values of nanoparticle concentration, water salinity, ratio of CO2/water flow rates, the overall flow rate and temperature. With deionized water, stable foams formed at nanoparticle concentrations as low as 0.05 wt%. Larger particle concentrations were required to maintain foam stability at larger salinities, e.g., 0.5 wt% particle concentration for 4% NaCl brine. Foam stability was independent of CO2/water volume ratio for ratios between two and eleven, but the normalized mixture viscosity increased with the increase in ratio. When foam was generated, it had two to eighteen times more resistance to flow than the same fluids without nanoparticles. Foams were generated at temperatures up to 95 °C. Foam generation by co-injection of the fluids appears to require a threshold shear rate.

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