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Abstract

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.

Introduction

Despite its wide use for enhanced oil recovery wherever CO2 is available at low cost, the critical weakness for CO2 flooding is the poor volumetric sweep efficiency. This is due to the preferential channeling of CO2 through high-permeability layers because of its very low viscosity, and to its gravity segregation because of its low density. Extensive research has been carried out on the use of surfactant-stabilized CO2 foams to remedy the above problems by lowering the CO2 mobility (e.g., Kim et al., 2005). Successful field tests on the foam application have also been reported (see for review, Rossen, 1996). Surfactant-stabilized CO2 foams, however, have some weaknesses. Because foam is by nature ultimately unstable, its long-term stability during field application is difficult to maintain. This is especially true when the foam contacts the resident oil (Hanssen and Dalland, 2000). Under high-temperature reservoir conditions, surfactants generally tend to degrade before they fulfill their long-term duty, even though some high-cost, specialty surfactants are available.