This paper investigates CO2-foam stabilized with nanoparticles (NP) as an enhanced oil recovery (EOR) agent in carbonate reservoirs with high temperature and salt content. Under these conditions surfactants, widely used to stabilize foams, becomes unstable and will not generate strong foam. Our objective is to use NP as a stabilizing agent for surfactant based foam through an integrated process where the surfactants generate foam and the NP stabilizes the foam. Aqueous NP solutions (1500 ppm) were injected through porous media at high temperature (120 °C) to evaluate solution stability for a range of brine salinities, pH and resident times. Stable solutions were subsequently co-injected with supercritical CO2 to create foam and increase the apparent viscosity of CO2.

High temperature and salinity are challenging for most surfactants; however it is a common reservoir condition in many carbonate fields. The nanoparticles had a high degree of stability with little precipitation or gelation in the presence of salt content as high as 23 wt.% NaCl as long as the pH was sufficiently low. The stability of the nanoparticles decreased with increased temperature, leading to gelation and injectivity problems in limestone core samples when the injection water had native pH. The problem was less pronounced in sandstone core samples. Nanoparticle stability increased with decreasing pH, and it was possible to achieve steady-state conditions at 120 °C for longer periods of time. CO2 was shown to be a viable stabilizing agent by decreasing the pH of the injection brine and had the added benefit of creating stable foam for enhanced oil recovery. This is particularly important in limestones and carbonates where the pH of the injection water tends to increase as it interacts with the rock. The nanoparticles can be used alone or in combination with surfactants to increase the stability the surfactant generated foam. Surfactant based foam is generally more effective at generating foam, but is less stable at harsh conditions than nanoparticles.

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