CO2-Soluble Ionic Surfactants and CO2 Foams for High-Temperature and High-Salinity Sandstone Reservoirs

The sweep efficiency of CO2 enhanced oil recovery can be improved by forming viscous CO2-in-water (C/W) foams that increase the viscosity of CO2. The goal of this study is to identify CO2-soluble ionic surfactants that stabilize C/W foams at elevated temperatures up to 120 degrees C in the presence of a high salinity brine using aqueous phase stability, static and dynamic adsorption, CO2 solubility, interfacial tension, foam bubble size, and foam viscosity measurements. An anionic sulfonate surfactant and an amphoteric acetate surfactant were selected to achieve good thermal and chemical stability, and to minimize adsorption to sandstone reservoirs in the harsh high-salinity high-temperature brine. The strong solvation of the surfactant head by the brine phase and surfactant tail by CO2 allows efficient reduction of the C/W interfacial tension, and the formation of viscous C/W foams at high salinity and high temperature. Furthermore, the effect of temperature and methane dilution of CO2 on foam viscosity was evaluated systematically in both bulk and porous media. High temperature reduces the stability of foam lamella, which leads to lower lamella density and, therefore, lower foam viscosity. Methane dilution of CO2 reduces the solvation of surfactant tails and makes the surfactant less CO2-philic at the interface. The consequent increase of the interfacial tension decreases the stability of foam lamella, as seen by the increase in foam bubble size, thereby reducing foam viscosity.