Switchable Amine Surfactants for Stable CO2/Brine Foams in High Temperature, High Salinity Reservoirs

Abstract Ethoxylated amine surfactants of the form C12-14N(EO)x have been characterized for foam generation to improve the sweep efficiency for CO2 enhanced oil recovery (EOR) up to 120 °C in the presence of high salinity brine of 22%TDS. These surfactants are switchable from the nonionic (unprotonated amine) state in dry CO2 to cationic (protonated amine) in the presence of an aqueous phase with a pH below 6. Potentiometric titration of two surfactants with 2 or 15 EO groups was conducted at 90 °C to test the switchability of the surfactant from nonionic to cationic forms at different pH values. Ethoxylated amine with 2 EO groups showed more affinity for protonation when compared to 15 EO due to the hinderance effect of the larger molecule that covers the ionizable nitrogen head. For C12-14N(EO)2, viscous C/W foams were produced by injecting the surfactant either in the CO2 phase or the brine phase, which indicates good surfactant transport to the interface in either case. The optimum foam quality was observed at 90% and 95% when the surfactant was injected from brine phase or CO2 phase, respectively. C12-14N(EO)2 was effective in lowering the interfacial tension between water and CO2 at 120 °C and 22%TDS from ~ 40 mN/m to 5 mN/m with CMC value of 0.038 mM. This significant reduction in interfacial tension enhances foam generation and foam stability.