Selection of switchable amine surfactants for stable CO2 – in – water foams for high temperature CO2 mobility control
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The generation of stable carbon dioxide-in-water (C/W) foams at high temperatures is hindered by lack of surfactant solubility in the aqueous phase and acceleration in various foam destabilization mechanisms. Herein, fundamental understanding of the aqueous and CO2 phase behaviors, aqueous phase rheology and interfacial properties of the surfactant enabled the formation of stable and viscous C/W at conditions never achieved before. Switchable amine surfactants in the protonated state were soluble up to 120 °C and salinities up to 22% TDS. The addition of a second amine in the head and methyl substitution as in C16-18N(CH3)C3N(CH3)2 increased the aqueous solubility and eliminated cloud point precipitation associated with hydroxyethyl (EO) substitution. The unprotonated surfactants were soluble in CO2 at 120 °C and 3400 psia. The affinity for both aqueous and CO2 phases resulted in high adsorption of the surfactant at the C-W interface and effective reduction of the interfacial tension. Furthermore, low C-W partition coefficients favored the formation of C/W foams. The surfactant structure was tuned to form a highly viscoelastic aqueous phase by transforming the micellar morphology from spherical to entangled wormlike micelles. For C16-18 N(CH3)C3N(CH3)2, the longer tail compared to C12-14N(EO)2 or smaller head vii versus C16-18N(EO)C3N(EO)2, given methyl instead of hydroxyethyl substitution on the amine, increased the packing parameter and enabled the formation of a viscoelastic aqueous phase.