Surfactant stabilization of CO₂-in-water foams at high temperatures
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The interfacial properties of a surfactant in a CO₂-aqueous system at a temperature above 100 °C, and how they influence foams are essentially unknown. A cationic surfactant, C₁₂₋₁₄N(EO)₂ in the protonated state below pH 5.5, was demonstrated to be soluble in an aqueous phase with up to 22% total dissolved salt (TDS) at 120 °C. Moreover, the strong solvation in brine (high cloud point) and simultaneous affinity for CO₂ led to significant adsorption of the surfactant at the CO₂-water interface. Given that the surfactant favored the brine phase over the CO₂ phase, the preferred curvature was a CO₂-in-water (C/W) macroemulsion (foam). The surfactant stabilized foam in the presence of crushed calcium carbonate at ~ pH 4 upon suppressing the dissolution of calcium carbonate upon addition of Ca²⁺ and Mg²⁺ according to the common ion effect. Cationic alkyltrimethylammonium surfactants with an alkyl tail of average carbon number less than 15 were soluble in 22% TDS brine up to 120 oC. The head group was properly balanced with a C₁₂₋₁₄ hydrocarbon tail for a sufficiently dense surfactant layer at the CO₂-water interface to reduce the interfacial tension. For C₁₂₋₁₄N(CH₃)₃Cl the solubility in brine and the surfactant adsorption were sufficient to stabilize C/W foam at 120 °C in both a crushed calcium carbonate packed bed (76 Darcy) and a capillary tube at the downstream of the bed. The stability of the foam at high temperature may be attributed to the high surfactant adsorption at the interface. The use of nonionic surfactants as a foam stabilizer is usually limited by their poor aqueous solubility at elevated temperatures, particularly at high salinity. A nonionic surfactant C₁₂₋₁₄(EO)₂₂ with high degree of ethoxylation gave higher salt tolerance at elevated temperature. The surfactant stabilize C/W foam at 80 °C in the presence of 90 g/L NaCl brine in a 30 Darcy sand pack, which has not yet been reported by a nonionic surfactant. Both the formation of strong foam in the porous media and the low of oil-brine partition coefficient suggest C₁₂₋₁₄(EO)₂₂ is a potential candidate for a CO₂ EOR field trial.