Synergism in binary surfactant mixtures containing a pH-responsive surfactant towards enhanced foam stability in brine at high pressure and high temperature conditions

Abstract

The use of liquid foams in enhanced oil recovery (EOR) applications requires the formation of stable foams under harsh reservoir conditions, such as a wide pH range, brines containing divalent cations, and high temperature and pressure. In this work, mixtures of the pH-responsive surfactant bis-(2-hydroxyethyl) cocoalkylamine (C12) with nonionic (tallow fatty amine, TFA20), zwitterionic (cocamidopropyl hydroxysultaine, CAHS) and anionic (sodium olefin sulfonate, AOS) surfactants were evaluated to obtain foams with improved stability under conditions relevant to CO2-based EOR. The presence of TFA20 in the foaming formulation increased the solubility of C12 at basic pH values, but impaired foam stability. In contrast, the surfactant mixtures containing CAHS exhibited a significant synergy regarding surface tension reduction and foam stabilization. The synergy was confirmed by the negative value of the interaction parameter, indicating strong attraction, likely electrostatic, between the surfactant molecules at the interface. This effect was evidenced in the decrease in the bubble growth rate obtained with the C12:CAHS (1:2) mixture, compared to the individual components, suggesting a reduction in Ostwald ripening and coalescence mechanisms of foam destruction due to higher packing density of surfactants in the interfacial film. The presence of the zwitterionic surfactant in the foaming mixture containing C12 allowed obtaining foams with improved stability at pH 4, under high pressure (100 bar) and high temperature (65 °C) conditions. The results obtained in this study showed the potential of using synergic mixtures to tailor the solubility and foaming properties of pH-responsive surfactants for enhanced performance in foam-based EOR methods.