Laboratory Study on Switchable Alkylamidine Surfactant for Daqing Oilfield

Surfactants are used in a variety of emulsion-based applications. Usually, stabilization of emulsion is only required temporarily. In the case of crude oil displacement from reservoirs, subsequent destabilization of produced water/oil (w/o) fluid is required during the separation of water and oil, but it is often a challenging task. Furthermore, the more effective is the surfactant in stabilizing the initial emulsion, the more difficult is the destabilization processes. To address this issue, novel switchable surfactants have been developed. They are molecules that can be reversibly converted between surface active and inactive forms by application of triggers including pH, ozone, ultraviolet light, CO2, N2 and heat. The reversibility concept is favorable environmentally and economically due to the possibility of recovering and reusing the compounds in different beneficial applications. The characteristic properties make switchable surfactants sustainable chemicals and simplify the treatment of produced fluid during the Alkali-Surfactant-Polymer (ASP) flooding processes. Presently, more than 20% recovery could be achieved by ASP flooding after water flooding in Daqing oilfield. However, due to high w/o interface activity of ASP surfactants such as petroleum sulfonate, the rapid breaking of produced emulsion is still a big challenge. In this paper, the switchability of CO2 triggered long-chain alkyl amidine surfactant was investigated. The hydrophobic C18 alkyl amidine (inactive) was protonated to become water-soluble surfactant upon the addition of CO2 in aqueous media, and the amidinium ions (active) could then be deprotonated by bubbling N2 through the surfactant solution. The reversibility of the conversion from C18 alkyl amidine to its amidinium bicarbonate was demonstrated by bubbling CO2 followed by N2 through solutions and measuring the change in conductivity of solution. The conductivity increased almost immediately when CO2 was bubbled through the solution and deceased slowly again when sparged with N2.The critical micelles concentration (CMC) was studied to compare the surface activity of inactive amidine and active amidinium bicarbonate. The approximate CMC values were measured at 20 mg/L for the latter. Interface tension (IFT) measurements between formation water and crude oil were conducted before and after CO2 treatment employing the amidine/petroleum sulfonate complex surfactants. The emulsification effect of amidine before and after CO2 treatment was also studied. The amidinium bicarbonate was a stronger emulsifier compared to inactive amidine for the examined ASP formula. Switchable surfactants are attractive for both economic and environmental considerations. As the switchable surfactant can undergo fully reversible interconversion between active and inactive forms at a specific desired stage, the development of novel switchable surfactants with active forms in the reservoirs and inactive forms in the central treating stations will spark further interest in ASP industrial application.