During the past 20 years, alkali-surfactant-polymer (ASP) chemical combination flooding has become a successful EOR technique for the mature reservoirs with high water cut and high recovery percent in Daqing oilfield. Based on the results of field tests, more than 20% recovery could be achieved by ASP flooding after water flooding in Daqing oilfield. Heavy alkyl benzene sulfonate (HABS) and petroleum sulfonate were employed as the main strong alkali and weak alkali surfactants, respectively. However, due to the high w/o interface activity of such surfactants, severe emulsion has also been found with the ASP produced fluid. Currently, rapid breaking of these emulsion fluid is still a big challenge.
In recent years switchable surfactants that will "switch" their interface characters, have sparked interest in a variety of emulsion-based applications. Switchable surfactants are attractive for both economic and environmental considerations. The switchable surfactants are molecules that can be reversibly converted between surface active and inactive forms by application of triggers. A range of possible "triggers" for the decomposition have been screened including pH, ozone, ultraviolet light, CO2, N2 and heat. All of those have been used to switch the polar head from the non-polar tail. These characteristic properties make switchable surfactants sustainable chemicals and simplify the produced fluid treatment of ASP flooding processes i.e. the emulsions need to be stabilized only temporarily and have to be demulsified at the end.
In this paper, the feasibility study of ASP switchable surfactant for Daqing oilfield was carried out. A long-chain alkylamine was employed as the switchable surfactant and CO2/N2 were triggers. This alkylamine surfactant was exposed to CO2 to produce the mono-carbamate form, afterwards the carbamate was exposed to N2 in order to revert it to the original amine form. The reversibility of the conversion from alkylamine to carbamate form was demonstrated by bubbling CO2 followed by N2 through solutions and measuring the change in conductivity of the solution. The conductivity decreased almost immediately when CO2 was bubbled through the solution and increased again when sparged with N2. The critical micelles concentration (CMC) was studied to compare the surface activity of amine form and its corresponding carbamate form. The results showed that the CMC values were similar to each other. Interface tension (IFT) measurements between formation water and crude oil were conducted employing the amine/petroleum sulfonate complex surfactants. Little difference in IFT was found with amine/PS mixture surfactants before and after CO2 treatment. However, the amine could not be functioned as ASP surfactant without petroleum sulfonate. The emulsification effect of amine before and after CO2 treatment was also studied. The carbamate was a stronger emulsifier compared to amine for the examined ASP formula.
As the switchable surfactant can undergo fully reversible interconversion between active and inactive forms at a specific desired stage, the development of novel ASP switchable surfactants with active forms in the reservoirs and inactive forms in the central treating stations will become a new research direction of chemical combination flooding.