Dr. Shell Global Solutions International B.V. Amsterdam, Netherlands
Alkyl Alkoxy Sulfate (AAS) surfactants are more tolerant for divalent ions than sulfonated surfactants and can be preferred for enhanced oil recovery chemical flooding applications as water softening is not required. A potential limitation of AAS surfactants is their thermal stability. At higher temperatures, AAS degradation occurs by hydrolysis of the C-O bond of the sulfate group which is acid-catalysed at lower pH. The conventional wisdom has been that reservoir temperatures above about 60°C will give increasing AAS decomposition making them unsuitable for these conditions. In EOR applications, the injected surfactant solution will be in long-term contact with reservoir rock while it travels from injector to producer. The hypothesis tested in this work is that the reservoir may provide buffering capacity maintaining higher pH and giving improved AAS thermal stability.
To simulate the anaerobic conditions sub-surface, AAS solutions were prepared under oxygen-free conditions in multiple tubes, with and without rock present, and then stored oxygen-free in ovens at 80° and 105°C. The surfactant concentration of individual tubes was determined by surfactant titration and surfactant composition of some tubes was measured using Time of Flight Mass Spectrometry (ToF-MS).
It is concluded that reservoir rock clearly provides buffering capacity and keeps the surfactant solutions at a pH of around 8 for solutions stored at 80° and 105°C for at least 12 months. It is also demonstrated that the stability of the AAS surfactants tested is improved in the presence of rock. Furthermore, even the stability of the AAS surfactants tested without rock present appeared to be better than expected from data published earlier.