Chemical enhanced oil recovery techniques progressively emerge as a means to increase mature oil fields production. In particular, surfactant flooding allows increasing oil production by lowering the interfacial tension between injection water and crude oil. The effectiveness of surfactants depends on their chemical stability over an extended period of time, which could be impaired by reservoir conditions in terms of temperature, salinity and oxidative conditions.
Within the frame of this work, we monitored the chemical stability of several families of anionic surfactants in water, including alkyl ether sulfates, alkyl glyceryl ether sulfonates, alkyl benzene sulfonates and internal olefin sulfonates. Analytical methods were first developed to evaluate the chemical stability of these surfactants, including high pressure liquid chromatography (HPLC) and mixed-indicator titration techniques. The thermal stability of industrially representative surfactants was then monitored as a function of time at high temperature.
Anionic surfactants designed to withstand high temperature conditions proved to be stable in oxygen controlled environments over a very long storage time. Alkyl benzene sulfonates, internal olefin sulfonates and alkyl glyceryl ether sulfonates (or alkyl alkoxyl glyceryl sulfonates) notably showed a very good stability over a year–long storage at 100°C. Alkyl ether sulfates (or alkyl alkoxy sulfates) showed by contrast a poor stability at 100°C as all the active contents were degraded in twenty four hours in deionized water and around four months in an alkaline buffer. The hydrolysis of the sulfate moiety at high temperature is probably responsible for the limited stability of alkyl ether sulfates at high temperature.
This study highlights the strong benefit of using sulfonated anionic surfactants in EOR processes applied at high temperatures. In particular, it demonstrates the advantage of replacing alkyl ether sulfates by alkyl glyceryl ether sulfonates in order to insure the long term stability of a formulation developed to be injected during several years.