Alpha olefin sulfonate (AOS) adsorption on calcite and dolomite: Role of active complex formation and electrical double layer capacitance

The surfactant injection process is used in the oil industry for various purposes, such as changing the wettability of the rock surfaces, decreasing the interfacial tension, and consequently increasing the oil production, using it as a de-emulsification agent and base material for foam formation. Usually in surfactant injection processes, one of the important issues is surfactant loss which can affect the economic and operational feasibility of the method. One of the phenomena involved in surfactant loss is its adsorption on the rock. In this study, the effective factors of the surfactant adsorption phenomenon in an electrolyte-mineral solid system were investigated. For this purpose, the mechanisms of surface potential formation, the electrical double layer capacitance and its rela-tionship with the zeta potential, repulsive and attractive forces between suspended particles, and changes in the Gibbs energy of the multi-component system as a result of adsorption were explained. Laboratory measurements of adsorption of an anionic surfactant, alpha olefin sulfonate (AOS), on calcite and dolomite surfaces were performed. For this purpose, ultraviolet and infrared spectroscopy measurements were conducted for each test. Results showed that increasing the salinity, from 0.001 M to 0.1 M, reduced the adsorption of AOS on calcite by about 50%. The presence of the divalent cation of magnesium, and the divalent anion of sulfate, causes the release of the sulfonate anion from the unstable complex of calcium sulfonate and consequently adsorption reduction. The magnesium sulfonate complex is stable and more active than the calcium sulfonate complex. Therefore, the adsorption of sulfonate on dolomite was negligible. Considering that sulfonates are one of the most common surfactants used in enhanced oil recovery processes, the results of this study can be effective in controlling its adsorption phenomenon. Also, the results of this study can be used in the design of wettability alteration processes.