Role of Water Chemistry on the Adsorption Behavior of a Saponin-based Biosurfactant on the Sandstone Surface

Summary Biosurfactants have been received more attention in oil recovery due to their low-cost and environmental friendly characteristics. In this study, we investigated adsorption behavior of a saponin-based biosurfactant named SUTBS which was extracted from the leaves of one of the Iranian plants growing in the southern part of the country. To this end, adsorption analysis of the SUTBS solution was performed on a clayey sandstone surface in the presence of different seawater solutions under high temperature that represents reservoir conditions. Role of water chemistry on the surfactant adsorption was addressed by changing the composition of seawater (SW) through tuning ionic strength and also manipulation of divalent ions. Adsorption analysis was then performed by contacting surfactant solution with the rock powder at 80 °C while tracking surfactant concentration using UV-spectrophotometer. Results showed that surfactant adsorption in the presence of SW brine increases with surfactant concentration up to 1500 ppm and beyond that it levels off to a plateau value that represents a Langmuir-type isotherm. In order to explore the effect of ionic strength, surfactant solutions were prepared by SW brine and its different dilution (0.5SW and 0.1SW). As to results, a lower surfactant adsorption was observed for diluted brines such that at surfactant concentration of 1500 ppm surfactant adsorption in the presence of 0.1SW was almost half of the SW brine. To check the role of aqueous ions on the adsorption behavior, a brine solution without any divalent ions was synthesized while keeping ionic strength equal to the SW brine only by adjusting NaCl concentration. It was found that removing divalent ions from brine solution caused a reduction in the surfactant adsorption level by 35%. Finally, to get further insights into the role of divalent ions (Mg2+, Ca2+ and SO42-), three SW brines were prepared such that in each solution only one of the divalent ions became present and the two other ones were excluded. Ionic strength of all these solutions was kept constant by adjusting NaCl concentration. As to results, adsorption level in the presence of sulphate-rich brine decreased as compared to the native SW brine. However, a higher adsorption level was observed for the case of divalent cation-rich brines. These observations could be discussed in terms of physicochemical interactions among aqueous ions/surfactant molecules/rock minerals. Findings from this study shed light on the importance of water chemistry to design an appropriate surfactant flooding.