Wettability Alteration of an Oil-Wet Sandstone Surface by Synergistic Adsorption/Desorption of Cationic/Nonionic Surfactant Mixtures

Various experimental methods including atomic force microscopy, scanning electron microscopy, zeta potential measurement, and contact angle measurement were used to analyze the mechanisms of wettability alteration of an oil-wet sandstone surface by cationic/nonionic surfactant mixtures in this work. Due to the synergies between cationic surfactants and nonionic surfactants, head groups of CTAB and TX-100 interact with each other, making the cmc of the cationic/nonionic surfactant mixtures lower compared to the single surfactant CTAB or TX-100. Ion pairs are produced by the carboxylic substances and the aggregates formed by CTAB and TX-100, which are irreversibly desorbed from the quartz surface and are solubilized into the mixed micelles formed by the CTAB/TX-100 mixture. The CTAB molecules are preadsorbed on the oil-wet sandstone surface by electrostatic attraction, acting as anchor particles, and the aggregates are formed by TX-100 and CTAB through hydrophobic interaction, thereby increasing the adsorption amount of CTAB on the oil-wet sandstone surface. The ability to form ion pairs for CTAB and the carboxylic substances in the presence of TX-100 and the solubilization ability of the mixed micelles are all enhanced, making the desorption capacity of ion pairs stronger. Thus, the CTAB/TX-100 mixture is more effective than the single surfactant CTAB in altering wettability of the oil-wet sandstone surface toward a more water-wet condition.