Wettability alteration of carbonate rocks by surfactants: A mechanistic study

A considerable quantity of the worldʹs oil reserve is located in naturally fractured carbonate reservoirs, with very low oil recovery efficiency, due to their wettability and tightness of matrixes. Recovery efficiency can be improved considerably, if the reservoir rock wettability is changed from mostly oil-wet to water-wet, thus enhancing water imbibition into the oil saturated rock. In this experimental work, an extensive mechanistic study is performed utilizing different analytical tools to study the effects of surfactants on the sample rockʹs wettability. The results indicate that the surfactants act in different manners according to their structure. Cationic surfactant C12TAB, tends to irreversibly desorb stearic acid from the dolomite surface via ionic interaction. Nonionic surfactant TritonX-100 is adsorbed on the surface by the polarization of π electrons and ion exchange, releasing more stearic acid from the solid surface. The released stearic acid is then adsorbed as a new layer on the surface, through hydrophobic interaction between the tail of adsorbed surfactants and the non-polar part of the stearic acid. Anionic surfactants, such as SDS, are adsorbed on the surface via hydrophobic interaction between the tail of surfactant and the adsorbed acid, thus changing the wettability of the surface to a neutral wet condition.