Mechanistic Study of Wettability Alteration of Carbonate Rocks Using New Quaternary Ammonium Surfactants

The wettability of subsurface carbonate reservoirs is a critical factor influencing the pore-scale displacement of fluids. As such, wettability plays a key role in enhanced oil recovery and greenhouse gas geo-sequestration. Wettability affects the interaction between the crude oil–brine-rock system and affects the distribution of fluids within pore spaces. Oil recovery can thus be improved by altering the rock surface wetting properties from an oil-wet state to a water-wet state. The adsorption of cationic surfactants can induce wettability alteration and thus improve water imbibition in oil-wet carbonate rocks. In this experimental work, solutions of tetramethylammonium chloride and hexadecyltrimethylammonium chloride were prepared in seawater for the purpose of evaluating the wettability alteration and examining their ability to enhance oil recovery. Wettability was monitored by contact angle measurements that were applied on Indiana limestone samples aged in crude oil at 90 °C. Spontaneous imbibition experiments were performed at 70 °C in oil-wet limestone cores. Time-of-flight secondary ion mass spectrometry was employed to semiquantify the adsorption of the cationic surfactants in order to support the findings of the spontaneous imbibition experiments and the contact angle measurements. Carbonate rock samples with and without adsorbed stearic acid were prepared in order to document whether the cationic surfactants remove stearic acid from the carbonate rock surfaces. The cationic surfactant hexadecyltrimethylammonium chloride was observed to significantly remove stearic acid from the surface, and tetramethylammonium chloride was observed to have an insignificant impact on adsorbed stearic acid. This aligns well with the fact that hexadecyltrimethylammonium chloride results in an oil recovery that is more than 5-fold greater than when tetramethylammonium chloride is used, which is supported by the contact angles.