Impact of Acid and Base Numbers and Their Ratios on Wettability Alteration of the Calcite Surface

Alteration of rock wetting preference toward a more favorable wetting state has been identified as the primary mechanism of oil recovery from intermediate and oil-wet carbonate rocks. In order to produce a favorable wetting state, it is critical to have a clear picture of the initial wetting properties of the rock surface. This study aimed to improve the understanding of carbonate wettability by examining the impact of acid, base, and their mixture on wettability of calcite surfaces using different analytical tools (contact angle measurements, zeta potential test, Fourier transform infrared spectroscopy, interfacial tension measurements, and fluorescence ultraviolet spectroscopy). The acid number (AN) and the ratio of the basic component (quinoline) to the acidic component (stearic acid) were systematically varied in model oil solutions to elucidate the effect of simultaneous presence of the acid and base on the wettability alteration process. The results showed that as the acid concentration increases, the wettability alteration toward the oil-wet state is enhanced. However, the impact of the base was significant at high concentrations, where the wettability was changed toward the neutral state. Addition of the basic compound to the acidic model oil hindered the impact of stearic acid on wettability alteration. Although the impact of the basic component on wettability alteration of calcite seems to be weaker than that of the acidic component, presence of the base can alter the wetting characteristics of the surface. Acid and base concentrations and their ratio dictate the extent of calcite wettability alteration. Slight impact of the base on the wettability alteration process was observed for a system with an AN of 0.25. However, at higher ANs, an increase in the base number (BN) to AN ratio from 0 to 1 resulted in a remarkable reduction in the contact angle; while the contact angle increased again when the BN to AN ratio was 4. Further investigations (for systems with AN = 1) revealed that at the BN to AN ratio of 0.25 and 1, adsorption of the basic component on the surface was unlikely to happen. Formation of an acid-base complex could hinder the access of stearate ions to the calcite surface. At the BN to AN ratio of 1, the quinoline concentration was equal to that for stearic acid, which may enhance the acid-base interactions; hence, more stearic acid would be involved in acid-base complexes. However, once the base concentration became higher than the acid concentration (BN to AN = 4), an excess amount of the base, which was not involved in the complex mixtures with acid molecules, might adsorb on the surface.