Experimental Investigation of the Effect of Acid and Base on Wettability Alteration of a Calcite Surface

Wettability alteration toward a favorable wetting condition is one major mechanism of enhancing oil recovery in carbonates. Obtaining the desired wettability stipulates that a clear understanding of the original wetting properties of the rock surface be provided. In fact, the aim of this study is to elucidate the effect of acid and base upon the wettability alteration of a calcite surface by investigating the individual effects of an organic acid (stearic acid) and a base (N,N-dimethyldodecylamine, N,N-DMDA) at different concentrations as well as their combinations on calcite wetting properties. Contact angle, ζ-potential, Fourier transform infrared spectroscopy, and thermogravimetric analysis (TGA) were conducted to determine changes in wetting properties of the calcite surface. Moreover, interfacial tension (IFT) measurements were used to evaluate the interfacial activity of the model oil systems. Overall, the results revealed that by increasing the acid concentration, wettability of a calcite surface shifted toward more oil-wetness and adsorption of stearic acid on a calcite surface increased. Treating calcite surfaces with the base revealed that a low concentration of N,N-DMDA was able to shift the calcite wettability from a water-wet state to a slightly oil-wet state. In addition, TGA results showed low adsorption of the base with a high surface area occupied by the base molecule, indicating a horizontal orientation of the base on the calcite surface. For acid-base mixtures, at a constant acid concentration, the wettability alteration toward oil-wetness increased in the presence of the base, as the base-To-Acid ratio increased up to 1, as a result of which oil-wetness decreased. Moreover, IFT measurements indicated lower IFT of acid-base mixtures compared to those of individual acidic and basic model oil solutions, which could be attributed to the synergistic effect between the acid and base by forming acid-base complexes. The presence of the base hindered acid adsorption on the calcite surface. However, the basic component could contribute to wettability alteration by forming acid-base complexes. These complexes could adsorb on the calcite surface and affect its wetting preferences. In addition, the base molecules may adsorb on the calcite surface at high base-To-Acid ratios. The results suggested that the ratio of base to acid would determine the contribution of acid molecules, base molecules, and acid-base complexes in wettability alteration of calcite surfaces. Hence, possible mechanisms of wettability alteration by acid-base combinations were proposed at different base-To-Acid ratios.