A mechanistic comparison of formate, acetate, and glycine as wettability modifiers for carbonate and shale formations

Previous studies indicated the efficacy of the simplest amino acid, glycine, as a wettability modifier that enhances the imbibition of water in carbonate reservoirs. The objective of this research was to compare the performance of formate, acetate, and glycine as wettability modifiers for carbonate and shale formations. Formate and acetate were studied for the first time as novel wettability modifiers. Note that the aminomethyl and amino groups are the only structural differences between formate and glycine, and acetate and glycine, respectively. The experiments consisted of contact-angle measurements on oil-aged calcite and shale plates, and imbibition displacements (spontaneous and forced). The comparison among these additives was made with/without adding hydrogen chloride (HCl) for adjusting the pH of the solutions. Amott indexes of glycine and formate + HCl solutions were noticeably higher than the rest of the cases in the imbibition experiments. We found that formate can be very effective in altering the wettability of carbonate rocks to water-wet state when the pH of the solution is reduced. This indicates the synergy of pH adjustment and the attraction of formate to the rock surface, which resulted in an improvement of the oil recovery. Glycine showed a superior behavior as a wettability modifier in comparison to formate and acetate for all the experimental conditions of this study. This indicated that the amino group, in the presence of the carboxyl group, plays a key role in altering the rock wettability. We demonstrated that the ability of glycine to alter wettability comes from two factors: one is the chelate effect caused by the amino group as an electron donor in the presence of the carboxyl group. The chelate effect makes glycine entropically more favorable for binding to calcium cation in the brine, which triggers calcite dissolution, and/or attaching directly to the calcite surface. The other factor is the calcite dissolution induced by pH reduction, followed by the attraction of glycine to the rock surface.