The potential of tuned-composition waterflooding to enhance oil recovery from carbonate reservoirs has been widely investigated in the literature. The consensus is that wettability alteration occurs because of the electrostatic interactions between the carbonate rock surface and the potential determining ions, Ca2+, Mg2+, CO32, and SO42. Recently, glycine, the simplest amino acid, has also been investigated as a wettability modifier for carbonates that acts similarly as the sulfate ions in brine. The impact of wettability modifiers like glycine and calcite's potential determining ions has been described by surface complexation models (SCM) and the wetting-state of the rock has been related to change of the surface potential. However, determining the relevance of the geochemical reactions is obstructed by the complexity of the SCM. Moreover, the surface potential as a surrogate of the wetting-state of the rock does not correlate with the experimental results with glycine reported in the literature.

The present research analyzed the results of single-phase displacement using a SCM for calcite to determine the important surface complexation reactions. Then, wettability alteration is modeled as a set of anion exchange reactions between wettability modifiers, like SO42 and Gly, and adsorbed carboxylic acids. Finally, analytical solutions are presented for the coupled two-phase and multicomponent reactive-transport model with anion exchange reactions.

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