Wettability Change Related to the Adsorption of Organic Acids on Calcite: Experimental and Ab Initio Computational Studies

We have investigated the interactions between organic acids and a calcite powder by adsorption from an organic phase (benzoic acid and lauric acid in toluene) and from an aqueous phase (benzoic acid and lauric sodium salt. In addition to the experimental study, ab initio quantum chemistry calculations were performed for cluster models, simulating the interactions of molecules with calcite surface sites. At the toluene/calcite interface, acids chemisorb on surface calcium ions, replacing surface hydroxyls resulting from the previous surface hydration by water. This surface saponification phenomenon was predicted by ab initio calculations and confirmed by spectroscopic techniques. The adsorbed acids conferred a hydrophobic character to calcite, with the higher contact angles in water for the longest alkyl chains in the organic acid. At the water/calcite interface, organic acids do not displace water. As electrostatic forces determine the stability of thin water films between calcite and crude oils at reservoir conditions, from our calculations, we deduce that when crude oil fills the biggest pores of the reservoir rock, oil/calcite interfaces appear where the adsorption of organic acids can change the wettability to oil-wet. By contrast, the mineral should remain water-wet at places where the capillary pressure has not been able to thin the aqueous phase beyond a critical value at which the film would collapse. This can occur even if the aqueous phase contains dissolved organic acids transferred from crude oil. Mixed wettability patterns might therefore be related to the spatial distribution of pore sizes. Our results support the latter conclusion in the specific case of organic acids with calcite.