Modeling the characteristic thermodynamic interplay between potential determining ions during brine-dependent recovery process in carbonate rocks

Brine-dependent recovery processes in carbonate reservoirs have been receiving much attention for the past two decades, and wettability alteration has been identified among the dominant process mechanisms. Most studies suggest that the wettability alteration in carbonate rocks is caused by desorption of oil acid groups from rock surfaces by the adsorbed sulfate, with concomitant co-adsorption of the divalent cations. Although such symbiotic interaction between active ions (Ca2+, Mg2+, and SO42−) and the rock surface has been established, their systematic interplay under different conditions remains to be better explored and understood. In this study, we propose a reactive transport model that considers various reaction sets to investigate the affinity of these active ions toward the rock surface. The important thermodynamic properties were obtained by using the model to interpret single-phase experiments. The model results were in excellent agreement with the produced ion histories reported from single-phase flow through experiments. Using the same thermodynamic parameters, our model also replicated the produced ion histories and oil recoveries obtained during brine-oil flooding experiments. The established thermodynamic parameter can be applied to predict various brine-dependent recovery processes in different carbonate lithology as no significant difference was observed for the interplay between active ions and either chalk or limestone rocks. Accordingly, the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of surface forces was used to rationalize active ions interactions and to evaluate the contribution of each force component to the wettability change in the oil-brine-rock system. The results emphasize that disjoining pressure and electrostatic interaction energy are sensitive to SO42−/Ca2+ ratio as well as brine salinity.