New Insights into the Effect of HCO3-, CO32- and SO42- Ions on the Zeta Potential of Intact Carbonate Rock Sample

Zeta potential is an important interfacial property that controls electrostatic interactions between mineral, water, and non-aqueous phase fluids. These interactions play an important role in defining the wetting state of reservoir rocks and transport of ionic species through porous media. The zeta potential is shown to be an efficient means for a broad range of applications including monitoring of single- and multi-phase flows in subsurface settings, characterization of fracture networks, efficiency of CO₂ sequestration, hydrogen underground storage and enhanced oil recovery. It is widely agreed that the zeta potential in carbonate rocks is controlled by the concentration of potential determining ions (PDI), but the understanding of the underlying mechanisms is still limited as there are little experimental data on quantitative characterization of the dependence of the zeta potential on concentration of negative potential determining ions (PDI) such as SO₄^2-, CO₃^2-, HCO₃^-, especially when their concentration is high and exceeds that of the positive PDIs.

In this study, the streaming potential method is used to investigate the zeta potential of natural carbonate rock samples in contact with natural aqueous solutions of low-to-high ionic strength and with varying concentration of sulphate (SO₄^2-) and carbon (C₄) related (HCO₃^-, CO₃^2-) ions. In each set of experiments the total ionic strength was kept constant to eliminate the impact of concentration on the zeta potential so that the increasing/decreasing concentration of negative PDI was adjusted by decreasing/increasing concentration of indifferent Cl^- ions. The study probed the concentration of negative PDIs that has never been reported before, with their respective lowest concentration consistent with previously reported equilibrium values, and the highest concentration being equal to the maximum achievable through stripping the tested solutions of Cl^-.

Our results demonstrate for the first time that magnitude of the negative zeta potential increases linearly with log₁₀ of C₄ concentration, however its dependence on the log₁₀ of SO₄^2- concentration is non-linear suggesting varying mechanisms of this PDI’s specific adsorption. Moreover, the results demonstrate that the zeta potential strongly depends on the total ionic strength, interpreted from slopes of the linear regressions for each negative PDI in different background solution. This observation suggests that equilibrium constants of negative PDI specific adsorption may be affected by the total ionic strength. Our findings improve the current understanding of the complex physicochemical processes that take place at calcite-water interface and provide important experimental data for surface complexation modelling of carbonate-brine systems.