Evolution of Modelling Zeta Potential: Impact of Brine Compositions and Concentration

Electrochemical interactions of calcite with brines in natural subsurface settings have received ample attention in the last decades due to the broad range of their applications. These interactions can be described by an electrical property termed the zeta potential. Many numerical simulation studies using surface complexation modelling (SCM) have been performed to investigate the relationship between the zeta potential and a wide range of salinities and complex brine compositions. Although most of the simulated results, especially in low salinity conditions, successfully match the experimentally measured zeta potential, the simulated zeta potential for high salinity conditions is still poorly understood.

In this study, we present a new approach of SCM to simulate the zeta potential by considering the actual molecular-scale phenomena at the calcite-brine interface. Unlike previous SCM studies, our model considers the hydrated diameter of ions as the distance of approach, which depends on salinity. We also consider the permittivity of the Stern layer as a function of salinity, which is consistent with previous unrelated studies. We calculate the capacitance for each salinity based on the relationship between the hydrated diameter of ions and the permittivity of the Stern layer. Moreover, all calcite-brine surface reactions are described by new equilibrium constants independent of salinity and composition of brines.

Our results show that the simulated zeta potential which is obtained from our SCM at a broad range of salinities is successfully matched with the published experimental data for two different carbonate rock samples as long as the salinity dependence of the hydration diameter and electrical permittivity is accounted for. We find that the potential determining ions (Ca²⁺, Mg²⁺, SO₄^2-, HCO₃^-,CO₃^2-) play a dominating role compared to the indifferent ions (Na⁺, Cl^-) in the calcite-brine surface reactions. The Implications of our findings are significant for wettability evaluation, characterisation of shallow and deep aquifers and CO₂ geological sequestration.