(669f) Measurement and Surface Complexation Modeling of Calcite Zeta Potential in Mixed Brines for Carbonate Wettability Characterization

We presents experimental measurements and surface complexation modeling (SCM) of synthetic calcite zeta potential in various conditions. The systematic zeta potential measurement and the proposed SCM can provide insight into the role of four potential determining ions (Mg²⁺, SO₄^2- , Ca²⁺ and CO₃^2-) and CO₂ partial pressure in calcite surface charge formation and facilitate the understanding of calcite wettability alteration. Brines with varying potential determining ions (PDI) concentration in two different CO₂ partial pressure (P_CO2) are tested in experiments. A double layer SCM is then developed to model zeta potential measurements. Then we propose a definition for contribution of charged surface species and quantitatively analyze the variation of charged species contribution when changing brine composition. After showing our model can accurately predict calcite zeta potential in brines containing four PDIs, we apply it to predict zeta potential in ultra-low and pressurized CO₂ environments for potential applications in carbonate enhanced oil recovery including miscible CO₂ flooding. Model prediction reveals that calcite surface with no impurity will be positively charged in all investigated brines in pressurized CO₂ environment (>1atm). Moreover, the sensitivity of calcite zeta potential to CO₂ partial pressure in the various brine is found to be in the sequence of Na₂CO₃ > Na₂SO₄ > NaCl > MgCl₂ > CaCl₂ (Ionic strength=0.1M).