Dynamic interactions of inorganic species at carbonate/brine interfaces: An electrokinetic study

Adjusting brine chemistry during waterflooding of carbonate and clastic reservoirs shows favorable impact on oil recovery. In carbonates, inorganic ions in the injection water play a key role on altering rock-fluid interactions and subsequently influencing rock wettability. Emphasis in this study is given to the direct measurements of carbonate particles zeta potential at different ionic content and aging times. A disturbance at interfaces was observed for most brine solutions especially within the first hour of interactions. The zeta potential of particles became steady with time as a result of attaining chemical equilibrium at interfaces. Individual ions including cations, and anions altered carbonate zeta potential and interacted differently at interfaces, in spite of having an identical salinity. Monovalent ions, for example sodium, significantly reduced the absolute surface potential of limestone particles. The hardness ions, calcium and magnesium, reversed the zeta potential of both calcite and limestone particles towards the positive side. The electrical properties, in general, were found to be affected by the ionic content and cation to anion ratio in the injection water. The dissolved divalent cations play a role in the interactions at the Stern layer boundary, which eventually impact the zeta potential at interfaces. In view of these zeta potential results, specific brines - SmartWater (10-times diluted seawater), key ions (calcium, magnesium and sulfates) and salt solutions containing strictly sodium or sulfate - were able to create electrical repulsions between oil/water and carbonate-particles/water interfaces. As a result, such tailored brines will alter the rock wettability to water-wet and therefore can enhance the oil recovery in carbonate reservoirs. This further understanding of ions specific impact on surface potential, hence wettability, provide essential insights for the successful design of optimal SmartWater recipes.