Electrokinetics of Limestone and Dolomite Rock Particles

High-salinity water such as seawater, or formation brines, is frequently injected in carbonate reservoirs. Ion interactions between injection water, reservoir fluids, and rock surface are quite complex. It has recently come to be believed that the chemistry of injection water can significantly enhance oil recovery. Several reaction mechanisms were suggested, including rock dissolution, change Of surface charge, and/or sulfate precipitation. This study attempts to characterize the electrokinetics of limestone and dolomite suspensions at 25 and 50 degrees C. In addition, reaction mechanisms at the water/rock interface were established. Synthetic formation brine, seawater, and aquifer water were chosen from Middle East reservoirs. Carbonate particles were soaked in high- and low-salinity water. A phase-analysis-light-scattering (PALS) technique was used to determine the zeta potential (surface charge) of carbonate particles over a wide range of pH, ionic strength, and temperature. Zeta potential of limestone particles was significantly affected by calcium ion. Low-salinity water created more negative charges on limestone and dolomite particles by expanding the thickness of the diffuse double layer. Individual divalent cations decreased the zeta potential of limestone particles in sodium chloride solutions, while sulfate ions showed a negligible effect. Limestone particles in high-salinity water had decreased zeta potential. The solubility of calcium ions increased as temperature was increased and thus created additional negative charges. The absence of sulfate in aquifer water strongly influenced the dolomite surface charge. In summary, surface-charge adjustment from positive to negative can alter the wettability of carbonate rock from preferentially oil-wet to water-wet. As a result, residual-oil saturation should be decreased.