The kinetics of calcite precipitation from a high salinity water

Calcium carbonate is one of the most common and important scale-forming minerals in oilfield produced water, but the kinetics of CaCO₃ precipitation has been ignored in most scale prediction models because of the lack of reliable precipitation rate model. There are none in the open literature for oilfield conditions (temperature > 100°C, pressure > 200 bar and salinity > 0.5 mol kg− 1). In this work the kinetics of calcite (CaCO₃) precipitation from high salinity waters (up to 2 mol kg⁻¹) have been studied by a pH-free-drift method in a closed water system. This method. is much easier to operate than the often used steady-state method. The experimental results indicate that the calcite precipitation rate is not only affected by the solution CaCO₃ saturation level, but also by the solution pH, ionic strength and the concentration ratios of Ca to HCO₃₋ ions (C_Ca²⁺/C_HCO3⁻). When the concentration ratios of Ca to HCO₃⁻ ions are close to their chemical stoichiometric ratio of 0.5, the calcite growth from a supersaturated solution is believed to be surface reaction controlled. However, at higher C_Ca²⁺C_HCO3⁻ ratios, the transportation of the lattice ions to calcite crystal surface has to be considered.