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 (CCa2+/CHCO3−). 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 CCa2+CHCO3− ratios, the transportation of the lattice ions to calcite crystal surface has to be considered.