Influence of Mg2+ on CaCO3 formation-bulk precipitation and surface deposition

The formation of CaCO3 mineral scale is a persistent and expensive problem in oil and gas production. The magnesium ion, present in formation and injection waters in downhole conditions, is a key determinant in CaCO3 scale formation. In the work reported herein, the kinetics of calcium carbonate scale formation is studied both in the bulk solution and on a metal surface. The effect of the Mg2+ ion on the scale formed on the metal surface has been studied systematically in this paper including aspects of the kinetics, morphology, and Mg/Ca ratio of the deposit on the metal surface. The Mg2+ ion adsorbs onto the deposited crystals and the ratio of Mg2+ in the deposit formed on the metal surface is proportional to the ratio of Mg/Ca in the scaling water. The distribution coefficient in surface deposition and in bulk solution is a constant and independent of the concentration of Mg2+ ions in the bulk solution. Simultaneously, the Mg2+ ion accelerates the crystal transformation from vaterite to calcite and adsorbs on the surface of vaterite and calcite causing an increase in surface roughness in addition to distortion of crystals.