A laboratory study of CaCO3 scaling kinetics has been undertaken to obtain information useful for prediction of CaCO3 scale formation. Thermodynamic carbonate scale prediction models have only a limited use since the rate of CaCO3 precipitation can be very slow at lower temperatures (< 60°C). The induction time for precipitation has been determined in a series of experiments where SR, T and the ionic composition of the water from which precipitation occurs, have been varied. The effect of sand from North Sea Oil Field reservoirs on the rate of CaCO3 precipitation has also been investigated by adding such sands to the water under investigation. Our data clearly show that oil field sands enhance the rate, and reduce the induction time for Calcite precipitation. The formation of solid CaCO3 has a window of metastability, which can vary from SR=aCa2+aCa32Ksp(CaCO3)= 7.0, 2.9 and 2.7 at 80, 100 and 120°C, respectively, with no sand in the water, and 4.8-4.9, 2.7 and 2.6 with sand present. An even more drastic effect was observed when Calcite scale was added to the water. At 100-120°C the precipitation at SR = 1.6 was observed after a few seconds. This is reasonable since the Calcite crystals suspended in the solution act as crystallisation sites for the dissolved carbonate. It was also observed that Mg2+ and SO42- ions in the solution increased the induction time and retarded the CaCO3 crystallisation. The precipitating phase seemed to be Vaterite when Mg2+ and SO42- ions were not present in the solution. Vaterite even started to form on Calcite seeds when these were added to the homogeneous supersaturated solution. This implies that compounds, which block active sites for Vaterite crystallisation, may inhibit CaCO3 precipitation. With Mg2+ and SO42- ions in the solution Aragonite was formed.

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