Kinetics of CaCO3 Scale Formation. The Influence of Temperature, Supersaturation and Ionic Composition

Abstract

Abstract 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 (<60o°). 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 = (Equation) = 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. Introduction Mineral precipitation or mineral scaling, during oil production is of considerable concern for the oil industry. Calcium carbonate is one of the most common of the various minerals precipitating from formation waters. Calcium carbonate can decrease the permeability in the near well area, adhere to the inside of the production tubing, and clog valves and other equipment. All of which will lead to a loss in production. Calcite scale is relatively easy to treat, since it is soluble in acids. The pressure drops as oil and water are produced from the reservoir. This pressure drop may lead to increasing supersaturation of CaCO3 and higher risk of calcite precipitation and the formation of scale. The supersaturation of CaCO3 can be calculated using thermodynamic models and oil and water analysis data. CaCO3 can, however, at rather high supersaturation, be in a metastable state in the solution. It will therefore not precipitate despite being thermodynamically unstable. When assessing whether CaCO3 scaling will be a problem, the kinetics of its formation ought to be taken into account. The induction time for precipitation is according to Mullin [1] defined as the time from supersaturation is created until critical nuclei for crystal growth are formed. However, if the induction time for a given supersaturation is longer than the retention time of the oil and water in the production tubing, scale might not form. The aim of this study has been to find the induction time for a given supersaturation at various conditions that can contribute to scale formation. Temperature, pressure, the ionic composition of the water phase, sand in the water and the flow rate are important parameters. The change in pH of the solution has been used to measure the experimental induction time at 80, 100 and 120°C. Experiments have been run with and without sand suspended in the solution by stirring. The sand was from two oil reservoirs in the North Sea. The effect of Mg2+ and SO42- ions on the induction time and crystal growth have also been studied.