Injection of inhibitors for gas hydrate formation during transportation of wet gas/condensate in pipelines strongly affects the tolerance for produced formation water. The use of monoethylene glycol (MEG) as hydrate inhibitor can be combined with pH increase (pH stabilisation) as corrosion control for carbon steel pipelines. The pH increase promotes precipitation of iron carbonate which forms a protective layer for corrosion at the steel surface. However, the elevated alkalinity downstream of the MEG injection point, combined with calcium ions from produced formation water, might result in the precipitation of calcium carbonate. The Fe2+ formed by corrosion will accumulate in the aqueous phase down the pipeline. Even with low corrosion rate, the Fe2+ concentration at the pipeline outlet is typically 10 – 100 ppm.

This work presents the impact of MEG on the precipitation rate of calcium- and iron carbonate, and how small amounts of Fe2+ affects calcium carbonate precipitation. The induction time for precipitation of both iron- and calcium carbonate are significantly prolonged by increased concentration of MEG. This effect is a result of retarded crystal growth and not reduced nucleation rates. The presence of 1-40 ppm Fe2+ also prolongs the induction time for calcium carbonate precipitation and the effect increases with concentration.

The presence of MEG and Fe2+ also affects the polymorphism of CaCO3. Vaterite forms at high MEG concentration (>90 wt%). Its growth is retarded by the presence of traces of Fe2+ and the induction time for precipitation increases with increasing iron. Lower MEG concentrations give aragonite mainly. Iron also lowers the growth rate of this polymorph, although not to such an extent as for vaterite.

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