Produced waters are increasingly found to contain high levels of dissolved iron, with typical ferrous iron concentrations ranging from a few ppm to several hundred ppm. The presence of iron can cause issues in production, one problem being a detrimental effect on the performance of scale inhibitors. The aim of this work was to investigate scale inhibitor chemistries with improved iron tolerance, and apply a new product in the field to address a severe inorganic scale issue that had been encountered.
Using static bottle tests to assess brine compatibility and anaerobic dynamic scale loop tests to assess scale inhibition efficiency, a wide variety of scale inhibitor chemistries containing different functional groups were screened. The aim was to identify an inhibitor which would give the best performance against calcium carbonate scale in the presence of up to 100ppm Fe2+. Previous studies have shown that the inhibition of calcium carbonate scale is more adversely affected by the presence of iron than the inhibition of barium sulfate scale, and as calcium carbonate was the main challenge in the field case the emphasis was placed on inhibiting this scale type.
Initial compatibility studies revealed the additives with the best brine compatibility, and around nine additives were taken forward for performance testing. It was found that acrylic acid based copolymers demonstrated reasonable scale control at 5-20 ppm Fe2+, but at higher iron the high dose levels required meant that the limit of compatibility was reached before complete scale control had been achieved. The best performing additive for calcium carbonate was found to be a phosphonate derivative. A field trial was conducted in a predominantly calcium carbonate scaling environment as a proof of concept and scale inhibitor residuals were monitored over a 5-month period. After this successful study, further lab experiments were performed with the chosen inhibitor to demonstrate good calcium carbonate control in the presence of up to 100 ppm Fe2+.
A comprehensive investigation of different scale inhibitor types resulted in an optimum chemistry to control calcium carbonate scale in the presence of high concentrations of ferrous iron. Applying this chemistry in the field has demonstrated better scale control than was being achieved with the previous scale inhibitor.