Ferrous-sulfide (iron II sulfide) scale typically forms in the wellbore under anaerobic conditions and may cause a reduction in well productivity. It has been proposed that ferrous ions react with bisulfide formed from dissolved hydrogen sulfide to form a complex iron bisulfide intermediate, which further reacts to produce black solid ferrous sulfide. Treatment of this type of scale typically involves mineral acids or chelants, which themselves provide additional challenges for pipeline integrity.
The Weatherford flow assurance laboratory carries out tests that determine the best inhibitors for scale problems. Compared to calcite, barium, and gypsum scales, little work has been done to study prevention of ferrous sulfide scale. For calcite, barium, and gypsum scales, the dynamic-flow loop is commonly used. However, for ferrous-sulfide scale, there is a challenge for a quick and reliable test method. The preparation of ferrous sulfide under sour conditions is not straight forward due to both the difficulty in achieving sufficiently anaerobic conditions and the associated hazards of managing H2S. The presence of oxygen will result in ferric sulfide instead of ferrous sulfide.
This paper describes in detail laboratory experiments to safely prepare ferrous sulfide scale using H2S and also test potential inhibitors. Comparisons were made between using metal sulfide instead of H2S. Using the H2S method to prepare ferrous sulfide provided a quick and robust way to select potential inhibitors under various conditions, which can be used to improve flow assurance assessments.
This paper will demonstrate selected chemicals compared to tetrakis (hydroxymethyl) phosphonium sulfate (THPS) in preventing the formation of ferrous sulfide under sour conditions and ambient or elevated pressures and temperatures when applied at low concentrations. Some inhibitors that are successful under sour ferrous sulfide preparations are not effective when metal sulfide ions are used instead of H2S, which means potential inhibitors are being missed due to inappropriate test methods.
Overall, a technological advancement was made with an anaerobic and sour method to test effectively ferrous sulfide inhibitors that would perform at low concentrations.