Inhibition and Interaction between Iron Sulphide, Zinc Sulphide and Lead Sulphide

Iron sulphide (FeS), zinc sulphide (ZnS) and lead sulphide (PbS) scales have been observed in many sour oil and gas wells. Sulphide scales often form alongside other scales such as calcium carbonate and barium sulphate and such scales can be removed using chemicals like hydrochloric acid (HCl) and chelating agents. However, there are several drawbacks associated with the removal of sulphide scales, for example, HCl acid, which outperforms other dissolvers has a high corrosion rate and generates hydrogen sulphide (H2S) gas as a byproduct. Other dissolvers, including chelating agents, have very low dissolution rates. Therefore, FeS inhibition is much preferred to its removal, if this can be achieved efficiently and economically. The objective of this paper is to investigate the inhibition efficiency of inhibitors/dispersants for preventing FeS, ZnS and PbS scales. Different scale inhibitor chemistries have been examined over a wide range of parameters, including temperature, salinity, pH and concentrations of Fe, Zn, Pb and sulphide. Static formation and inhibition experiments were conducted and the progress of the reaction was monitored by ion tracking using inductively coupled plasma (ICP) analysis and pH monitoring. Also, filter-blocking inhibition tests were carried out to examine the impact of scale inhibitor concentration on the scaling time. Polymeric scale inhibitors showed a high inhibition efficiency for ZnS while no inhibition was observed for phosophonate based scale inhibitors. Unlike ZnS, none of the tested scale inhibitors inhibited FeS. High molecular weight scale inhibitors performed well even at high temperature and salinity and maintained the particles suspended in solution for several days. However, high scale inhibitor concentrations were required to prevent the deposition of FeS, particularly when the iron concentration was raised to 100 ppm and above. In SI-1 solutions, it was easier to inhibit PbS and ZnS when they formed concurrently rather than forming PbS followed by ZnS. These results are in line with the difference in the MIC (Minimum Inhibitor Concentration) observed in SI-2 solutions. Based on these results, the tested polymeric scale inhibitors managed to inhibit ZnS and PbS but failed against FeS under the same conditions despite the fact that the solubility of ZnS and PbS is less than FeS. It was also found that, cation displacement and the sequence of the scale formation had a significant impact on the inhibition efficiency. The size of scale particles was dependent on the type of scale inhibitor and concentration and this was reflected in the scaling time in the dynamic inhibition tests.