Iron sulphide (FeS), zinc sulphide (ZnS) and lead sulphide (PbS) are considered to be among the most challenging scales in terms of inhibition and removal. They can form by direct reaction of aqueous sulphide species with dissolved Fe, Zn and/or Pb and by the exchange between aqueous sulphide species with preformed iron compounds, such as iron oxide hydroxide. These existing iron compounds may have formed during production and/or intervention, such as an acid treatment. Similarly, PbS and ZnS can form by extracting sulphide from a more soluble sulphide scale i.e. Zn exchanging with Fe in FeS. The objective of this work was to investigate FeS formation and inhibition under a range of conditions including pH, temperature, salinity and proposed mode of formation. In addition, the interaction between iron, zinc and lead within solutions containing sulphide species was investigated
The majority of this study was conducted under anaerobic conditions, with the scale formation and/or inhibition experiments being monitored by inductively coupled plasma (ICP) analysis, pH and particle size measurements. Among the tested scale inhibitors, two showed high efficiency against iron sulphide, however high pH and salinity had a detrimental impact on the performance of one of these products. Interestingly, these scale inhibitors prevented iron sulphide deposition even under aerobic conditions i.e. iron hydroxide partially preformed. Moreover, at sufficiently high concentrations of scale inhibitor, the deposition of zinc sulphide and lead sulphide was prevented even when these scales were formed via cation displacement i.e. zinc and lead displaced sulphide ions from pre-formed iron sulphide. The route of formation for FeS, ZnS and PbS was seen to have a significant impact on the inhibition process.
The particle sizes of inhibited (suspended) FeS were significantly lower than the blank FeS samples, with this effect increasing with increased scale inhibitor concentration. This difference in particle size may have an important influence on in-line filter blocking tests and produced water quality issues.