Zinc and lead interactions in combined sulphide scales

Zinc sulphide (ZnS) and lead sulphide (PbS) have both been observed as mineral scale deposits in hightemperature high-pressure HT/HP oil and gas fields. ZnS and PbS deposition can pose safety hazards and have serious economic consequences. Some field results have revealed the formation of mixed ZnS/PbS scales despite sulphide being the limiting reactant to individual Pb and Zn ions, while contrary results have been obtained by using scale prediction and lab testing under the same conditions. The objective of this study was to investigate the interaction between Zn and Pb within solutions containing sulphide over a wide range of conditions including varying salinity (sulphate-free North Sea Seawater (SFNSSW) and Glenelg formation water (GFW)), temperature (50 and 95°C), time and Zn:S, and Pb:S molar ratios. Static scale formation experiments coupled with inductively coupled plasma (ICP) analysis, pH monitoring and particle size measurements have been used to analyse the system. When a mixed Zn/Pb brine was combined with sulphide brine, Pb ions were completely stripped from solutions where sulphide was in excess to Pb. In this region, sulphide was partially consumed by Pb and thus the remaining sulphide reacted with Zn to form ZnS. On the other hand, when both Zn and Pb existed in excess to sulphide, single PbS formed despite the fact that both scales have very low solubility. Similar behaviour was observed when the temperature was increased from 50 to 95°C and GFW was used instead of SFNSSW. This study showed that, under the conditions examined, PbS will always form preferentially to ZnS in a mixture even at high Zn:Pb molar ratios. Understanding the interaction between Zn and Pb in the presence of sulphide is essential for accurate interpretation of scale inhibition results and scale formation predictions.