ZnS and Pbs Scale Formation in H₂S-Containing Environments and Their Influence on Fluid Chemistry and Corrosiveness (C2026-00048)

Eiman Saleem, Olawale Frank, Frederick Pessu, Wassim Taleb

In H₂S-containing geothermal brine systems where metal ions are present, sulphide scales such as lead (PbS), zinc (ZnS), and iron (FeS) precipitate from solution, causing flow assurance challenges. Metals used are therefore affected by both surface sulphide scale deposition and H2S-induced corrosion.
This study investigates the effect of controlled bulk pH on the kinetics of PbS formation, including its evolving characteristics and the influence of the Pb: Zn ratio on sulphide scaling tendencies and the potential corrosiveness of the environment. Experiments were carried out at 80 °C in NaCl brine saturated with a gas mixture containing 10 mol.% H₂S and 90 mol.% CO₂, with 100 ppm of dissolved Pb²⁺. Pb:Zn ratios varied between 0 – 100 ppm for Zn. Bulk precipitation of sulphide scales was assessed by Atomic Absorption Spectroscopy (AAS), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX), and X-ray Diffraction (XRD) techniques.
The data show that controlling the fluid pH influences Pb²⁺ speciation in the bulk phase: at pH values ≥ 4, speciation is initially hindered, resulting in reduced Pb²⁺ availability for PbS formation. At lower pH (≤ 3), this effect is reversed, promoting PbS precipitation. PbS formation also contributes to the acidification of the bulk solution.