Scale formation is mostly governed by scaling ion concentrations and fluid conditions (pressure, temperature, and pH). Sulfide scale formation is most commonly initiated through the mixing fluids containing scaling cations (Fe2+, Zn2+ and Pb2+) or sulfidic anions (H2S(aq), HS- and S2-), or, more rarely, in a single fluid containing both ions which is undergoing physical condition changes, such as a pressure drop or pH change. The literature has extensive reviews of sulfide scales formed by mixing two fluids in both static and dynamic tests. The self-scaling of metal sulfides in a single fluid, however, has been less investigated.
An experimental setup and procedure have been developed to investigate the impact of various factors, such as pH (0 - 10), sulphide and metal ion concentrations and salinity (3.5 - 20 wt. %), on the formation of sulfide scales in general and iron sulfide (FeS) in particular. This new setup provides anaerobic conditions to isolate and prevent the interference of atmospheric oxygen, while retaining aqueous and gaseous sulfide in solution. The setup is comprised of airtight vials and Hungate-type tubes equipped with septum-caps to facilitate the gas-tight liquid transfers required in such experiments. The concentrations of sulfide ranged from 100 to 1,000 mg/L, and iron, zinc and lead were studied at levels in the range of 50 - 100 mg/L.
The formation of sulfide scales was measured by monitoring the depletion of cation concentration in aqueous solution at various pH values. The excess amount of sulfide concentration significantly affected the formed iron sulfide by affecting the pH at which initial cation depletion occurred. The higher sulfide excesses gave an FeS precipitation onset at lower pH levels, and larger FeS particle size than lower levels of sulfide excess.
These findings directly affect the scale inhibition design, as most sulfide scale control chemicals are dispersants. Therefore, particle size is very relevant to these dispersants in terms of the inhibitor loading and efficiency. The assumption that sulfide scale is principally reliant on the cation concentration, particularly if limiting, is inaccurate, and sulfide excess must also be quantified and taken into consideration in the inhibition design.