Iron Sulfide Scale Formation:A New Anaerobic Setup and New Insights

摘要

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 fluidscontaining scaling cations(Fe2+,Zn2+ and Pb2+)or sulfidic anions(H2S(aq),HS-and S2-),or,more rarely,in asingle fluid containing both ions which is undergoing physical condition changes,such as a pressure drop orpH change.The literature has extensive reviews of sulfide scales formed by mixing two fluids in both staticand 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 ofsulfide scales in general and iron sulfide(FeS)in particular.This new setup provides anaerobic conditionsto isolate and prevent the interference of atmospheric oxygen,while retaining aqueous and gaseous sulfidein solution.The setup is comprised of airtight vials and Hungate-type tubes equipped with septum-caps tofacilitate the gas-tight liquid transfers required in such experiments.The concentrations of sulfide rangedfrom 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 inaqueous solution at various pH values.The excess amount of sulfide concentration significantly affectedthe formed iron sulfide by affecting the pH at which initial cation depletion occurred.The higher sulfideexcesses gave an FeS precipitation onset at lower pH levels,and larger FeS particle size than lower levelsof sulfide excess.These findings directly affect the scale inhibition design,as most sulfide scale control chemicals aredispersants.Therefore,particle size is very relevant to these dispersants in terms of the inhibitor loading andefficiency.The assumption that sulfide scale is principally reliant on the cation concentration,particularlyif limiting,is inaccurate,and sulfide excess must also be quantified and taken into consideration in theinhibition design.