Investigation of sulphide scale formation and inhibition in mixed H2S and CO2 - containing environments

Summary - Developing Issues related to corrosion and scale induced sulfide in H2S-containing energy productions environments

Energy production from geothermal and oil and gas systems are usually characterized by a complex mixture of gaseous and ionic species. Brine chemistries have significant influences on these species, with consequential effects on flow assurance and corrosion related phenomena at the metal - fluid interface, especially with the use of carbon steel. Some of the key gaseous species includes mixed H2S and/or CO2.

There have also been increasing flow assurance related concerns due to the presence of alkaline earth and/or transition cations present within brine chemistry. Some of these cations; Ba2+, Ca2+, Zn2+, Pb2+ etc have the potential to influence surface and bulk scale deposition kinetics in addition to corrosion characteristics of the metal interface. The presence of dissolved sulphide species and the kinetics of FeS formation from corrosion process is likely to induce a complex interaction with transition cations.

Such interactions could influence the surface and bulk deposition kinetics and adhesion of sulphide species such as PbS and ZnS. Cations incorporated in FeS films could substantially transform the corrosion properties of FeS scales at the metal interface. While attempts have been made to mitigate such deleterious interfacial and bulk phenomena with corrosion and scale inhibitors (CIs and SIs), there is still a lack of understanding of how these CIs/SIs interact with corrosion pathways to mitigate these interfacial phenomena; particularly: 1) in conditions where a complex mixture of various Iron sulphide (FeS) morphologies exist; 2) when FeS is potentially incorporated with transition cations which are competing for the same interface.

In the presence of Zn2+ and Pb2+ cations, the formation of ZnS and PbS in the bulk fluid which the potential to deposited on the surface are likely to induce corrosion and flow assurance issues. None of these complex phenomena have been fully investigated. Fundamental research has shown that the formation of FeS in simple single phase environments is linked to pitting corrosion damage as a result of the electronic properties of FeS.

The complexity of FeS formed in the presence of alkaline earth and/or transition cations in multiphase systems may also transform the electronic properties of FeS scales and may influence the nature of pitting corrosion characteristics. While there is clearly an urgent need to optimize the functional properties of CIs and/or SIs in interaction with sulphide scales of complex chemistries at metal-brine interfaces, these interfaces need to be initially understood and accurately characterized.