Inhibition mechanisms for sulphide scales

Sulphide scales have over recent years become increasingly common in many oil and gas producing regions. The main sulphide scales are iron, zinc and lead, with the latter two often occurring together. This paper presents experimental results from a series of static iron sulphide (FeS) inhibition efficiency tests using 2 scale inhibitors (SI), a phosphonate and a polymer. The paper also highlights the methodology which has been developed for assessing iron sulphide scale inhibition. The objective of this preliminary iron sulphide inhibition efficiency study was (a) to establish which of the conventional SI tested inhibit FeS and under which test conditions; (b) to determine the mechanisms through which SIs inhibit FeS; and (c) to determine if it is possible to determine a "minimum inhibitor concentration" (MIC) with this methodology. This preliminary study shows that: (i) The conventional SIs tested in this study do inhibit FeS scale formation; (ii) A clarification phenomenon was observed after sulphide formation using conventional phosphonate which has not been observed/reported previously. This involves the formation of black FeS and the clarification that follows after 24hrs, which indicates subsequent "inhibition", although probably by direct chelation rather than by threshold (sub-stoichiometric) inhibition; (iii) It is possible to obtain an "MIC" from the static bottle test.