Iron sulfide and zinc sulfide inhibition and scale inhibitor consumption

Iron sulfide (FeS) and zinc sulfide (ZnS) scales have been observed in many sour oil and gas wells. FeS often forms alongside other scales such as calcium carbonate and barium sulfate and such scales can be removed using chemicals like hydrochloric acid (HCl) and chelating agents. However, there are several drawbacks associated with the FeS removal by acid. For example, HCl acid, which outperforms other dissolvers has a high corrosion rate and generates hydrogen sulfide (H₂S) gas as a byproduct. Other dissolvers, including chelating agents, often have low dissolution rates. Therefore, FeS inhibition is preferred as a strategy rather than allowing it to form followed by its removal. The objective of this paper is to investigate the inhibition efficiency of various inhibitors for preventing FeS and ZnS deposition. Different scale inhibitor (SI) chemistries have been examined over a wide range of parameters, including temperature, salinity, pH and concentrations of Fe, Zn and sulfide. Static formation and inhibition experiments were conducted and the progress of the reaction was monitored by inductively coupled plasma (ICP) analysis and pH. In addition, the inhibitor consumption in sulfide scale solutions has also been investigated; i.e. the inhibitor concentration has been monitored during the sulfide inhibition process to determine the role and fate of the SI itself. Polymeric scale inhibitors, including phosphino polycarboxylic acid (PPCA), showed a high inhibition efficiency for ZnS in different salinity and temperature conditions. On the other hand, some polymeric scale inhibitor that prevented the deposition of ZnS completely failed to inhibit FeS. It was found that, in mixed FeS and ZnS systems where both sulfide scales deposited, the precipitation of FeS had a negative impact on the inhibition efficiency for ZnS. By analogy, the FeS formation can affect the inhibition efficiency for other scales such as calcium carbonate and barium sulfate. In addition, in our SI consumption experiments we found that the scale inhibitor was consumed in ZnS solutions while there was no decrease (no SI consumption) in the scale inhibitor concentrations in FeS solutions. Based on these results, we demonstrate that it is easier to inhibit ZnS rather than FeS under the same conditions. The presence of FeS had a negative impact on the performance of the scale inhibitor for ZnS and similar effects might occur in FeS/conventional scale systems. For the first time in a sulfide scaling system, this work examines if the scale inhibitor remains at its original dosage in solution or if it declines. There were two distinct behaviors, the scale inhibitor was consumed in ZnS but not in FeS solutions.