A systematic investigation of sulphide scale formation and inhibition
Alharbi, Bader Ghazi
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This thesis presents an investigation of the formation, inhibition and interaction of iron sulphide (FeS), zinc sulphide (ZnS) and lead sulphide (PbS) scales. Several scale inhibitors were tested for their inhibition efficiency against these scales, including a selection of commonly used scale inhibitors (SIs), such as phosphonates and polymers, as well as proprietary high molecular weight sulphonated co-polymers. An extensive series of sulphide scale formation and ion displacement experiments were also carried out using slightly modified experimental sequences in the testing procedures. The purpose of these latter experiments was to establish whether the precise details of the experiment, such as the sequence of scale formation, affected the overall efficiency of the SI. Of the tested SIs, only two showed reasonable promise for inhibiting all three sulphide scales, while some polymeric scale inhibitors prevented the deposition of ZnS and PbS in 3.5 wt% NaCl but failed to inhibit FeS. The impact of pH, temperature, salinity and scale inhibitor concentration on the inhibition efficiency and particle size of inhibited scale was also investigated. Increasing the pH and salinity had a detrimental impact on the performance of one of the proprietary scale inhibitors i.e. SI-3. None of the tested phosphonate scale inhibitors showed any field-appropriate inhibition effect on any of the sulphide scales. On the other hand, some tested polymeric scale inhibitors did inhibit ZnS and PbS, but none of them prevented the deposition of FeS; however, a significant reduction in the particle size of the FeS was observed for some SIs. In the presence of SI-1 (polyphosphino carboxylic acid, PPCA), it was noted that scale inhibitor “consumption” (i.e. the SI was removed from the solution) took place for ZnS and PbS solutions but not in FeS solutions. It was seen to be easier to inhibit ZnS and PbS when they formed concurrently rather than by subsequent formation of PbS then ZnS. In cation displacement experiments, PbS deposition was prevented provided that the preformed ZnS was inhibited.