Carbonate and sulphate scales are the most common types of mineral deposit associated with the recovery and processing of hydrocarbon and associated produced water. In high temperature fields (>140°C) within the North Sea, more unusual scale types have been observed, namely zinc and lead sulphide. Limited studies have been focussed on zinc sulphide (ZnS), and mechanisms for scale inhibitors to control this scale are still relatively unknown.

In laboratory studies, the relatively small amount of ZnS deposited and ‘soft’ scale characteristics make it difficult to test ZnS using traditional test methods, especially at high temperature. A novel dynamic loop test combining the traditional test coil and novel test filter system has been developed to evaluate ZnS inhibitors and was published in SPE paper 169810.

The mechanisms of the inhibitor interactions with ZnS suggested from this previous study have been further evaluated and will be reported in this paper. The investigation of the inhibitor mechanisms have been addressed by scanning electron microscope evaluation of scale texture and size distribution of scales recovered from static bottle tests conducted at 70°C.

Several different generic types of ZnS scale inhibitor chemistries were selected based on the previously published study. Type 1 inhibitor is dispersion/nucleation inhibitor, Type 2 is nucleation/growth inhibitor, and Type 3 has no inhibition function on ZnS. The test results at 70°C agree well with the mechanism suggested in the previously published tube / filter blocking study, and the inhibition mechanisms are reinforced. The difference in particle size, particle quantity and crystal morphology shows how different generic types of inhibitors interact with the ZnS scale.

This paper is a comprehensive study of the mechanism of ZnS formation and inhibition via a range of experimental test methods to fully understand the impact that inhibitor functional groups have on crystal growth inhibition, nucleation inhibition and dispersion mechanism.

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