Carbonate-sulphide mixed scale problems have been reported when water is produced from carbonate reservoirs. The mechanistic understanding of CaCO3 co-precipitation with sulphide scales has not been very extensively studied to date. This paper presents a study of the interactions of lead/zinc sulphides co-precipitating with calcite. The relationship between bulk precipitation and deposition on a metal surface of the mixed carbonate-sulphide scales is examined when the mixed scales begin to nucleate, agglomerate and grow. CaCO3 crystals appears to transform from a stable rhombohedral crystal to an orthormbic variant of this crystal form and then to an extreme acircular crystal in the presence of sulphide scales. These transformations are more significant in the presence of the polymeric scale inhibitor, PPCA, where the CaCO3 crystals formed on the metal surface are highly distorted, while those formed in bulk solution solution are hardly changed. The nature of these mixed crystals may indicate that mixed sulphide scales tends to prefer forming in solution rather than adhering to metal surfaces in protected zones. Thus, the mixed sulphide scale may be controlled in one area but unprotected in other regions.
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