Zinc sulfide (ZnS) is one of the notoriously exotic scales formed in the high temperature oil fields. Its formation can potentially cause severe scale issues by itself or combined with conventional CaCO 3 scale. Limited publications are available on ZnS deposition/inhibition, let alone the combined ZnS and CaCO 3 scales. The mechanisms of these scale interactions are still unknown.The relatively small amount of ZnS precipitated that adheres to surfaces and its 'soft' scale characteristic make it difficult to test ZnS using traditional scale inhibitor evaluation test methods, especially at high temperature. A new methodology with modified dynamic loop test combining the traditional test coil and novel test filter system has been developed and allows evaluating ZnS and combined ZnS/CaCO 3 at high temperature in the absence and in the presence of scale inhibitors.Co-deposition of ZnS/CaCO 3 and the interference between ZnS and CaCO 3 have been studied at elevated temperature. The mechanisms have been addressed by both scale prediction and laboratory tests. Findings from this study show that ZnS dramatically accelerated CaCO 3 formation within the test brine and made the scaling condition extremely harsh, even with a very small amount of ZnS present.Several types of inhibitor chemistries were short listed for this study based on a previously published mechanism study to assess their ability to prevent ZnS and co-deposition of ZnS/CaCO 3 at elevated temperature. Type-1 inhibitor is dispersion/nucleation, type-2 is nucleation/growth and type-3 has no inhibition function on ZnS. The test results conducted at 170°C agree well with the mechanism assumed by the previous mechanism study, and the inhibition mechanisms are reinforced. The minimum inhibitor concentration (MIC) of scale inhibitors has been evaluated for CaCO 3 and co-deposition of ZnS/CaCO 3 at 70°C and 170°C. The difference of MIC gives an indication of the negative effect that very small amounts of ZnS can have on the inhibition of co-deposition. The performance of environmentally acceptable inhibitors is also presented for this challenging environment.This paper presents a comprehensive study of the challenge of ZnS and co-deposition of ZnS/CaCO 3 under conditions similar to that of many of the current HT/HP field developments in the North Sea and recent developments in the Gulf of Mexico.

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 (Ͼ140°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.

Zinc sulfide (ZnS) is one of the notoriously exotic scales formed in the high temperature oil fields. Its formation can potentially cause severe scale issues by itself or combined with conventional CaCO3 scale. Limited publications are available on ZnS deposition/inhibition, let alone the combined ZnS and CaCO3 scales. The mechanisms of these scale interactions are still unknown. The relatively small amount of ZnS precipitated that adheres to surfaces and its ‘soft’ scale characteristic make it difficult to test ZnS using traditional scale inhibitor evaluation test methods, especially at high temperature. A new methodology with modified dynamic loop test combining the traditional test coil and novel test filter system has been developed and allows evaluating ZnS and combined ZnS/CaCO3 at high temperature in the absence and in the presence of scale inhibitors. Co-deposition of ZnS/CaCO3 and the interference between ZnS and CaCO3 have been studied at elevated temperature. The mechanisms have been addressed by both scale prediction and laboratory tests. Findings from this study show that ZnS dramatically accelerated CaCO3 formation within the test brine and made the scaling condition extremely harsh, even with a very small amount of ZnS present. Several types of inhibitor chemistries were short listed for this study based on a previously published mechanism study to assess their ability to prevent ZnS and co-deposition of ZnS/CaCO3 at elevated temperature. Type-1 inhibitor is dispersion/nucleation, type-2 is nucleation/growth and type-3 has no inhibition function on ZnS. The test results conducted at 170°C agree well with the mechanism assumed by the previous mechanism study, and the inhibition mechanisms are reinforced. The minimum inhibitor concentration (MIC) of scale inhibitors has been evaluated for CaCO3 and co-deposition of ZnS/CaCO3 at 70°C and 170°C. The difference of MIC gives an indication of the negative effect that very small amounts of ZnS can have on the inhibition of co-deposition. The performance of environmentally acceptable inhibitors is also presented for this challenging environment. This paper presents a comprehensive study of the challenge of ZnS and co-deposition of ZnS/CaCO3 under conditions similar to that of many of the current HT/HP field developments in the North Sea and recent developments in the Gulf of Mexico.