Mechanisms, New Test Methodology and Environmentally Acceptable Inhibitors for Co-deposition of Zinc Sulfide and Calcium Carbonate Scales for High Temperature Application

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.