Thermal Decomposition of Thiocyanate Corrosion Inhibitors: A Potential Problem for Successful Well Completions

Abstract Salts of thiocyanate (SCN-) have been widely used as corrosion inhibitors in halide-based brine completion/packer fluids for many years. When applied, they have unique characteristics over conventional amine-based inhibitors (excellent solubility in high-density brines and ability to control corrosion of brines on carbon steels and low alloy steels at high temperatures). However, SCN- may decompose at high temperatures to produce H2S or free sulfur. When SCN- thermal decomposition occurs in zinc bromide (ZnBr2)-based completion/packer fluids, H2S or byproduct of free sulfur can react with zinc ion to form zinc sulfide (ZnS) scale. The ZnS scale can cause formation damage and plug downhole tubular assemblies. Additionally, sulfide (S2-) can induce sulfide stress corrosion cracking (SSC) of high-strength and high-alloy steels under downhole environments. The presence of ZnS scale and SSC of downhole tubulars requires subsequent workovers to restore well productivity. This paper presents the results of laboratory evaluation on thermal decomposition of SCN- inhibitors. Evaluation primarily focuses on the effect of temperatures on decomposition of thiocyanate inhibitors in ZnBr2 brines. Heat-aging tests of ZnBr2 brines inhibited with SCN- inhibitor, along with SEM/EDS techniques, were conducted to determine the decomposition temperature of SCN- and its potential to form ZnS scale. Results indicate SCN- thermally degrades at temperatures lower than those published in the literature. Evaluations of stress corrosion cracking (SCC) behavior of a high-strength and high-alloy steel in the presence of NaBr and CaBr2 brines treated with a SCN- inhibitor also are presented. These results reveal the decomposition of SCN- could cause SCC of high-strength and high-alloy steels even in chloride-free brines such as NaBr and CaBr2.