Carbonate and Sulfide Mixed Scale: Corrosion, Prediction and Control

Abstract

Abstract Sulfide and carbonate mixed scales are ubiquitous in oilfield production and injection system with low corrosion resistant carbon steel equipment and pipeline. Previously, such conditions were generally treated as a corrosion problem rather than a scale problem, and the interactions between steel corrosion and mineral scale formation remain unclear. In this study, the iron sulfide and iron carbonate mixed scaling and corrosion behaviors were investigated simultaneously under simulated produced water environments. The influence of carbonate to sulfide ratios and calcium concentration in the brine was investigated. It was found that the iron sulfide (FeS) scale was always formed on the mild steel surface under various brine compositions due to the fast kinetics of the sulfide scale precipitation and sour corrosion. While with the presence of calcium, the carbonate scale was easier to form, and this carbonate scale layer would be crucial to promoting calcium carbonate deposition. The iron carbonate (FeCO3) precipitation kinetics was a diffusion-controlled reaction that can be accelerated by higher temperature and calcium concentrations. The conventional scale inhibition and corrosion inhibition methods were also tested. The combination of scale inhibitor, corrosion inhibitor, and dispersant chemical combos successfully prevents the deposition formation on the mild steel surface and significantly reduces the corrosion rate. This study demonstrated the complicated interaction between the sulfide and carbonate scale and also bridged the scale and corrosion, which could help to develop a better scale and corrosion control strategy under complicated field conditions.