Modelling Development and Experimental Methodology Design for Iron Sulphide (FeS) Scale Prediction

Iron sulphide (Fes) scale is widely present in both oilfield and geothermal systems and causes a range of production and Health and Safety problems. Although substantial progress has been made, continued efforts are needed to precisely understand and predict FeS scaling. The aim of this work is to present a simple robust model for FeS scale experiments in the laboratory. This model predicts saturation ratios (SRs) and masses of the formed iron sulphide scale, final solution compositions and final pH levels, for specific lab experiments. This model is verified by comparing results with carefully designed experiments which are monitored by a range of analytical experimental techniques, e.g., ICP-OES, ESEM/EDX and XRD (explained in the text). These analytical methods allow us to analyse for all the components present, such as initial and final [Fe2+], aqueous [H2S] levels etc, and they also give direct information on the morphology of any precipitates formed, either as crystalline or amorphous solids. Experiments were performed in an anaerobic chamber since we were using iron (II) ions (Fe2+) from both iron (II) chloride tetrahydrate and also ammonium iron (II) sulphate hexahydrate. The latter, known as Mohr salt, is thought to be a more reliable source of Fe2+. In fact, we found different crystallographic types of FeS scale precipitate from each of these 2 irons (II) salts. In addition, we observed that when FeS particles are precipitated from the solution, then under some circumstances some FeS particles can remain in colloidal suspension. This has implications for the level of measured "[Fe2+]" by ICP which measures the total Fe in solution, i.e., the free Fe2+ ions as well as any suspended colloidal FeS. The results show that there is a quantitative agreement between the experimental results and the predictions of the model in determining final pH of solution, final [Fe2+] and mass of FeS precipitate. However, it was also noted in some cases where discrepancies occurred – e.g., in [Fe2+] level – this may be ascribed to the colloidal nature of FeS scale. The information presented in this study will help production chemists to understand the chemical formation of FeS in laboratory testing, and this will assist in the selection and design for future scale inhibitor treatments. It should be also noted that the novelty of this research study is to (i) contributes to our understanding the morphology of generically different structures of FeS which can assist production engineers with selection and design for future inhibitor treatments. (ii) compares a stand-alone model with commercial prediction software and highlights some differences between them, (iii) then compares both models directly with experiment, and (iv) it gives the lab experimentalist a simple tool which models the FeS formation quite well (and is freely available as a spreadsheet). Furthermore, the codes of commercial scale prediction software are not "open source", and the precise formulation of the sulphide chemical equations is often not accessible to the user. However, in our model, the users of code can get access to the equilibrium equations simply and use it as a handy method to predict FeS scale formation before performing their bottle tests.