Laboratory Evaluation and Mechanistic Understanding of the Impact of Ferric Species on Oilfield Scale Inhibitor Performance

Scale inhibitor chemicals are widely used in oilfield operations for mineral scale control. However, the presence of iron species in oilfield produced water can considerably impair the performance of scale inhibitors. To date, few studies have been conducted to experimentally investigate the mechanism of iron effect on scale inhibitors. Although Fe(II) is the major form of iron species in oilfield produced water, Fe(III) can be formed in produced waters due to oxidation of Fe(II). In this study, Fe(III) effect on various scale inhibitors was evaluated by examining the inhibitor performance to control barium sulfate (barite) scale formation. This study finds that Fe(III) can significantly impair the performance of both phosphonate and polymeric inhibitors with an iron concentration below 1 mg L–¹. Moreover, the mechanism of the influence of Fe(III) on scale inhibitors was studied by investigating the adsorption capacity of ferric hydroxide solid of phosphonate scale inhibitor and also examining the efficacy of the unadsorbed inhibitor in aqueous solution. It can be concluded that the Fe(III) impact on phosphonate inhibitor is due to the adsorption of inhibitor to the surface of ferric hydroxide solids. Furthermore, two common chelating chemicals (EDTA and citrate) were tested for their effects in reversing the adverse impact of Fe(III) on scale inhibitor. Experimental results suggest that citrate is more effective than EDTA in reversing the detrimental impact of Fe(III) despite the fact the EDTA is a stronger chelating agent. The mechanisms of these two chelating chemicals in terms of interacting with Fe(III) were discussed and compared. This study provides the theoretical basis and technical insights for oilfield iron control to minimize iron impairment on scale inhibitor performance.