Chemical inhibition is a common method for controlling erosion-corrosion in offshore mild steel pipelines, tubing and pipe fittings. The paper introduces a new approach for predicting inhibited erosion-corrosion in mild steel pipes including the effects of flow and environmental conditions, sand production, and an oil phase.

When sand is produced, sand particle impinging on piping surfaces can decrease the efficiency of corrosion protection systems such as iron-carbonate scale formation or chemical inhibition and result in severe corrosion and even pitting. The need to be able to predict inhibitor performance under sand production conditions is particularly acute when the wells are deep or off-shore because of the difficulty in running coupon tests to evaluate the inhibitor efficiency at critical points throughout a system. Research reported in this paper is aimed at providing producers with information that will help them make decisions on the design of the well given advanced knowledge of the inhibition options and their predicted effectiveness under sand production conditions. Inhibition mechanisms and the relation between inhibitor concentration and inhibitor coverage are described using adsorption isotherms. The Frumkin isotherm showed the best fit to experimental data for an imidazoline-based inhibitor used in sand-free conditions. Flow loop tests indicated that sand particle erosion decreased the efficiency of the inhibitor. However, Frumkin isotherms modified to handle effects of erosivity, temperature, and oil phase were successfully fitted to erosion-corrosion data. Inhibitor adsorption isotherms, for both sand-free and sand production conditions, were integrated into a mechanistic model for prediction of CO2 corrosion rates as a function of inhibitor concentration and good results were obtained as compared with data.

Results of this study show that the inhibitor adsorption isotherm, modified to handle effects of sand production, temperature, and oil phase can be a valuable tool for predicting inhibited metal loss rates under sand production conditions.

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