Studies of steel corrosion in oilfield brines

Abstract

Experiments used both simulated brines (3% NaCl + sodium acetate (NaOAc), containing free acetic acid (HOAc) or saturated with CO₂) and brine samples from the oilfields.  The project requires a detailed understanding of the speciation within the brines.  This was examined using the modified version of the computer programme (PHREEQC 2.2) available on the World Wide Web and thermodynamic data from the literature.  It was demonstrated quantitatively that the influence of other species in the brine, particularly bicarbonate, which is also a component of oilfield brines, must be considered.  The PHREEQC 2.2 computations have been tested by examining the voltammetry of the solutions at a Pt rotating disc electrode RDE) and a good fit between the predicted and experimental limiting current densities was found.  The voltammetric studies demonstrated that the solutions containing NaCl + NaOAc + CO₂ and NaOAc + HOAc with the same pH behave in the similar way.  It was confirmed that the rate of corrosion is largely determined by the cathodic current at the corrosion potential.  The cathode reaction leading to corrosion is hydrogen evolution that results from several parallel electron transfer reactions, the reduction of proton, undissociated acetic acid, carbonic acid and carbon dioxide through hydration mechanism.  The acetate-enhanced corrosion will be important when the contribution to the corrosion current from the reduction of acetic acid is the dominant process.

Just recently it was found that the acetate can influence the rate of corrosion in two different ways – it can lead to the formation of the free acetic acid and it can also catalyse the hydration of carbon dioxide. As a result, two different acids can form, acetic acid and carbonic acid.  Both are electroactive and can act as proton donors.  Under different conditions they can predominate over each other in the overall cathodic reduction process enhancing the rate of steel corrosion.  At low concentrations of acetate, which corresponds to the conditions met in many oilfields, the carbon dioxide reduction via hydration is the dominant process.  At high concentrations of acetate, the reduction of acetic acid takes priority.

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