Why So Low Free Acetic Acid Thresholds in Sweet Corrosion at Low P_CO2? Available to Purchase

In condensed water, the free acetic acid content (HAc) is physically dissolved from the gas (or oil) phase, and this induces an overacidification of the CO₂ solution. When HAc becomes the dominant acid, this increases the solubility of corrosion products, i.e. the iron content Fe_sat at the saturation of corrosion products in FeCO₃. The empirical field thresholds of 1 and 0.1 mM HAc actually correspond to the same acidifying power as 2 and 0.2 bar of pure CO₂, i.e. nearly the API limits of 1958… which nobody has ever considered as low !

In reservoir water, the free HAc is chemically produced from the total acetate content (Ac_tot) and dissolved CO₂, and this does not practically change anything on the Fe_sat. It just occurs that in strongly buffered waters, the HAc value in mM is always close to that of Fe_sat in ppm, so that the free HAc is not too bad a marker of the corrosion products solubility. Thresholds are then low because the corrosion layers can be sufficiently protective only at low Fe_sat.

Since the morphology and location of corrosion damage in downhole corrosion had long shown an ubiquitous effect of this solubility, as well as the more recent mechanisms on the protectiveness of corrosion layers, it is therefore suggested to replace the HAc thresholds by Fe_sat thresholds in the field data bases, and to look for experimental protocols in the laboratory which would be sensitive to the same Fe_sat thresholds as the documented experience on downhole or Top of Line corrosion.

In this respect, the famous Wytch Farm failure is not related to any acetic acid issue, but to a solubilization of corrosion products induced by a CaCO₃ saturation in the gathering line. This indeed comes from an unprecedented combination of high Ca and low bicarbonate content B₀ in the reservoir (high Ca/В₀ ratio) and particularly low P_CO2 levels in a trunk line under "full well stream transfer".