The enhancement of brine corrosivity due to the presence of acetate (or anions of other weak acids) in CO2 containing waters has been discussed frequently in recent literature. It has been confirmed that the observed increase in corrosivity is related to the equilibrium concentration of undissociated acetic acid (HAc) in the water and this in turn can be determined via solution speciation calculations – providing the composition of the water is known.

As with all weak acids (and regardless of the in-situ pH), organic acids will be present in both their undissociated form and dissociated form (anions) and since propionic and butyric acids essentially share the same pKa as acetic acid, their molar concentrations can be typically summed under the generic name of the dominant species, i.e., acetate ions (Ac-) and acetic acid (HAc).

This paper discusses the implications of significant levels of organic acids in produced waters (> 1mM) with regards fluid corrosivity. Specifically, how can we account for this in corrosion prediction modelling, as well as ensuring any testing programs conducted correctly reflect the water chemistry and speciation? Our approach to water speciation calculations and subsequent use of this will be discussed in relation to a specific example. The rate of corrosion is not determined solely by solution composition, however, we can’t understand or estimate the corrosion rate without considering the solution speciation.

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