Water sample analysis and data interpretation are critical to internal corrosion prediction, especially with respect to CO2/H2S and organic acids corrosion. Misunderstanding of the water data is a frequent source of error in corrosion prediction. This paper provides guidelines on using water chemistry data for corrosion prediction, in particular, the issue of how to deal with organic acids and alkalinity including sampling analysis and interpretation.
• The theoretical background of pH calculation and water chemistry is introduced briefly.
• Current issues with water analyses and their interpretation for corrosion prediction are reviewed.
• The advantage and disadvantages of various pH calculation options with different water data inputs in the corrosion model are introduced.
• Different water sample analyses methods are reviewed
• Guidelines on interpreting lab data and approaches for calculating total alkalinity are provided for practical use.
Various corrosion prediction tools for CO2/H2S corrosion have been developed in the past thirty years [1]-[5]. For corrosion analysis in oil and gas production, the water chemistry largely determines the corrosion rate which is mainly driven by in-situ pH.
The in-situ water or brine is pressurized with acid gases (CO2/H2S) which results in a decrease in pH and typically an increase in the corrosion rate. Water samples are typically taken from the process stream depressurized (with acidizing by CO2 and H2S reversed) and analyzed. So, there are two items that require attention:
a) What are the pressurized conditions (use as input conditions)?
b) What is the water chemistry of the water sample?
A key element for water analysis is total alkalinity, which can be explained as the capacity of an aqueous electrolyte to resist acidification. This factor determines the effect of the pressurization with acid gases on the shift in pH (from a water sample to in-situ conditions).
Another relevant aspect is the presence of organic acids species. Organic acids are typically analyzed as their total (which covers both ion and anion /acid and acetate). Organic acid anions are part of the alkalinity. Their concentrations are dependent on the pH of the water which is impacted by the partial pressures of the acid gases. Moreover, corrosion triggered by organic acids has some different effects compared to CO2/H2S corrosion only. The last part of the water analysis is the influence of salts and salinity (TDS Total dissolved solids). These trigger different effects on the corrosion:
• General presence of iron (Fe2+) reduces the corrosion rate, increases pH, and reduces corrosion product solubility (less effect on H2S corrosion due to the very low solubility of FeS).
• High TDS can reduce the solubility of H2S and CO2 in brines (salting-out effect)
• Chlorides impact localized corrosion in sour regime.
• Chlorine can influence corrosion significantly at low O2 conditions (<100ppb)
• TDS, sulphate, carbon, nitrogen, and microbial activity are all relevant to MIC (microbiologically influenced corrosion).
• Biocide and inhibitor concentrations measured in the brine can be a good indicator of the efficiency of corrosion mitigation (residuals and other aspects can have an impact on CI-IOW[Integrity Operation Window]). This is out of scope here and is addressed elsewhere[6],[7].
• Ca2+ at saturation conditions with CaCO3 indicates precipitation of CaCO3 and higher alkalinity would be expected upstream.
