Measurement and interpretation of corrosion inhibitor residuals in a mature offshore gas/condensate field could not be reconciled with field data leading to the identification of a potential infrastructure integrity threat that mandated understanding. The field had recently transitioned from buffered pH operation to "natural" pH operation of the monoethylene glycol (MEG) loop (alongside addition of corrosion inhibitor) due to carbonate scaling caused by formation water influx. An investigation was initiated to determine the corrosion inhibitors behavior throughout the production system with focus on demonstrating the effectiveness of the inhibitor. The investigation included extensive laboratory corrosion testing using field and synthetic fluids, residual determination in field samples using liquid-chromatography mass-spectrometry (LC-MS), field implementation and confirmation of appropriate actions.
Upon completion of the investigation it was found that the intended corrosion inhibitor active components were not concentrating up in the MEG loop but were strongly partitioning to the natural gas condensate phase. This was leaving the topside facilities "under-inhibited". Obscuring this conclusion was the concentration of other benign (not corrosion inhibitor) active components present in the inhibitor formulation at very low concentrations which were giving falsely high inhibitor residuals.
After changing the inhibitor injection philosophy from batch-wise to continuous, LC-MS residuals have continued to confirm the partitioning behavior in field operation without the introduction of an unmanageable secondary property concern due to the inhibitor. Further online and laboratory corrosion studies have confirmed the integrity of the production system as proof of the effectiveness of the inhibitor. These key lessons learned challenge operators and chemical vendors to consider the MEG circuit chemistry more carefully during chemical qualification to ensure that chemical behavior is understood both before field application and that it is confirmed once applied.