Corrosion inhibitor chemicals are an established successful means of protecting oil and gas producing assets from damage caused by internal corrosion.1,2  Over life of field, these assets will often have to operate under significantly different conditions compared to the original basis of design. Long term processes such as increasing water production and the break-though of injection water in producing wells typically result in a gradual change in system conditions and fluid compositions. Alterations or additions to the system, such as through brown-field development or addition of a subsea tieback, can result in significant and rapid changes to system conditions and fluid compositions.

Corrosion modelling software is often used to predict how these changes will affect the severity of the corrosion risk in the system.3  Although useful in predicting the anticipated corrosivity of the fluids, the models do not adequately take into account the effect which these changes may have on the corrosion inhibitors used to protect the system. Factors such as brine composition can have a minor influence on predicted corrosion rate but a significant effect on inhibitor performance; other factors such as composition of the hydrocarbon phase may dramatically influence inhibitor performance but are not accounted for in standard corrosion modelling software.

This paper describes a laboratory study evaluating the performance of corrosion inhibitors under North Sea oilfield conditions. The paper details how inhibitor performance is affected by changing brine composition over time as a consequence of injection seawater breakthrough from secondary recovery. The study also describes when the overall hydrocarbon composition is altered, due to commingling of fluids in the topsides process as a result of a new subsea tieback to a gas-condensate field being brought on-line. The data presented demonstrates the importance of evaluating the suitability of a corrosion inhibitor under both current conditions and potential future conditions when selecting the most suitable inhibitor for the application. It also highlights that changes in certain factors which do not significantly change fluid corrosivity as predicted by corrosion modelling, such as brine and hydrocarbon composition, can have a significant detrimental effect on corrosion inhibitor performance. Finally, the work reinforces the assertion that inhibitor revalidation is a key requirement, whether system conditions change gradually, for example with injection water break-through or instantaneously through drilling and production of new wells.

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