Learnings from reservoir souring treatment by nitrate injection in the Halfdan oil field

Over the last few decades, some of the seawater injection projects in the North Sea have been suffering from reservoir souring problem. The source of this problem has predominantly been the high concentration of sulfate in the seawater. In some research works nitrate treatment has been introduced as a good strategy for reducing reservoir souring, although in a few investigations it has shown to be ineffective. In this research work, a comprehensive investigation is conducted based on available literature and field data in order to illustrate the source of souring and performance of nitrate injection process. It is postulated that seawater does not initiate reservoir souring only by transporting sulfate ions, but also by transporting other essential components such as trace elements, and/or alteration of some parameters like temperature. Additionally, it is argued that nitrate treatment is an effective mitigation measure if a sufficient concentration of nitrate is added to the injection water. The treatment concentration can be defined as the concentration that provides a local minimum concentration of nitrate during the entire production lifetime wherever seawater exists in the chalk reservoir. This local value can be defined as the smallest nitrate concentration at which nitrate mitigation is active.