Currently, the application of nitrate/nitrite is considered one of the most promising souring mitigation solutions during Produced Water Re-Injection (PWRI). Norske Shell tested nitrate based souring mitigation as part of a PWRI feasibility study in the Draugen field. Prior to the pilot, the application of both nitrite and nitrate had been tested on Draugen using a Souring Mitigation Cabinet (SMC) specially developed to mimic the microbial activity in the near well reservoir. From this pre-study, the dosage of nitrate was selected based upon bio-available carbon and the required stoichiometric concentration of nitrate. During the PWRI pilot, corrosion rates were measured continuously Downstream (D/S) the Water Injection (WI) pumps in the High Pressure (HP) system, and after three months testing, significant increases in corrosion rates were seen. These were thought to be related with the addition of nitrate to the Produced Water (PW). To investigate this more closely, the SMC was modified by including a dedicated Low Pressure (LP) Corrosion Sidestream Monitoring (CSM) system. The results obtained in the CSM system verified the results obtained in the HP corrosion monitoring system in place on Draugen. The results of the Draugen PWRI pilot also showed that the addition of nitrate to the PW was efficient to control near-well reservoir souring. However, as mentioned above the corrosion rates increased logarithmically when nitrate was used. At the same time, a few ppm nitrite was seen in the nitrate treated PW water. The addition of biocide resulted in an instantaneous decrease in nitrite and corrosion rates to background levels. The result clearly indicates that bacterial activity, resulting from the addition of nitrate, was the causative agent of the increased corrosion seen in the Draugen PW treated for re-injection. It was concluded that the increase in corrosion rates most likely was Microbiologically Influenced Corrosion (MIC). Details of the PWRI pilot and the observed effects when applying nitrate are discussed in this paper. Because of the negative side-effects observed when applying nitrate to PW, Norske Shell re-evaluated the requirement to mitigate souring with nitrate. The testing during the PWRI pilot showed a low tendency to develop SRB activity, probably because of the low VFA concentration in the PW. Consequently it was decided to terminate the application of nitrate to PW on Draugen and control bacterial activity in the surface facilities with biocide. As nitrate is still promising to be applied in PW in other field applications, dedicated research has been initiated to learn more about the mechanisms leading to the increased corrosion rates seen when applying nitrate in PW. Introduction The Draugen field The Draugen field is situated in block 6407/9 at Haltenbanken and the platform is operated by A/S Norske Shell E&P (Exploration and Production). Haltenbanken is considered to be an environmental sensitive area on the Norwegian Continental Shelf (NCS). The water depth varies between 240 and 290 meters. Oil and gas is produced from a sandstone reservoir, consisting of the Garn and the Rogn formations (Figure 1) and is situated at 1610 meters the below seafloor.The bottom-hole temperature is 71°C and the pressure is 165 bars. Oil production started in October 1993 and is predicted continuing until year 2025. Seawater injection, at an average of 40 000 m3/day, was initiated in 1994 to maintain the pressure in the reservoir. In 2002, significant water production on a continuous basis started, and in September 2006, 50% water cut was reached. The water cut in later field life is expected to reach >90%. The maximum water production is expected around year 2012 (34–37 000 m3/day) and a rather flat profile is expected until 2024. As the water production increases, it is expected that the existing produced water system would not meet the zero discharge goals on the NCS. PWRI was considered a promising solution. As part of the Norwegian Government Commitment to Zero Harmful Discharges (ZHD) by 2005 and the Shell Group Minimum Standard Targets incorporating PW injection, A/S Norske Shell adapted a produced water management strategy with a ZHD objective. As a result, a PWRI pilot was planned in the Draugen field.