Souring of Reservoirs by Bacterial Activity During Seawater Waterflooding

Abstract

Abstract Waterflooding with seawater for pressure support is to be employed in many of the deepwater Gulf of Mexico fields. Seawater flooding has been employed in the North Sea oilfields for over thirty years and, therefore, this region serves as a valuable source of case histories of the problems that can be encountered. It is now accepted that due to inadequate control of sulfate-reducing bacteria (SRB) the majority of the North Sea seawater floods resulted in souring of the reservoir. In some cases, hydrogen sulfide concentrations in the production wells have risen to the point where either the well has had to be shut-in or sulfide scavenging chemicals have had to be applied in order to prevent excessive corrosion. Where souring was not predicted, some production systems have had to be upgraded for sour service materials. These remedial actions have resulted in very significant unplanned costs. Although there is no formal published economic data, it is clear that over the history of the North Sea, the cost of lost production and remedial action has been $ 100's millions. This paper reviews the North Sea experience and discusses the models developed in an attempt to predict reservoir souring. The paper discusses the application of chemical biocides attempted to kill the SRB; the injection of nitrate to stimulate other bacterial populations to compete with the SRB for essential nutrients; and the potential for sulfate removal technologies to reduce sulfate and minimise sulfide production. A strategy for the development of future models to help predict and avoid reservoir souring in future seawater floods is also discussed.