Reservoir Souring: Mechanisms and Prevention

Reservoir souring is an example of a process that is initiated at the microbiological level, yet exerts an effect over an entire reservoir and its produced fluids within the production lifetime of a field. A characteristic of reservoir souring is that not all production wells show the same increases in hydrogen sulfide (H2S) concentration at the same time. There have been several biotic and abiotic mechanisms proposed for reservoir souring, such as thermochemical sulfate reduction and pyrite dissolution. The practice of produced water reinjection (PWRI) as part of a waterflood has the potential to increase H2S production beyond seawater-induced souring. Monitoring injection water sour water concentration can be particularly insightful if it is accepted that injected water contributes most of the H2S observed at production wells. Methods of controlling microbial reservoir souring are of three types: those that attempt to deal with the H2S after it has been generated and produced from the reservoir, those that attempt to prevent H2S from being formed, and those that reduce the mass of H2S that is generated. The surveillance of individual wells allows the relative degree of souring to be mapped for all of the producing wells in the field. A partial cure for souring may be achieved by selecting production chemicals that are added to the injection water to exclude those that provide additions to the nutrient pool available for sulfate-reducing bacteria (SRB). Treatment of injection water with nitrate is a relatively new technology for control of souring.