Abstract

Sulfate-reducing bacteria (SRB) can be commonly found in oil reservoirs causing H2S formation and subsequently corrosion and souring issues in associated production/injection water systems. To control and mitigate the microbial H2S formation, the production-injection loop should be monitored and the results should be implemented for effective control of H2S formation. One mitigation measure can be the injection of a biocide into produced and/or injected water killing or inactivating SRBs and other microorganisms responsible for the souring and fouling. However, the challenge often occurs to define the suitable injection spot and the required biocide concentration; not well defined biocide concentration and/or injection location can even favor SRB proliferation causing for example fouling of tanks or filters, which themselves can continously "inoculate" the system.

At the water treatment plant of of one of Wintershall's onshore oil field in Northern Germany, formaldehyde injection was started in the end of 2015. The scope of the project was to investigate the effectivness of the formaldehyde regarding the bacterial- and SRB count and to analyse the MIC (Microbially influenced corrosion) and souring potential in and around the treatment plant including the waterflood injection line. Water samples were taken under sterile and anoxic conditions at various points including production lines from the three fields, selected spots of the water treatment system and injection lines to the fields. The samples were analysed for key chemical- (pH, salt content, sulfate, formaldehyde, organic acids, H2S) and microbial parameters (bacterial, archaeal via qPCR (quantitative polymerase chain reaction) and SRB cell numbers via MPN (Most Probable Number). The obtained data can not only help the operator to optimize the biocide injection to reduce souring and fouling, but also to better understand the complex microbial growth occuring in the production facilities.

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