A North Sea Field has been water flooded with commingled produced and aquifer water since 1997. Low- sulphate source waters were selected for injection in order to mitigate barium sulphate scaling and reservoir souring development. Small volumes of seawater are included in the injection water due to operational reasons, but the average sulphate concentration of the overall injection water does not exceed 20 mg/l. Aquifer and produced water has historically been injected with minimal treatment due to the high permeability of the clastic reservoirs.

Despite the low sulphate concentration of the injection water there has been souring development over field life. The reservoir temperature of 104 F is sufficiently low to allow microbiological activity throughout the reservoir. Microbiological monitoring in the produced fluids indicates the presence of sulphate-reducing bacteria (SRB), but the low sulphate availability limits SRB activity.

The injection water was treated with continuous biocide with the intention of controlling SRB activity within the reservoir, and reservoir souring simulations were carried out to investigate the effectiveness of this treatment. The simulation results indicated that the levels of H2S present in the produced water were consistent with the availability of sulphate in the injection water, and that the biocide treatment was not limiting SRB activity. The continuous biocide treatment was discontinued and no subsequent increase in H2S generation was observed in the field, demonstrating that souring development was sulphate limited.

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