The Impact of Dissolved Organic-Carbon Type on the Extent of Reservoir Souring

Abstract

Abstract A reservoir souring forecasting model was presented previously. This model utilized a generic algorithm to determine, via history match, the extent of H2S biogeneration by sulfate-reducing bacteria (SRB) within the waterflooded reservoir such that the production to the surface could be forecasted. The generic algorithm assumed a decline in SRB-usable organic nutrients as a function of water flow within the reservoir. However, nutrient utilization by non-SRB (e.g., nitrate-reducing bacteria) could not be differentiated from the SRB with this model. The current study has incorporated stoichiometry of microbial sulfate and nitrate reduction utilizing both volatile fatty acids (VFA) and BTEX components as dissolved organic carbon (DOC) substrates. This paper presents the updated algorithms and discusses partitioning of the DOC components between oil and water within the reservoir. VFAs such as acetate have historically been assumed to be the favored nutrient source by SRB, but recent field experience has suggested that other DOC sources are contributors. BTEX components, especially toluene, are shown with the model to potentially have a large impact on souring in spite of their limited solubility in the reservoir water. While nitrate usage to control reservoir souring is becoming a "standard" practice, H2S generation in reservoirs with conditions that support BTEX as an SRB nutrient might not be sufficiently inhibited with nitrate.