Abstract
Nitrate treatment has been widely used in various seawater injection projects to treat biologic sulfate reduction or reservoir souring. To design a promising nitrate treatment plan, it is essential to have a comprehensive understanding of reactions that represent the microbial communities of the reservoir and mechanisms through which the souring process is inhibited. We employ a new approach of evaluating different reaction pathways to design reaction models that reflect governing microbial processes in a set of batch and flow experiments. Utilizing the designed models, we suggest dissimilatory nitrate reduction to ammonium is the main reaction pathway. Additionally, we illustrate nitrite inhibition is the major mechanism of nitrate treatment process; independent of nitrate reduction being autotrophic or heterotrophic. We introduce an inhibitory nitrate injection concentration that can inhibit souring regardless of nitrite inhibition effect and the distance between injection and production wells. Furthermore, we demonstrate that the ratio of the nitrite-nitrate reduction rate can be used to estimate nitrate treatment effectiveness. Our findings in regard to importance of nitrite inhibition mechanism and the inhibitory nitrate concentration are in accordance with the field observations.
Highlights
While seawater injection in oil reservoirs is considered one of the most successful recovery methods, in some cases, high concentrations of sulfate leads to microbial induced reservoir souring by sulfate reducing bacteria (SRB), a process in which some amount of hydrogen sulfide (H2S) appears in the producing fluid from a reservoir that was initially sweet[1,2]
Observation of ammonium in production water samples cannot be regarded as sole evidence for dissimilatory nitrate reduction to ammonium (DNRA), since ammonium in oil reservoirs can be derived from abiotic origins such as presence of clay minerals and/or alkali-feldspars[20,21]
In the case of sulfate reduction, we examine if sulfate is directly reduced to sulfide, or if it is first reduced to elemental sulfur and sulfide
Summary
While seawater injection in oil reservoirs is considered one of the most successful recovery methods, in some cases, high concentrations of sulfate leads to microbial induced reservoir souring by sulfate reducing bacteria (SRB), a process in which some amount of hydrogen sulfide (H2S) appears in the producing fluid from a reservoir that was initially sweet[1,2]. Cheng et al.[16] used TOUGHREACT to simulate experimental work of Engelbrektson et al.[17] They considered denitrification pathway for nitrate reduction by NRB or NRSOB and they assumed sulfide is only oxidized to sulfate.
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