MEOR (microbial enhanced oil recovery) is known as low-cost and easy to apply EOR technology. It uses the fact that natural occuring microrganisms in the oil reservoir can be stimulated to create effects that may result in mobilizing oil and sweeping it to the production well. During bacterial stimulation, there is always the risk that the growth of sulfate-reducing microorganisms, specifically bacteria (SRB), which are present in many oil reservoirs, is also triggered and toxic and highly corrosive H2S could be produced. The paper reports an investigation of microbiological H2S production and its mitigation. The project is jointly run by Wintershall and BASF in the context of a MEOR project in a German oil field.
Different SRB inhibitors (one of them being nitrate) have been tested. The inhibitors have different modes of action regarding H2S inhibition, bacteriocidal activity and also show other side reactions. In growth experiments with original reservoir water, it was observed that some inhibitors can prevent H2S formation. It was concluded that very high nitrate concentrations (100 – 500 mM) are necessary for a long-term suppression of H2S up to 41 days. Furthermore, in cultures with nitrate, iron and calcite precipitations were observed as the result of chemical reactions induced by bacterial activity. Reactive flow simulations were performed using Toughreact to predict the impact of those precipitations on the permeability of the reservoir. In contrast to these findings, alternative inhibitors could successfully mitigate H2S production in long-term experiments without any complications. Based on this data and predictions, the use of an alternative SRB inhibitor is preferred over nitrate in the upcoming field pilot.