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Methanogenesis from Oil and the Impact of Nitrate on the Oil-field Sulfur Cycle
a Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
Key Words: oil field • sulfur cycle • sulfate-reducing bacteria • nitrate-reducing bacteria • souring • methanogenesis • sulfide
Address for correspondence: Department of Biological Sciences, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, T2N 1N4, Canada. voordouw{at}ucalgary.ca
Our society depends greatly on fossil fuels, and the environmental consequences of this are well known and include significant increases of the CO2 concentration in the earth's atmosphere. Although microbiology has traditionally played only a minor role in fossil-fuel extraction, two novel key discoveries indicate that this may change. First, the realization that oil components can be converted to methane and CO2 by methanogenic consortia in the absence of electron acceptors (oxygen, nitrate, sulfate) explains how much of the world's oil has been biodegraded in situ. In addition to inorganic nutrients, only water is needed for these methanogenic conversions. Hence, continued methanogenic biodegradation may have shaped the heavy-oil reservoirs that are so prevalent today. The potential to exploit these reactions, for example, by in situ gasification, is currently being actively investigated. Second, injection of nitrate in oil and gas fields can lower sulfide concentrations. High sulfide concentrations, caused by the action of sulfate-reducing bacteria (SRB), are associated with increased risk of corrosion, reservoir plugging (through precipitated sulfides), and human safety. Nitrate injection into an oil field stimulates subsurface heterotrophic nitrate-reducing bacteria (hNRB) and nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB). Nitrite, formed by these NRB by partial reduction of nitrate, is a strong and specific SRB inhibitor. Nitrate injection has, therefore, promise in positively controlling the oil-field sulfur cycle. There is now more interest in and potential to apply petroleum microbiology than there has been in the past, allowing microbiologists to contribute to a sustainable energy future.
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