Sulfate-reducing Bacteria and Their Activities in Oil Production

Abstract

Summary. This paper presents an overview of the microbiology of sulfate-reducing bacteria (SRB) and their detrimental effects in oil technology and summarizes a study on SRB in an oil field. SRB are a group of specialized microorganisms that occur in aqueous environments in the absence of oxygen. The main nutrients for SRB are simple organic acids and molecular hydrogen (H2) from decomposing natural organic matter. The nutrients are oxidized, with sulfate being reduced to sulfide (hydrogen sulfide, H2S). The formed H2S is the principal agent in the disastrous effects caused by SRB. It contaminates gas and stored oil, precipitates ferrous sulfide that plugs injection wells, and promotes precipitates ferrous sulfide that plugs injection wells, and promotes corrosion of iron and steel in the absence of oxygen (anaerobic corrosion). Another principal mechanism by which SRB are involved in corrosion is their ability to depolarize iron surfaces by consumption of cathodically formed hydrogen. The postulated mechanism in anaerobic corrosion are briefly explained. As an example for a microbiological study of SRB in oil technology, examination of an oil treater in a field in northern Germany is presented. On the basis of measured growth characteristics of the SRB, presented. On the basis of measured growth characteristics of the SRB, possibilities for controlling their activity are discussed. possibilities for controlling their activity are discussed. Introduction Biological sulfate reduction by SRB is the only known process by which, in aquatic environments of moderate process by which, in aquatic environments of moderate temperatures (O to 75 degrees C [32 to 167 degrees F]), H2S is formed from sulfate. In sediments of ponds, lakes, and marine environments, SRB are usually part of the indigenous community of microorganisms and are rather inconspicuous in nonpolluted waters. In oilfield water systems, however, SRB cause serious problems:corrosion of iron in the absence of air (anaerobic corrosion),precipitation of amorphous ferrous sulfide that, by precipitation of amorphous ferrous sulfide that, by plugging, diminishes the in injectivity of water injection wells, plugging, diminishes the in injectivity of water injection wells,contamination of fuel gas with H2S, andcontamination of stored fuel oil with H2S. Furthermore, H2S is extremely toxic if inhaled: it easily escapes from contaminated waters and may accumulate under poorly ventilated conditions. It is usually recognized by its distinctive, unpleasant odor, but high concentrations anesthetize the sense of smell. The objective of this paper is to present an overview of" the biological features of SRB and of their activities in oil technology with emphasis on anaerobic corrosion. We also include results from our studies on SRB in an oil field in northern Germany. Microbiology of SRB SRB are an assemblage of specialized bacteria that thrive in the absence of oxygen and obtain energy for growth by, oxidation of organic nutrients, with sulfate being reduced to H2S. The biological significance of this form of life is best understood within the overall natural decomposition process carried out by living organisms. Processes in Biological Decomposition. The natural Processes in Biological Decomposition. The natural decomposition of organic material in our biosphere through a food chain of oxygen-breathing (respiring) organisms-namely, animals, fungi, and bacteria-is a well-known process. Biochemically, respiration is a transport of reducing power (hydrogen, "electrons") from the organic nutrients (organic substrates, electron donors) being oxidized to oxygen (electron acceptor) being reduced (Fig. la). Respiration liberates the energy that has been originally conserved in the organic matter during photosynthesis by green plants and cyanobacteria photosynthesis by green plants and cyanobacteria (bluegreen algae). In the oxygen-breathing organisms, the liberated energy is used for maintenance of their living structures and for growth-i.e., a net synthesis of their own cell material from the nutrients. Thus every organic substrate of a respiring organism is partly decomposed for obtaining energy and partly converted into new cell material. These functionally distinctive reactions in living organisms are designated catabolism or dissimilation (energy metabolism) and anabolism or assimilation (cell synthesis), respectively. An amount of biomass initially synthesized by photosynthesis is diminished more and more by passing through the food chain because of respiratory losses. The final result is a reoxidation (mineralization) of the chemically complex biomass to CO7, H2O, and other minerals (Fig. 1a). These inorganic end products are used by green plants and cyanobacteria for products are used by green plants and cyanobacteria for photosynthesis of new organic substances (the natural cycle photosynthesis of new organic substances (the natural cycle of matter). The total reoxidation of biomass is possible only if the conditions are aerobic- i.e., if sufficient oxygen is present. If biomass gets into stagnant or rather closed water present. If biomass gets into stagnant or rather closed water systems where the gas exchange with the atmosphere is limited, dissolved oxygen may be completely consumed. JPT p. 97