Abstract

The effect of nitrate injection on microbial communities from either oil field production water facilities or from a simulated laboratory production water community and its associated bio-films was monitored with reverse sample genome probing (RSGP). Genomic DNA samples from these communities were labeled and used to probe hybridization membranes with 47 genomic DNAs, including thirty sulfate-reducing, one sulfide-oxidizing (CVO), and sixteen heterotrophic bacteria. These had been previously isolated from production waters and characterized with respect to microbial properties, degree of genomic cross hybridization, and partial 16S rRNA gene sequence for identification. The community genomic DNA probe was spiked with an internal standard (phage λ DNA) which was also immobilized on the filter in defined concentrations to allow for quantitative analysis with a phosphor-imaging analyzer. Environmental nitrate injection resulted in a large increase in the amount of the Campylobacter-like, sulfide-oxidizing bacterium, CVO and a decrease in the concentration of sulfide in the production waters. There was no increase in the amounts of the other 46 bacteria, including those capable of nitrate utilization but not sulfide-oxidation. The data indicate that the introduced, high-potential electron acceptor, nitrate, is used primarily for the recycling of sulfide to more oxidized forms.

Introduction

Oil production and recovery processes can be affected by the metabolic activities of complex microbial communities present in oil fields and their production facilities. Although these communities generally have an abundance of electron donors, such as aliphatic and aromatic hydrocarbons, electron acceptors are usually in short supply. Thus, metabolic activities can be influenced by the introduction of electron acceptors into the community (in order of increasing redox potential: carbon dioxide, sulfur and sulfate, ferric ions, nitrate, and oxygen). Water injection, used to improve oil recovery, can introduce sulfate into the community, subsequently boosting the fraction of sulfate-reducing bacteria (SRB). This in turn results in an increase in the sulfide producing capabilities of the population and can lead to well souring and corrosion. Although this is an undesirable side effect of water flooding, it demonstrates the metabolic potential available in the resident microbial community.

Other potential activities of the community include microbial production of gas, emulsifying biopolymers, blocking non-productive subsurface channels or decreasing sulfide levels to combat well souring and corrosion1. These types of activities are referred to as microbial enhanced oil recovery, or MEOR, and could potentially be realized by introducing an alternate electron acceptor into the community. Well souring and sulfide related corrosion could be managed by encouraging growth of the sulfide-utilizing component of a resident microbial community2.

It is known that there are organisms in oil fields that can couple sulfide oxidation to nitrate reduction2,3,4. Nitrate injection could stimulate these bacteria to scavenge sulfide2,3. However, some SRB are also known to use nitrate as an electron acceptor5,6,7. Furthermore, sulfide oxidation via nitrate reduction may result in increased sulfate and/or sulfur levels. Thus, injecting nitrate into an oil formation may increase the fraction of SRB in the community, thereby increasing the sulfide-producing capabilities of the community.

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