Q-193. Toluene in Crude Oil Is the Dominant Electron Donor for Denitrifying Bacteria in a Low Temperature Oil Reservoir Receiving Nitrate

A. J. Lambo, S. Larter, G. Voordouw;
Univ. of Calgary, Calgary, AB, CANADA.

The injection of nitrate to control reservoir souring by sulfate-reducing bacteria (SRB) is becoming widely accepted in the oil industry. Nitrate-mediated mechanisms for control of souring include competitive exclusion of SRB by heterotrophic nitrate-reducing bacteria (hNRB) for organic electron donors. Volatile fatty acids (VFA) are thought to be the dominant organic electron donor in high temperature reservoirs. This may not be so in low temperature oil reservoirs. The dose of nitrate required to eliminate sulfide is determined, not by the amount of sulfate but, by the amount of available organic electron donor. Thus, the use of oil as electron donor by hNRB from a low temperature reservoir that is being injected with nitrate was investigated. Oil-degrading hNRB enrichment was established in minimal medium with 10 or 11 mM NO3- by serial transfer of cultures initially inoculated with produced or source water. The reduction of NO3- to NO2-, NH4+, or N2 was determined in cultures initially amended with 2.3 to 10 mM NO3-/NO2-, and 2% light or heavy oil. NO3- reduction was also determined with 4 mM of selected hydrocarbons. The concentration of NO3-, NO2-, NH4+, and of VFA in produced water was determined by HPLC. The composition of oil and the concentration of toluene in oil were determined by GC-MS analysis. Reduction of NO3- occurred in cultures with produced water and oil. NO2- was the major product in cultures that reduced 11.2 mM NO3- with complete oxidation of toluene and partial oxidation of m-xylene. Significant oxidation of m-xylene and complete oxidation of toluene, in light oil, occurred in cultures that reduced 11.2 mM NO2- to N2. Higher concentration of NO3- inhibited NO2- reduction. Reduction of NO3- occurred with 4 mM toluene. Acetate and propionate (0.03 to 0.13 mM) were detected in produced water. Toluene concentration in light and heavy oil was 0.6 and 0.1 mM respectively. Based on the concentrations of nitrate reduced, the oxidizable toluene concentration in light oil was estimated to be 0.62 mM. These results confirm that VFA was negligible as electron donor. Instead, toluene appeared to serve as the main electron donor.