No abstract

Anaerobic bacteria were identified in live drilling muds and cores from nine different North Sea and Irish Basin oilfields, varying in depth from 3500 to 15000 ft, and at temperatures up to 150°C. The anaerobic bacteria may be introduced into the reservoir during drilling operations or injection of water, but in many cases the bacteria are indigenous to the oilfield reservoirs. Confirmation of the indigenous anaerobic bacteria was made using molecular biology techniques (16S rDNA sequence analysis), comparing microbial populations present in the blank drilling mud as supplied to wellsite, in the live drilling mud taken during coring, and in the live core. The role of anaerobic bacteria in oilfield diagenesis is not fully understood, though pyrite precipitation, and exopolymer and H2S gas production were noted in this study, up to temperatures of 95°C.

Several metabolic types of sulphate‐reducing bacteria, including mesophiles and thermophiles, were successfully obtained from four samples from two different North Sea oil fields. The Gram‐negative, rod‐shaped, sulphate‐reducing strains MM6, EF2, FM2, and GF2 were isolated from drain water, and from drilling muds E, F, and G, respectively. All four isolates grew on lactate, pyruvate, glycerol, and ethanol, with optimal growth temperatures between 25 °C and 35 °C and at salinities between 0 and 5% NaCl. They were capable of using sulphate, thiosulphate or sulphite, but not nitrate, as electron acceptors. These isolates were tentatively identified to be the same species of Desulfomicrobium based on physiological and biochemical characterization, and 16S rRNA gene analysis. Therefore, the same Desulfomicrobium species was present in different samples from distant oil fields. This result suggests that these microorganisms are likely to be widespread throughout oil field systems, and possibly play an important role in the generation of sulphide.

The sulphate-reducing bacteria (SRB) produce the toxic gas hydrogen sulphide (H2S) and cause numerous problems in the oil and gas industry. It has been suggested that these organisms are either introduced to the formation by drilling or water injection or that they may be indigenous. Little is known of the activity and survival mechanisms of the SRB under reservoir conditions. In this study a number of samples from hydrocarbon reservoirs were inoculated into sulphate-rich growth medium to determine the presence and activity of the SRB. The samples studied included freshly drilled core material and live drilling muds. The majority of samples analysed contained SRB which produced HzS to varying degrees following different incubation times. Organisms capable of producing H2S at temperatures up to 95~ were detected. Often long incubation periods, in excess of 6 months, were necessary before H2S was produced.