The presence of sulphate-reducing bacteria in live drilling muds, core materials and reservoir formation brine from new oil fields

The sulphate-reducing bacteria (SRB) produce the toxic gas hydrogen sulphide (H(2)S) 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 H(2)S to varying degrees following different incubation times. Organisms capable of producing H(2)S at temperatures up to 95 degrees C were detected. Often long incubation periods, in excess of 6 months, were necessary before H(2)S was produced. Many of the blank mud samples analysed (e.g. prior to use in the well) were already contaminated with sulphidogenic bacteria. Some of the cultures obtained to date have been studied in reservoir conditions core flood tests. It is hoped that the use of these laboratory tests will highlight any potentially damaging reactions prior to their occurrence in hydrocarbon formations.