The Contribution of Crude Oil Hydrocarbons to Sulphidogenesis under Sulphate- and Nitrate-Reducing Conditions
When assessing the potential of a reservoir to sour, often the only carbon sources considered are the volatile fatty acids (VFAs) present within the formation water. In this study the contribution of crude oil hydrocarbons to sulphide production in the presence of VFAs under sulphate-reducing or nitrate-reducing conditions was investigated. Six oils, ranging from light (API 43.1) to heavy (17.3) and with a range of TANs (<0.05–2.5), were incubated for 12 months at 30°C, 45°C or 60°C with produced water spiked with a number of known sulphate-, nitrate- and iron-reducing microorganisms. Samples were taken at 0, 6 and 12 months for gas chromatography-mass spectrometry (GC-MS), nitrate/nitrate, sulphate/sulphide, VFA, non-purgeable organic carbon (NPOC) and molecular microbiological analysis. All crudes, with the exception of one, exhibited sulphide generation in excess of the theoretical maximum from VFAs alone, indicating that carbon sources other than VFAs were being utilised for sulphate reduction. Analysis of nitrate-amended enrichments revealed that sulphidogenesis was impaired in all enrichments; nitrate had been depleted by more than 98% in all enrichments at all temperatures. Molecular microbial analysis was used to monitor changes in the microbial community composition over the course of the enrichment.