Studies on the Thermophilic Sulfate-Reducing Bacteria from a Souring North Sea Oil Field
Authors: Cochrane W. J. , Jones P. S. , Sanders P. F. , Holt D. M. , Mosley M. J. 
Published: December 2007

Sulphide souring of the Ninian oilfield reservoir was studied to ascertain the possible involvement of sulphate-reducing bacteria in this important phenomenon. The data presented in this paper demonstrate the presence and growth of thermophilic sulphide generating bacteria throughout this facility. A range of bacteria were detected in representative water samples from close to the injection wellbore. These bacteria included mesophilic sulphate-reducing bacteria (m-SRB), capable of growth between 20°C and 40°C and thermophilic SRB (t-SRB), capable of growth between 50°C and 70°C. Thermophilic SRB were also found throughout the oil production system, in both water and surface film samples. The presence of t-SRB in the water cut from the oil producing wellheads suggests that a significant proportion of reservoir souring in this case may be bacterial in origin.

Between 1984 and 1986, a number of microbiological surveys were carried out on the Ninian Field, initially to monitor the growth of SRB around injection wellbores. These surveys clearly showed that SRB were present in the formation close to the perforations of injection wells and that extended biocide treatment of 20 days continuous dosage was effective in reducing the numbers of these SRB. In addition there was an apparent reduction in hydrogen sulphide (H2S) concentrations measured in injection well backflow samples. This reduction in H2S could be caused either by cidal action against SRB or by the known H2S scavenging properties of the glutaraldehyde based biocide. There was thus indirect evidence that bacteria were generating at least some of the observed H2S in the reservoir. More recent studies have detected t-SRB in produced fluids from the producing wellheads and throughout the production system.

This new evidence of t-SRB activity in the formation, and their detection in hot oil production systems, has clear implications and ramifications for H2S induced corrosion and cracking and on hydrocarbon handling operations.

The field and laboratory evidence indicates that the previously unrecorded t-SRB have a wide temperature and nutritional range. They are able to produce sulphide under the temperature and carbon source conditions of the oil formation. A simple model for bacterial colonisation and growth in oilfield reservoirs is presented, together with a discussion of the implications for remedial measures to control microbiological growth. The core of this model is that seawater flooding completes the balance of sulphate, temperature and organic nutrient status in the reservoir. This favours the growth of t-SRB at a considerable distance from the injection well perforations. Such widespread microbiological growth indicates that souring control by organic biocide application would be difficult to achieve, and this is supported by the results of laboratory biocide trials.

The model predicts that a finite concentration of sulphide can be produced by this mechanism. Bacterial sulphide generation is limited by both nutrient source availability and sulphide toxicity. In the case of the Ninian Field, laboratory data and theoretical assessments indicate that sulphide toxicity would inhibit bacterial activity above approximately 100ppm dissolved sulphide.