Hydrogen sulfide souring in oil and gas reservoirs due to sulfate-reducing bacteria (SRB) has been well documented, and the awareness of the problem is widespread in the industry. Reser-voir engineers often specify preventive subsurface biocide treatment where progressive souring has the potential to occur. This strategy has the advantage of attaining early control of microbial communities that may eventually cause souring. The disadvantages include cost, environmental, and worker-safety considerations. In addition, the reservoir engineer cannot be certain that down-hole treatment is necessary without specific chemical and microbiological information on the system treated.

In a study sponsored by Northern Natural Gas (an Enron Company), a group of wells in a natural gas storage field having a history of souring was evaluated. Wells in the reservoir had been biocide treated for many years and some eventually went sour. The wells studied remained sweet although they had been expected to eventually go sour. Microbiological and chemical analysis of these wells revealed that, unlike the sour counterparts, little potential existed for them to go sour. The subject wells exhibited limited nutrients available for SRB, and other bacteria were identified in the produced water that naturally inhibit sulfide production. The study was extended to three additional natural gas storage facilities, utilizing the same formation, and similar correlations were shown between bacteria, nutrients, and sulfide production, although the fundamental water-quality parameters were different. The sponsor of the study has utilized these findings to curtail unnecessary treatment.

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