ABSTRACT
436The environments found in oil and gas operations are often ideal for promoting the growth of microbial populations capable of corrosion, fouling, and souring. As a result, microbial monitoring and application of biocides is a routine part of normal operations. A variety of different techniques, both old and new, have become available for monitoring bacterial populations. Each technique varies with respect to mechanism of detection, what component or feature of the bacterial cell is actually being detected, as well as the physiological status of the cells being detected. Since oilfield environments are often quite extreme, the ability of each method to be suitable under different sample physical parameters is also not known. For this study, data are presented from four very distinct O&G field case histories: (1) a sour, highly saline shale gas field in which microbial corrosion was not expected but determined to be present; (2) a natural gas tight sandstone field that was not experiencing MIC in contrast to culture-based analysis that indicated it should be; (3) a light oil shale field where rapid MIC was occurring in the extreme temperature on the surface of fire tubes; and (4) a tight shale gas field experiencing a wide range of microbial-associated problems, including downhole and surface MIC issues. In addition to inquiries about operations, the microbial populations at each location were analyzed using four distinct monitoring methods: (1) culture-based enumeration using bug bottles with indicator media for sulfate-reducing bacteria and acid-producing bacteria, (2) rapid bacterial cell enumeration based on an adenosine triphosphate quantification assay, (3) epifluorescence microscopy, and (4) molecular analysis of the microbial populations by DNA isolation and sequencing of 16S amplicons. Samples from location 4 were also analyzed using a direct bacterial cell counting system and by an assay that quantifies specific enzymatic activity. The usefulness of each method varied from location to location. Considering the variations in physical and chemical characteristics of each field site, as well as the very high bacterial diversity that is present in oil and gas locations, such variations are not unexpected. Microbial population analysis allowed for a diagnosis and course of action to be suggested to the field operators. The relative value of each data set for diagnosing microbial contribution to corrosion issues in the fields is discussed. Ultimately, there is not one method that can be used to accurately monitor microbial populations in all locations. Understanding the strengths and limitations of each method provides the most solid foundation for microbial monitoring and educated decision making in the field.
