THE EFFECTS OF BIOCIDE USE ON SUBSURFACE MICROBIAL POPULATIONS DURING HYDRAULIC FRACTURING IN THE EAGLE FORD FORMATION, TEXAS, USA

During hydraulic fracturing (HF), a fluid is pumped into low permeability shales to allow for enhanced hydrocarbon extraction. A consequence is the inoculation of the subsurface with surficial microorganisms, many of which could take advantage of the nutrients and carbon released during HF. Current practice is to control microbial growth by adding biocide to HF fluids. However, biocide is a significant cost and has important consequences for health, safety, and the environment.

Here, we assess the effectiveness of the biocide tributyltetradecyl phosphonium chloride (TTPC) on the Eagle Ford Shale (EF) in Central Texas. Temperatures in the EF are >160°C downhole, with conditions that cannot support viable microorganisms. However at lower temperatures, such as along the wellbore, viable microbial communities may be supported. Freshwater used in HF fluids as well as produced water were collected from a well site and characterized for their solution chemistry and microbial community. Field samples were also cultured in the laboratory in representative subsurface conditions at 128°C and 60°C, with and without TTPC. Cultures were assessed for viable cells, community composition, and geochemistry. At temperatures below 128°C, cells are viable despite TTPC treatment though TTPC killed a significant number of the cells. After a two-week incubation with TTPC, nearly 50% of the total cells in cultures were still viable. Microbial diversity in TTPC-treated cultures was much lower than controls and likely contain a community of biocide-resistant organisms mostly represented by the classes Alphaproteobacteria, Actinobacteria, Gammaproteobacteria, and Cytophagia. There is little knowledge about how these organisms may impact production. In contrast, environmental PW samples contained a diverse microbial community despite TTPC treatment, and these organisms may be implicated in bioclogging, enhanced oil recovery, and corrosion such as Crenothrix, Microbacterium, and Thiobacillus. Our studies suggest that that TTPC will not completely sterilize the subsurface microbial community and that further characterization of biocide is necessary to determine if it is truly useful in HF operations.