(355d) Case Study on Microbiological Control Program for Hydraulic Fracturing Operations Using Nitrate-Reducing Bacteria

Hydraulic fracturing rarely uses potable water as fracturing fluid because of cost considerations or unavailability. Produced water is more readily available, but contains various types of bacteria. Insufficient treatment and reuse of produced water allows these organisms to concentrate in the fracture tanks. Sulfate reducing bacteria (SRB) introduced by means of fracturing fluids can proliferate in the reservoir, particularly in locations, such as the Marcellus Shale, where the formation temperature can be low (between 38 and 66 °C). SRB produce hydrogen sulfide (H₂S) by reducing sulfate ions leading to souring of production fluids, iron sulfide formation, and microbiologically influenced corrosion (MIC).

Various chemical biocides are applied to fracturing fluids to control bacterial population. However, traditional biocides inherently possess some degree of toxicity. Hence, a treatment system consisting of live nitrate reducing bacteria (NRB) combined with a sodium nitrate solution was used as an environmentally acceptable alternative microbiological control program. NRB mitigate SRB activity by at least three possible mechanisms—competition for available carbon source if the source is limited, evolution of nitrites, which act as a metabolic inhibitor of SRB and direct oxidation of biogenic sulfides.

This study discusses a two-component NRB-nitrate program consisting of liquid slurry containing active, naturally occurring NRBs, which can survive under reservoir conditions, and sodium nitrate solution, which acts as terminal electron acceptor for the NRB metabolic pathway. The treatment efficiency was compared with chemical biocide that historically had exhibited good microbial control at the same location. This treatment was extended to over 1,000 wells and has been found to provide equivalent microbial control to conventional biocides. This NRB-nitrate program has low toxicity and provides an effective environmentally sensitive approach to microbial control in hydraulically fractured assets.