Determining Microbial Activities in Samples from the Shale Gas Field Compromising Water Reuse and Disposal

Abstract

Abstract Gas production from subsurface shales requires fracture technologies in which fracturing fluid, consisting of guar gum-suspended sand, is forced into the fractures to "prop" them open. The guar gum is easily degraded by bacteria both downhole and at the surface, compromising water reuse or disposal. Samples from the Pinedale shale gas field had high activity of mesophilic acid-producing bacteria (APB), converting guar gum to sugars and then to acetic and propionic acids and of heterotrophic nitrate-reducing bacteria (hNRB), using sugars or acids from guar gum as electron donor for nitrate reduction. Activity of sulfate-reducing bacteria (SRB) was considerably lower with guar gum, reflecting a low initial population size of SRB using the organic acids produced by APB for reduction of sulfate to sulfide. The low concentrations of sulfate in the samples (0–0.4 mM; 0–40 ppm) may be the root cause for this low SRB activity. Indeed, most probable numbers (MPNs) of SRB, determined on standard lactate-sulfate medium were 10- to 100-fold lower than those for APB, determined on standard phenol red-glucose medium. Interestingly, lactate-utilizing SRB appeared to be able to grow in APB medium, indicating that some SRB can also maintain themselves by fermentative metabolism, when sulfate is absent. Culture independent surveys of community composition confirmed that the microbial community at Pinedale samples was dominated by classes of fermentative bacteria (APB). Overall, we conclude that monitoring of the MPN of glucose-fermenting APB most accurately reflects microbial activity and associated biofouling at Pinedale. The success of biocide treatment to reduce microbial activity and associated biofouling is, therefore, also more accurately determined with the APB assay than with that for lactate-utilizing SRB.