Surfactants such as polyglycols and polyethoxylates are a broad group of nonionic surface-active agents widely applied as emulsifiers during hydraulic fracturing to enhance the solubility of additive chemicals and improve shale wettability for enhanced gas production. High volume hydraulic fracturing (HVHF) fluid formulations typically comprise mixtures of polyethylene glycols (PEGs) and/or alcohol polyethoxylates (AEOs) that vary in chain length and degree of ethoxylation. PEGs and AEOs biodegrade in the shallow subsurface, but little is known about their degradability under the higher temperatures and pressures, elevated salinities, and anoxic redox conditions encountered in deep fractured shale. Under surface conditions, the anaerobic biodegradation pathway for these compounds proceeds by successive de-ethoxylation which eventually leads to assimilation and/or mineralization of transformed alcohols and carboxylic acids. However, this biodegradation process produces acid metabolites that are be corrosive to installed equipment and a reduction in the surfactant’s efficiency during well completion and gas extraction. Here, we use chemical and genomic analyses to investigate AEO biodegradation in produced fluids collected from a Utica (Ohio) shale well. Metagenomic data reconstructed for Halanaerobium, the dominant bacteria detected in field samples through time, revealed genes associated with anaerobic AEO biodegradation in produced fluids including propanediol dehydrogenases and acetaldehyde dehydrogenases, which catalyze de-ethoxylation and carboxylation processes, respectively. Metagenomic pathways were supported by HPLC-HR-MS/MS chemical analyses, which revealed the disappearance of higher chain length surfactants (C4 through C8 AEOs) and concurrent formation of metabolites associated with the anaerobic biodegradation pathway of AEOs (e.g., acetaldehyde, propionaldehyde, propionate, and acetate). These findings indicate the degradation of alcohol ethoxylates occurs in deep shales after hydraulic fracturing by halotolerant bacteria, with implications for resource loss or souring, and the production of corrosive metabolites from chemical additives in hydraulic fracturing fluids.

In situ biodegradation of ethoxylated surfactants by halotolerant bacteria in a hydraulically fractured shale well

253rd National Meeting (2017)

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