Downhole Transformation of the Hydraulic Fracturing Fluid Biocide Glutaraldehyde: Implications for Flowback and Produced Water Quality

Hydraulic fracturing fluid (HFF) additives are used to enhance oil and gas extraction from unconventional shale formations. Several kilometers downhole, these organic chemicals are exposed to temperatures up to 200 °C, pressures above 10 MPa, high salinities, and a pH range from 5-8. Despite this, very little is known about the fate of HFF additives under these extreme conditions. Here, stainless steel reactors are used to simulate the downhole chemistry of the commonly used HFF biocide glutaraldehyde (GA). The results show that GA rapidly ( 1/2 < 1 h) autopolymerizes, forming water-soluble dimers and trimers, and eventually precipitates out at high temperatures (∼140 °C) and/or alkaline pH. Interestingly, salinity was found to significantly inhibit GA transformation. Pressure and shale did not affect GA transformation and/or removal from the bulk fluid. On the basis of experimental pseudo-second-order rate constants, a kinetic model for GA downhole half-life predictions for any combination of these conditions within the limits tested was developed. These findings illustrate that the biocidal GA monomer has limited time to control microbial activity in hot and/or alkaline shales, and may return along with its aqueous transformation products to the surface via flowback and produced water in cooler, more acidic, and saline shales.