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 (t1/2 < 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.