Polyacrylamide (PAM) based friction reducers are a primary ingredient of slickwater hydraulic fracturing fluids. Little is known regarding the fate of these polymers under downhole conditions, which could have important environmental impacts including decisions on strategies for reuse or treatment of flowback water. The objective of this study was to evaluate the chemical degradation of high molecular weight PAM, including the effects of shale, oxygen, temperature, pressure, and salinity. Data were obtained with a slickwater fracturing fluid exposed to both a shale sample collected from a Marcellus outcrop and to Marcellus core samples at high pressures/temperatures (HPT) simulating downhole conditions. Based on size exclusion chromatography analyses, the peak molecular weight of the PAM was reduced by 2 orders of magnitude, from roughly 10 MDa to 200 kDa under typical HPT fracturing conditions. The rate of degradation was independent of pressure and salinity but increased significantly at high temperatures and in the presence of oxygen dissolved in fracturing fluids. Results were consistent with a free radical chain scission mechanism, supported by measurements of sub-μM hydroxyl radical concentrations. The shale sample adsorbed some PAM (∼30%), but importantly it catalyzed the chemical degradation of PAM, likely due to dissolution of Fe2+ at low pH. These results provide the first evidence of radical-induced degradation of PAM under HPT hydraulic fracturing conditions without additional oxidative breaker.
Bibliographical noteFunding Information:
This research was funded by a Penn State College of Engineering Innovation Grant and a seed grant through the Center for Collaborative Research in Intelligent Gas Systems (CCRINGS) program funded by General Electric (GE). Additional funding comes from Pennsylvania Water Resources Research Center Small grants program. We thank Weatherford Inc. for providing the synthetic chemical additives. We acknowledge the Kappe Environmental Engineering laboratories for providing access to TOC measurement instrumentation and technical assistance provided by David Jones. We acknowledge help from Sydney Stewart on the iron and anoxic tests, and the Penn State Materials Characterization Lab (MCL) staff, specifically Nicole Wonderling (XRD), Julie Anderson (EDS), Jeff Shallenberger (XPS), Josh Stapleton (ATR-FTIR) and Ekaterina Bazilevskaya (TGA). We also thank Prof. Alexey Silakov (Penn State Department of Chemistry) for guidance on free radical measurements.
© 2017 American Chemical Society.