As part of an ongoing program to study interactions among compounds used by petroleum industry operators and the chemical environments of hydrocarbon-bearing shales, we investigated reactions involving ammonium peroxydisulfate (APS). Oil and natural gas producers commonly thicken hydraulic fracturing fluids with borate-crosslinked polysaccharide gels. This supports the transport of proppant materials to cracks in deep rock formations into which their wells are bored. Once well stimulation is complete, the gels are thinned (“broken”) to permit commodity oil and/or gas and other produced fluids to flow back to the earth’s surface. This is the final chemical reaction that is carried out prior to well operators receiving the flowback of commodities and wastes and APS is among the most cost-effective reagents used to trigger it. To model this system, we added ammonium peroxydisulfate to a heated reaction vessel combining methane with Marcellus shale-buffered bromide and chloride brines. Products isolated from the reaction vessel, chilled distillate collection flask and triethylene glycol-filled bubbler were analyzed by UV-VIS and 1H-NMR spectroscopy, ion chromatography, and reaction with chromotropic acid. Our results indicate that in addition to the intended reactions of APS with borate crosslinks to disrupt the polysaccharide gel matrix, peroxydisulfate also engages in unintended reactions with methane, shale and brines to produce undesirable by-products that include formaldehyde.
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