Indications of Transformation Products from Hydraulic Fracturing Additives in Shale-Gas Wastewater

Abstract

Unconventional natural gas development (UNGD) generates large volumes of wastewater, the detailed composition of which must be known for adequate risk assessment and treatment. In particular, transformation products of geogenic compounds and disclosed additives have not been described. This study investigated six Fayetteville Shale wastewater samples for organic composition using a suite of one- and two-dimensional gas chromatographic techniques to capture a broad distribution of chemical structures. Following the application of strict compound-identification-confidence criteria, we classified compounds according to their putative origin. Samples displayed distinct chemical distributions composed of typical geogenic substances (hydrocarbons and hopane biomarkers), disclosed UNGD additives (e.g., hydrocarbons, phthalates such as diisobutyl phthalate, and radical initiators such as azobis(isobutyronitrile)), and undisclosed compounds (e.g., halogenated hydrocarbons, such as 2-bromohexane or 4-bromoheptane). Undisclosed chloromethyl alkanoates (chloromethyl propanoate, pentanoate, and octanoate) were identified as potential delayed acids (i.e., those that release acidic moieties only after hydrolytic cleavage, the rate of which could be potentially controlled), suggesting they were deliberately introduced to react in the subsurface. In contrast, the identification of halogenated methanes and acetones suggested that those compounds were formed as unintended byproducts. Our study highlights the possibility that UNGD operations generate transformation products and underscores the value of disclosing additives injected into the subsurface.

Effect of maturity and mineralogy on fluid-rock reactions in the Marcellus Shale

“…Low molecular weight carboxylic acids, in particular, acetate and formate, have been observed in both produced water and effluent from similar fluid-shale experiments (Lee & Neff, 2011, Zhu, et al, 2015, Hoelzer, et al, 2016. The proliferation of these compounds is believed to be related to the labile ester linked carboxyl functional groups attached to the organic matter within the shale formation.…”

“…On average roughly 40% of this fluid returns to the surface upon completion and is then considered produced fluid. Produced fluid has a chemical signature distinctly different from the initially injected HFF (Strong et al, 2014, Hoelzer, et al, 2016, Marcon, et al, 2017 and the copious amounts of fluid generated from thousands of wells drilled within the Marcellus Shale necessitate a better understanding of reactions occurring between HFF and the shale reservoir. Recycling and remediation efforts of produced fluid are of great concern to industry, environmental scientists, and the general public.…”

“…The breaker creates SO4 2free radical ions at elevated temperatures (Tasker et al, 2016), which react with the linkages of the polymer structure, breaking up the compounds and allowing the fluid to flow back to the surface. Both experimental and field samples have proven that the chemical signature of produced fluid can be significantly different from the initial injection fluid (Strong et al, 2014, Hoelzer, et al, 2016, Marcon, et al, 2017. Produced water is typically characterized by its relatively high TDS,…”

Alternative Management of Oil and Gas Produced Water Requires More Research on Its Hazards and Risks

“…Produced water is a waste stream generated along with oil and gas production. This wastewater is often hypersaline and varies in composition during the life of a well and can contain native formation water from oil‐ or gas‐bearing layers, hydraulic fracturing and maintenance chemicals, as well as any reaction byproducts formed via abiotic or biotic processes (Hoelzer et al ; Oetjen et al ; Rosenblum et al ). Nearly 900 billion gallons of produced water are generated each year from current onshore extraction of conventional and unconventional resources in the United States (Veil ).…”

“…Limited characterization studies of produced water have identified nearly 600 chemicals in produced water associated with hydraulic fracturing (USEPA ). More recent studies using high‐resolution mass spectrometry indicate that the number of potentially detectable chemicals may be much greater (Hoelzer et al ; Luek and Gonsior ; Luek et al ; Piotrowski et al ).…”