Both conventional and unconventional oil and gas wells can co-produce aqueous fluids with high levels of total dissolved solids (TDS, up to 300 g/L in the Appalachian Basin) along with hydrocarbons. These brines must be disposed of in an environmentally benign manner, through wastewater treatment, injection into deep disposal wells, or mixing with fresh water for future hydraulic fracturing. In addition, certain components of these produced waters can interfere with hydrocarbon extraction (e.g., Ba forms barite scale). While the origin of conventional produced waters is well understood, the ultimate source of high-TDS produced waters in unconventional wells has not been resolved. Potential explanations for some or all of the dissolved solids include (1) extraction of exchangeable and/or carbonate-bound components in reservoir rocks or wellbore cement by hydraulic fracturing fluids, (2) diffusion of salt from capillary-bound brines; (3) dissolution of drilling mud barite; and/or (4) incursion of formation waters from within or outside of the target formation. In order to understand the origin of brines from unconventional wells in the Appalachian Basin, we are using a suite of metal isotopes as direct and indirect tracers for downhole fluid sources and water-rock interaction. The radiogenic Sr isotope signature (87Sr/86Sr) of a brine reflects the interaction of fluid with rock units of different ages and/or compositions, while the Li stable isotope system (7Li/6Li) can be affected by differences in isotopic composition of the source rock as well as mass fractionation accompanying clay formation. Recent application of stable Ba isotopes (138Ba/134Ba) suggests that this system could be a powerful tool for tracking the source of Ba in high-TDS fluids prone to scale formation. Our results to date show that produced water from unconventional wells in the Middle Devonian Marcellus Shale yields significantly different isotope ratios than immediately overlying Upper Devonian/Lower Mississippian conventional produced waters, suggesting the existence of a long-lived barrier between these units in parts of the basin. The data are currently most consistent with a formation water source for unconventional Marcellus produced waters.

Origin of conventional and unconventional produced water brines in the Appalachian Basin, USA, using radiogenic and stable metal isotopes

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.