Produced brines and flowback waters comprise the largest waste stream associated with shale gas extraction. Proper management of these wastewaters is essential to preventing contamination of surface freshwater resources and is challenging due to the high salinities and elevated levels of naturally occurring radioactive materials (NORM) present in these brines. Ra-226 and Ra-228 comprise the majority of NORM activity in shale gas produced water and may be concentrated during treatment processes through co-precipitation with barite. Although such co-precipitation removes Ra from solution, it may also create a new waste management challenge in form of a low-level radioactive solid waste byproduct. Having knowledge of which shale gas wells may produce brines with elevated Ra activities could help operators and regulators optimize wastewater treatment and disposal strategies to best manage risks associated with shale gas produced NORM.<br/>Shale formations, especially organic-rich shales, tend to contain a higher content of U than other sedimentary formations due to accumulation of reduced U mineral phases, which are highly stable under the reducing conditions commonly found in organic-rich shales. In this study, we seek to link the U and Th content in several shale gas reservoirs to the expected Ra activity in concomitant produced brine. Measured Ra activity in produced brine from a series of Antrim Shale wells and from flowback water for a single Utica-Collingwood Shale well in Michigan will be discussed. We measured Ra activity via a combination of ICP-MS and gamma spectroscopy. Higher Ra activities were observed in Antrim produced brines than in the Utica-Collingwood flowback, which correlates well with the higher U content of the Antrim Shale (19.8 ppm) relative to the Utica-Collingwood (3.5 ppm). Also, the theoretical ratio of Ra-226/Ra-228 activities in the brine calculated from measured shale content of U-238/Th-232 agrees well with the measured Ra-226/Ra-228 ratio of the Utica-Collingwood and Antrim field samples of 1.2 and 10.1, respectively. The main processes believed to be responsible for transferring Ra from the rock matrix to the formation water include chemical leaching, alpha-recoil, adsorption/desorption, and the equilibration of Ra in solution with barite. We will present a survey of geochemical data from other shale gas plays in the U.S. correlating shale U, Th content with produced brine Ra content.

Exploring the origin of radium in shale gas produced water

251st National Meeting (2016)

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