Activity concentrations of 238U and 226Ra in two European black shales and their experimentally-derived leachates

• Published in: Journal of environmental radioactivity 2018-10, Vol.190-191, p.122-129
• Main Authors: Wilke, Franziska D.H., Schettler, Georg, Vieth-Hillebrand, Andrea, Kühn, Michael, Rothe, Heike
• Format: Article
• Language: English
• Subjects: Batch experiments; Black shales; Flowback; NOR; Radioactivity; Unconventional gas production
• ISSN: 0265-931X; 1879-1700
• DOI: 10.1016/j.jenvrad.2018.05.005

The production of gas from unconventional resources became an important position in the world energy economics. In 2012, the European Commission's Joint Research Centre estimate 16 trillion cubic meters (Tcm) of technically recoverable shale gas in Europe. Taking into account that the exploitation of unconventional gas can be accompanied by serious health risks due to the release of toxic chemical components and natural occurring radionuclides into the return flow water and their near-surface accumulation in secondary precipitates, we investigated the release of U, Th and Ra from black shales by interaction with drilling fluids containing additives that are commonly employed for shale gas exploitation. We performed leaching tests at elevated temperatures and pressures with an Alum black shale from Bornholm, Denmark and a Posidonia black shale from Lower Saxony, Germany. The Alum shale is a carbonate free black shale with pyrite and barite, containing 74.4 μg/g U. The Posidonia shales is a calcareous shale with pyrite but without detectable amounts of barite containing 3.6 μg/g U. Pyrite oxidized during the tests forming sulfuric acid which lowered the pH on values between 2 and 3 of the extraction fluid from the Alum shale favoring a release of U from the Alum shale to the fluid during the short-term and in the beginning of the long-term experiments. The activity concentration of 238U is as high as 23.9 mBq/ml in the fluid for those experiments. The release of U and Th into the fluid is almost independent of pressure. The amount of uranium in the European shales is similar to that of the Marcellus Shale in the United States but the daughter product of 238U, the 226Ra activity concentrations in the experimentally derived leachates from the European shales are quite low in comparison to that found in industrially derived flowback fluids from the Marcellus shale. This difference could mainly be due to missing Cl in the reaction fluid used in our experiments and a lower fluid to solid ratio in the industrial plays than in the experiments due to subsequent fracking and minute cracks from which Ra can easily be released. •Release of U and Th from European black shales experimentally studied.•Activity concentration of 238U and 226Ra from experimental fracturing fluids measured.•Amount of U in the European shales is similar to that of the Marcellus Shale (US).•Behaviour of U and Ra seems to be similar in experiments and production sites.•Preferential release of Ra