Release of gases and organic compounds during heating of Opalinus Clay from the Mont Terri rock laboratory

The storage of high-level nuclear waste will introduce a transient heat source in subsurface repositories due to the energy released during the ongoing decay of remaining radionuclides. This will increase the temperature not only of the waste canisters, but by heat transfer also in the technical barrier and in the host rock formation. The elevated temperature will induce changes in the geochemical porewater-host rock system, e.g., by carbonate dissolution processes or thermal decomposition of natural sedimentary organic matter in the host rock, resulting in the release of additional compounds into the porewater or into a gas phase. These dissolving compounds alter chemical parameters such as porewater pH or redox conditions, again inducing mineral dissolution or precipitation reactions, as well as changing speciation and, hence, transport properties of radionuclides. Some organic compounds, e.g., short-chain organic acids, could be metabolized by microbial communities in the repository. In addition, the buildup of a gas phase and the pressure increase in the latter are important parameters in the long-term safety case assessment (Diomidis et al. 2016). Usually, heating experiments are conducted in laboratories with retrieved rock material to identify the main chemical reactions occurring in host rocks (Elie & Mazurek 2008; Lorant et al. 2008). For some host rocks in-situ heater tests have been carried out by installing a heat source in dedicated boreholes, heating the host rock under in-situ stress conditions (e.g., the Mont Terri HE-D experiment). For these in-situ heater experiments, the quantitative observation of produced gases and organic compounds released into the porewater is not straightforward. Often, only an increase of porewater-pressure and temperature is monitored at certain locations of the heated rock volume. In this study of the Mont Terri HE-F experiment, we conducted experiments on core material from the Opalinus Clay (Mont Terri, Switzerland) in the Experimental Geochemistry Laboratory at the Federal Institute for Geosciences and Natural Resources (BGR) aiming for a detailed characterization of released gases and organic compounds. The results were compared to data obtained from rock material after an in-situ heater experiment in the same host rock in the Mont Terri rock laboratory. For this, two new cores of 13 m in length were retrieved from the rock volume of the former heater experiment, HE-D, perpendicular to the heater borehole. The retrieved core material had been heated to temperatures up to 75 °C for up to 9 months. The two parallel cores (horizontal distance of 0.5 m) have been drilled to verify findings of the rock analyses in this duplicate pair, of which each contains the imprint of the temperature transect from non-heated to heated to non-heated rock.