Experimental Design for in situ Characterization of the Callovo-Oxfordian Pore Water Composition at 85°C

AbstractWhen emplaced into deep geological disposal cells, the high-level radioactive waste packages will induce a transitory temperature increase in the surrounding geological environment. High temperatures will have an influence on the composition of water that will flow into the cells and come in contact with the disposal materials. An in situ experiment which aims at characterizing the Callovo- Oxfordian pore water at 85°C has been running since 2012. This paper presents the design of the experiment and the predictive thermo-hydro-mechanical (THM) modeling results

Insights on Desaturation Processes based on the Chemistry of Seepage Water from Boreholes in the Callovo-Oxfordian Argillaceous Rock

AbstractSeepage waters from boreholes were collected in the Meuse/Haute-Marne Underground Research Laboratory (NE France) in the Callovo-Oxfordian clay-rich host rock. Over the first months after the drilling of the boreholes, these waters were saltier than the waters collected later on, even in the boreholes that had been drilled without oxygen. These chemical composition variations give insights on the volume of rock affected by the loss of water around the boreholes. This data will help to characterize the desaturation processes in such a rock and the consequences with respect to the composition of the water that will fill the underground works after their closure

Coupled study of water-stable isotopes and anions in porewater for characterizing aqueous transport through the Mesozoic sedimentary series in the eastern Paris Basin

International audienc

Prefferred orientations and anisottropy in shales: Callovo-oxfordian shale (France) and opalinus clay (Switzerland)

Anisotropy in clay-rich sedimentary rocks is receiving increasing attention. Seismic anisotropy is essential in the prospecting for petroleum deposits. Anisotropy of diffusion has become relevant for environmental contaminants, including nuclear waste. In both cases, the orientation of component minerals is a critical ingredient and, largely because of small grain size and poor crystallinity, the orientation distribution of clay minerals has been difficult to quantify. A method is demonstrated that relies on hard synchrotron X-rays to obtain diffraction images of shales and applies the crystallographic Rietveld method to deconvolute the images and extract quantitative information about phase fractions and preferred orientation that can then be used to model macroscopic physical properties. The method is applied to shales from European studies which investigate the suitability of shales as potential nuclear waste repositories (Meuse/Haute-Marne Underground Research Laboratory near Bure, France, and Benken borehole and Mont Terri Rock Laboratory, Switzerland). A Callovo-Oxfordian shale from Meuse/Haute-Marne shows a relatively weak alignment of clay minerals and a random distribution for calcite. Opalinus shales from Benken and Mont Terri show strong alignment of illite-smectite, kaolinite, chlorite, and calcite. This intrinsic contribution to anisotropy is consistent with macroscopic physical properties where anisotropy is caused both by the orientation distribution of crystallites and high-aspect-ratio pores. Polycrystal elastic properties are obtained by averaging single crystal properties over the orientation distribution and polyphase properties by averaging over all phases. From elastic properties we obtain anisotropies for p waves ranging from 7 to 22%

Dissolved helium distribution in the Oxfordian and Dogger deep aquifers of the Meuse/Haute-Marne area

The 140-m-thick, clay-rich Callovo-Oxfordian (COX) layer of the eastern Paris Basin, France, is being considered by the French Nuclear Waste Agency (Andra) as a long-term underground nuclear waste repository. Andra has selected a 250 km2 area (transposition zone) to be further characterised, especially in view of the confinement properties exhibited by the COX. This study reports the helium concentrations and isotopic ratios of water samples from the aquifers above and below the COX, which are the Oxfordian and Dogger aquifers, respectively. The samples were collected from five drilling sites (2007-2008). Both the Oxfordian and Dogger groundwaters are of meteoric origin and have accumulated radiogenic He with a 3He/4He ratio of approximately 0.02 Ra. The He concentrations in the groundwaters are two orders of magnitude higher than in the air-saturated water in the Oxfordian and approximately 10 times higher in the Dogger than in the Oxfordian. One borehole was drilled down into the Triassic sediments, allowing a sample to be collected from the Bundsandstein aquifer. Here, the He concentration is of the same order of magnitude as those measured in the Dogger, but the isotopic ratio is slightly higher at 0.04 Ra. However, this ratio is approximately 10 times lower than those measured by Marty et al. (2003) in waters collected closer to the Trias recharge and tagged with mantle-derived He. This lower ratio is most likely due to a significant and rapid slowdown of the circulation in this aquifer, allowing substantial radiogenic He accumulation. A key conclusion of this study concerns the lateral non-homogeneity of the studied area: He concentrations are higher in the northern part of the transposition zone. This observation can be explained by a longer residence time of the waters and/or a higher input of He from the basement. A simplified 2D model of He transport shows that the second hypothesis alone does not allow fitting of the data, thus implying more stagnant water in this area. The indicative residence times derived from this coarse model are 0.3-0.5 Myr (Oxfordian) and 0.5-0.6 Myr (Dogger) for the borehole located in the centre of the investigated zone; for the northernmost borehole, they reach 0.8-1.4 Myr for the Oxfordian and 1.5 Myr for the Dogger