Since 1999, Andra has been investigating and characterizing the Callovo-Oxfordian clay layer which could host a reversible deep geological disposal for High-level and Intermediate-Level Long-Lived radioactive Waste (Cigéo project). The site selected is located on the eastern boundary of the Paris Basin (Meuse/Haute Marne region). Claystone formations, due to in particular a very low permeability and high confining properties, are indeed candidates to host a radioactive waste disposal. In this region, the Callovo-Oxfordian formation lies between around 420 m to 600 m deep, and its thickness is at least about 130 m. The Callovo-Oxfordian formation is surrounded by two main calcareous formations (Dogger and Oxfordian). The characterization studies are carried out thoroughly in a specific site where is implemented an Underground Research Laboratory within the Callovo-Oxfordian formation, called “Meuse/Haute Marne URL” (MHM URL).
More than 15 years of research and development works have been carrying out in this URL to support the main milestones and reports of the Cigéo project. The MHM URL will continue to play a key role for the next steps of the Cigéo project. Due to an important network of drifts available (Figure 1), the MHM URL could be open for experiments not focused on radioactive waste management such as CO2 studies in the framework of ECCSEL. The MHM URL has been granted to operate until 2030 allowing to envisage long term experiment if required.
Brief technical description with picture and/or diagram
The MHM URL consists in (Figure 1):
More than 11 000 sensors have been installed in the rock, inside boreholes, in the drifts, in the shafts, allowing a continuous monitoring at distance. The type of sensors tested is very large according to concerned Thermal, Hydraulic, Mechanical, Chemical and Radiological domains: from very classical sensors (ex. pressure/temperature) to the development of innovative optic fibre with distributed measurements and sensors for thermal and mechanical long term monitoring.
Specifically for fluids, experiences in continuous pH and redox measurements are online. Direct analyses (by chromatography or spectrometry) underground are also possible.
Figure 1: The current MHM URL network consisting in 1.7 km of drifts and about 850 boreholes (various colour lines).
Figure 2: Examples of equipment implemented in the MHM URL.
Hydro-mechanical and geochemical characterisations of clay (deep) formations are commonly assessed in the context of nuclear waste geological repository and could be easily adapted to CO2 storage experiments where clay plays the role of cap-rock in this case. Hydro-mechanical responses of clay solicited by gas injection have been conducted in the MHM URL in order to assess hydro-fracturing characteristics of the Callovo Oxfordian clay formation. This in situ large scale gas injection test called PGZ experiment, started in 2009, allows us to investigate gas migration mechanisms in clay materials and to define in particular:
Thus, N2 was injected at various pressure steps up to 98 bars and the data collected might be used for CO2 injection experiments. The MHM URL has also a good experience in the field of microbiological and geochemical experiments (cleaning procedures for drilling tools and borehole equipment, use of argon as drilling fluid to avoid oxygen impact, download equipment made exclusively of inert material, sampling protocols) and to study these processes with in situ conditions (PAC, BAC, POX experiments).