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
The MHM URL consists of (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.
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).
In France, the MHM URL is the only facility accessible for in situ experiments in a deep clay formation at 500 m depth, with a hydraulic pressure of 46 bars and a vertical stress of 12 MPa, allowing to perform different tests from small scale to large scale and in representative conditions of these ones of a cap-rock for capture and storage CO2 concept.
Andra has collected a large number of data concerning the properties of the Callovo-oxfordian clays at different scales (I.e. from the pore scale up to the scale of the geological formation) which will be available to support new projects. Surrounding calcareous formation were also investigated. Thus, the geological environment is already well defined and it would not be required to plan a lot of analyses to be characterised. The overall operating and maintenance works of the MHM URL is operated by Andra. This high cost will not be charged on ECCSEL projects.
The MHM URL benefits from the support of Andra’s technicians and engineers: 40 scientists of Andra specialized in geology, rock mechanic, hydrogeology, chemistry, fluid transfer, biology, corrosion, sensors and data acquisition system, work on site. A specific team provides support in designing and setting up experiments. Several contractors used to work in the MHM URL can give support in specific domains (drilling, geological survey and sampling, chemical, biological and mineral characterizations). It’s also possible to perform in situ gas and fluid analyses (by ionic chromatography, or Raman spectrometry…). Andra has also a hall for technological demonstrators on MHM centre that could be accessible to people involved in ECCSEL projects.
To achieve all objectives, Andra has developed an important network of collaboration with others scientific partners as experts in their domains (figure 3).