Clay-rich rocks are widely recognised as providing excellent barrier properties for radioactive waste containment. The two key characteristics of these geomaterials are i) very small pore sizes, and ii) significant clay-mineral content. The small pore sizes result in extremely low hydraulic conductivities, and the abundance of clay minerals is responsible for high retention and high self-sealing potential.
The Opalinus Clay has pore sizes in the nm range, and the clay-mineral content in all the siting regions is high throughout the formation (> 40 wt.-% when averaged over metre scale). These properties are similar to those of other clay-rich rocks, notably the Callovo-Oxfordian, the designated host rock for the French deep geological repository, and the Toarcian-Domerian at the Tournemire rock laboratory (OECD/NEA 2022). The mineralogy and basic properties of these rocks are also comparable with many top seals of hydrocarbon reservoirs (Fisher et al. 2023).
This chapter provides a detailed characterisation of the host rock in terms of the properties and processes relevant for its efficiency as a geological barrier to fluid transport. In the various sections, the clay-mineral-rich confining units are characterised in comparison with the Opalinus Clay to highlight their potential to contribute to the geological barrier of a repository in the Opalinus Clay. In a few cases, the comparison is extended to limestones of the aquifer units to better emphasise the favourable barrier properties of the Opalinus Clay and its related confining units.
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Section 5.2 explores the spatial variability of the mineralogy of both the whole rock and the clay fraction, and diagenetic mineral sequences. The clay-mineral content is of great relevance, because transport properties and the self-sealing potential directly correlate with it, as will be summarised in Section 5.10.
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Section 5.3 is dedicated to porosity and pore-size distributions, including correlations with mineralogy. The pore network controls transport properties but also clay mineral – porewater interactions, which in turn affect the porewater chemistry.
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As will be shown in Section 5.4, the porewater chemistry, rock mineralogy and porosity network provide a very stable geochemical environment in the Opalinus Clay.
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Section 5.5 summarises key geomechanical properties, deformation behaviour of and typical fault architecture in the Opalinus Clay. The volumetric behaviour, notably the volumetric expansion from unloading (swelling) is relevant for self-sealing (Section 5.7). The main features of the deformation behaviour of the Opalinus Clay are also put into context with the relatively low tectonic overprint.
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Sections 5.6 to 5.9 provide detailed information on the transport properties of the Opalinus Clay. The hydraulic conductivities (Sections 5.6) are low, and mass transport is dominated by diffusion (Section 5.8). The section on self-sealing (Section 5.7) provides further evidence and a conceptual explanation for the absence of significant fracture- or fault-related transmissivities in the rock.
For the various sections, the properties and behaviour of the Opalinus Clay were examined across different scales (e.g. microscale, core, borehole, tunnel). Considerable effort went into correlating geochemical, hydrogeological and geomechanical properties with basic properties such as density, porosity and clay-mineral content. A high clay-mineral content stands out as the fundamental characteristic ensuring favourable barrier properties. In contrast to the continuously clay-mineral-rich Opalinus Clay, the low-permeability confining units are characterised by a higher mineralogical variability, including layers with reduced clay-mineral content and thus limited self-sealing potential.
Although the chapter sequentially addresses the rock microstructure (porosity and mineralogy), deformation behaviour and transport properties, it is noted that all these aspects are strongly interdependent (hydromechanical coupling) as emphasised in Fig. 5‑1. This figure also provides an overview of the sections included in this chapter (see numbers in brackets).
Fig. 5‑1:Coupling between structure, deformation behaviour and transport properties in the Opalinus Clay
Numbers in brackets relate to sections dedicated to the various topics in this chapter.