For the long-term safety of a geological repository and for site comparison, the following aspects of the Opalinus Clay and its confining units are particularly relevant:

  • The Opalinus Clay exhibits excellent and comparable barrier quality in all three siting regions. The consistently high clay-mineral content (> 40 wt.-%) leads to a clay-matrix-supported microstructure, which in turn dominates the mechanical behaviour and transport properties with relatively low variability.

  • The porewater of the Opalinus Clay is of moderate ionic strength, pH near neutral and the redox conditions are buffered by pyrite and siderite equilibria to the reducing range. This translates to favourable geochemical conditions regarding the mobility of radionuclides and the stability of the engineered barriers.

  • The difference in current burial depth in the three siting regions (approximately 400 to 900 m) results in only minor differences in basic properties (e.g. porosity). The Opalinus Clay can therefore be considered comparable across all regions. Depth-dependence of mechanical properties (strength and stiffness) is accounted for by the well-established dependence of these properties on effective stress.

  • Deformation is accommodated in a more distributed manner than in more competent rocks with low clay-mineral content, with segmentation of fault planes and soft-linkage geo­metrically restricting continuous fracture flow paths.

  • Measurements of extremely low hydraulic conductivity and porewater investigations provide evidence that mass transport in the Opalinus Clay is diffusion-dominated.

  • The Opalinus Clay self-seals fractures at burial depths greater than approximately 200 m and therefore fractures do not compromise the barrier integrity over relevant timescales.

  • The validity of diffusion properties is established at different scales of space and time: Consistent properties result from small-scale lab experiments and in-situ experiments in the Mont Terri rock laboratory. The laboratory diffusion coefficients lead to plausible matches of the formation-scale profiles of natural tracers in porewater that evolved over geological timescales.

  • In the Opalinus Clay, the diffusion coefficient for the water tracer HTO shows very minor variation and is consistent across the different siting regions. As a result of differences in porewater ionic strength, the effective diffusion coefficient equation for chloride shows some site-specific differences, with lowest values in JO.

  • The confining units increase the effective containment of a future repository and also have very low hydraulic conductivities, but a lower self-sealing potential than the Opalinus Clay. Relative homogeneity, good spatial correlation and consistently high clay-mineral content are the major differences of the Opalinus Clay and its confining units, and a major advantage in terms of ease of characterisation.

Link of texture and clay-mineral content with key hydromechanical properties in Jurassic sediments of the siting regions (lower confining units to upper aquifers) The uppermost figure depicts a highly simplified transition from a grain-supported frame-work (clastics or carbonate skeleton, with low clay-mineral content partially filling the pores) on the left side, to a clay-matrix-supported framework on the right. Transition to the clay-matrix-supported framework at approximately 20 to 40 wt.-% is consistent with the low hydraulic conductivity (a; see Fig. 5 42) because of the high swelling capacity (c; see Fig. 5 22), which in turn is possible because swelling is not hindered by strong diagenetic bonds, as indicated by low strength (b; see Fig. 5 30). Note figure shows data from the Lias Group to the Malm Group. HR stands for Hauptrogenstein.

Fig. 5‑57:Link of texture and clay-mineral content with key hydromechanical properties in Jurassic sediments of the siting regions (lower confining units to upper aquifers)

The uppermost figure depicts a highly simplified transition from a grain-supported frame­work (clastics or carbonate skeleton, with low clay-mineral content partially filling the pores) on the left side, to a clay-matrix-supported framework on the right. Transition to the clay-matrix-supported framework at approximately 20 to 40 wt.-% is consistent with the low hydraulic conductivity (a; see Fig. 5‑42) because of the high swelling capacity (c; see Fig. 5‑22), which in turn is possible because swelling is not hindered by strong diagenetic bonds, as indicated by low strength (b; see Fig. 5‑30). Note figure shows data from the Lias Group to the Malm Group. HR stands for Hauptrogenstein.

 

The above observations are conceptually explained in relation to the clay-mineral content, and more specifically to the distribution of clay minerals at the microstructure scale (Fig. 5‑57). At low clay-mineral content, the larger clastic grains (typically carbonates or quartz) are load-bearing and the microstructure can be referred to as grain-supported, whereas at higher clay-mineral content the microstructure is clay matrix-supported with clastic grains enclosed in clay minerals (Section 5.2). The transition between a grain-supported and matrix-supported microstructure for shales was interpreted to be around 30 to 40 wt.-% of clay and considered a threshold for robust barrier function with increasing clay-mineral content (Bourg & Ajo-Franklin 2017, Revil & Cathles 1999). This value is in good agreement with an increasing self-sealing potential observed from the confining units towards the Opalinus Clay, most notably with an increase in the swelling index (Fig. 5‑57c).

In Fig. 5‑57 only Jurassic sediments are shown (which have low matrix permeability), where elevated transmissivity can be related to fracture-dominated flow. It is noted that the hydraulic conductivity of the intact material is very low (<< 10-10 m/s) in all formations shown in Fig. 5‑57, in limestones of the Malm Group ("Aquifers") as well as in the Opalinus Clay. But while cementation of pores plays a dominant role in ensuring low permeability in intact limestones, flow in the clay matrix is controlled by the pore network of the small clay particles, i.e. a high tortuosity and narrow passages (constrictivity).

Fracturing in the Opalinus Clay has a much lower impact on changes in rock mass hydraulic conductivity because of the fundamentally different controls on its low permeability compared to limestones, where the brittle response during fracturing leads to relatively high surface roughness and localisation of deformation with hard linkage. As cement bonds and grains are broken during fracturing and swelling capacity is negligible, fracture permeability cannot easily be reversed.

Whereas the clay-mineral content in the Opalinus Clay is consistently high, the confining units show a larger variability, with clay-mineral contents transitioning from grain-supported to clay matrix-supported microstructures. At a clay-mineral content of approximately 20 to 40 wt.-%, this transition is associated with a decrease in strength and an increase in the swelling index towards higher clay-mineral contents. In the Opalinus Clay, packer tests show consistently low hydraulic conductivity (< 10-12 m/s), regardless of whether clay-mineral contents are 40, 50 or even 70 wt.-%. In contrast, rocks with clay-mineral contents < 20 wt.-% exhibit rather high strength, negligible swelling indexes and partly very high, fracture-dominated hydraulic con­ductivities. Confining units in the transition zone with clay-mineral contents of 20 to 40 wt.‑% generally also show low hydraulic conductivities but can occasionally be 1 – 2 orders of magnitudes above those measured in the Opalinus Clay.