A comprehensive summary of evidence for self-sealing in argillaceous formations in the context of geological disposal of radioactive waste was provided by Bock et al. (2010).

Specific to the Opalinus Clay, there is both phenomenological and direct evidence from active testing for self-sealing (Tab. 5‑2). The phenomenological evidence stems from observations in tunnels and boreholes (see also Sections 5.6.3 and 5.6.4) that tectonic structures in the Opalinus Clay do not show significant changes in hydraulic properties in situ. In addition, experiments at the Mont Terri rock laboratory and in the RHE1 borehole in the ZNO siting region at potential repository depth provide direct evidence for self-sealing, as fissures are either generated or faults reactivated. These experiments have demonstrated that faulting or fault re-opening at low effective stress is associated with an increase in transmissivity by several orders of magnitude and followed by fault closure and a strong decrease in transmissivity over time and with an increase in effective stress.

The empirical findings are complemented by numerous laboratory experiments under controlled conditions, which describe the self-sealing behaviour in the Opalinus Clay phenomenologically and quantitatively:

  • Small-scale tests with artificially induced cracks in core material show closure of the cracks within hours to days in the presence of water and confining stress of a few MPa (e.g. Seiphoori 2019a, Wenning et al. 2021a, Voltolini & Ajo-Franklin 2020; see Fig. 5‑45).

  • Direct shear and triaxial tests with measurements of hydraulic properties exhibit rapid dis­aggregation of the rock in the vicinity of fractures (e.g. Zhang et al. 2008, Cuss et al. 2011, Monfared et al. 2012, Zhang 2013, Seiphoori 2019a), and eventually their closure, with a significant reduction in conductivity (several orders of magnitude) reaching values close to (or slightly higher than) the intact material.

  • Detailed studies of volumetric behaviour such as consolidation and swelling tests (Section 5.5.2) quantify the volumetric expansion or compaction due to effective stress changes as well as hydration.

Tab. 5‑2:Empirical findings and key experiments on self-sealing in the Opalinus Clay

EDZ: Excavation damage zone.

Locality / Area

Findings

Conclusion

References

Indirect evidence for self-sealing (phenomenological evidence)

Internal Jura

Geological gallery surveys in railroad and road tunnels: no dripping water or damp spots where overburden thickness greater than a few hundred metres (e.g. Mont Russelin with 300 – 400 m overburden).

Transmissivity of brittle structures at larger overburden thickness is very low, even in heavily faulted sections.

Gautschi (2001), Gautschi (2017)

Autochthonouscover, Alpine Foreland

At Lausen, Opalinus Clay outcrops beneath Quaternary. Drastic decrease in hydraulic conductivity within uppermost 30 m depth from the surface.

Low depth / stress sufficient to have a significant sealing effect.

Vogt et al. (2017) and Section 6.5.2

Central Molasse Basin

Strong tectonic overprint at Base Opalinus Clay in the Schafisheim borehole: no increased hydraulic conductivity.

Brittle structures do not differ significantly hydraulically from the rock matrix.

Gautschi (2017)

Siting regions

Diverse brittle structures of different sizes in numerous deep boreholes: no significantly increased hydraulic conductivity in packer tests.

Brittle structures do not differ significantly hydraulically from the rock matrix.

Gautschi (2017), Dossier VII of reports in Tab. 2‑1

Whole of Northern Switzerland

In the Opalinus Clay, vein fillings are rare and volumetrically not relevant.

No significant water – rock interaction associated with advective water flow.

Nagra (2002), Mazurek et al. (2018)

Mont Terri rock laboratory

"Main Fault": no increased hydraulic conductivity in packer tests prior to fault reactivation.

Under undisturbed effective stress conditions, the "Main Fault" is not hydraulically effective.

Nussbaum & Bossart (2004), Guglielmi et al. (2020)

In-situ experiments with direct evidence for self-sealing

Mont Terri rock laboratory

GS experiment: significant crack transmissivity is observed only at low effective normal stress (< 2 MPa).

Only low normal stress required for mechanical self-sealing of discrete deformation structures.

Nagra (2002)

Mont Terri rock laboratory

In the Selfrac experiment, the hydraulic conductivity of the EDZ decreased by about 2 orders of magnitude over a period of 2 years. At loads of up to 5 MPa, an additional decrease by 2 orders of magnitude is observed within 6 months.

Various other experiments confirm the findings (HG-A, MB etc.).

Open fractures in the EDZ seal by contact with water (swelling) and by increasing the normal effective stress.

Nagra (2002), Marschall et al. (2017)

Mont Terri rock laboratory

FS experiment: rapid reduction of trans­missivity by 2 orders of magnitude after stimulation (pressure pulse) at the "Main Fault", followed by a slow reduction and closure at sustained rate over a period of approximately 1 year.

Confirmation of the findings from the Selfrac experiment. Empirical evidence from EDZ can be transferred to larger fault zone.

Soom et al. (2021)

RHE1 deep borehole, 580 m depth

Head injection test: Increase in transmissi­vity by approx. 3 orders of magnitude by increasing pore pressure until reactivation of an existing disturbance. After reduction of the pore pressure, the transmissivity drops back to the level before stimulation (see Section 5.7.4).

Transmissivity only elevated near fracture opening pressure. High transmissivity only main­tained at high pore pressure.

Dossier VII of Nagra (ed.) (2023b) and Section 5.7.4 of this report

The knowledge base is further supplemented by analogue studies of similar rocks:

  • In particular, the Callovo-Oxfordian Claystone from the French investigation programme (e.g. OECD/NEA 2022, Wang et al. 2022, Zhang & Talandier 2023) with very similar hydro­mechanical properties to the Opalinus Clay.

  • Experiments in the Tournemire rock laboratory with Toarcian-Domerian claystones (OECD/NEA 2022, Donzé et al. 2023).

  • Very broad literature documenting clay-rich rocks (shales) with similar mineralogy and basic properties as to the Opalinus Clay acting as top seals or cap rocks of conventional hydro­carbon reservoirs over geological timescales (e.g. Fisher et al. 2013, 2023).

image203.png

Fig. 5‑45:Experimental evidence of self-sealing in laboratory experiments

X-ray images at the beginning (a), (b) and about 2 days after water injection (c), (d), with (b) and (d) illustrating the reconstructed 3D image of the fractures in red. The apertures of the original fractures (up to 200 mm) in (b) reduce partly to below the imaging resolution of about 30 mm in (d). Experimental details can be found in Seiphoori (2019a).

 

In contrast to cap rocks, the clay-mineral content and porosity in gas resource shales is much lower, and deformation response more brittle. However, successful production of gas from resource shales generally relies on injection of proppants to maintain flow after fracturing, demonstrating relatively rapid closure even in shales with low self-sealing potential (Fisher et al. 2013, 2023).