The mechanisms contributing to self-sealing are of both physicochemical and hydromechanical nature. Bock et al. (2010) provides an empirical basis and some conceptual background for self-sealing in clay-rich rocks considered as host rocks for a deep geological repository.
For the Opalinus Clay, self-sealing processes were described in an earlier report by Nagra (2002). Since then, the understanding has been enhanced significantly, particularly at the Mont Terri rock laboratory, and using core material from the Schlattingen-1 (SLA1) deep borehole, drilled near the ZNO siting region. The summary of process understanding presented hereafter is based on the above-cited works, as well as on Mitchell & Soga (2005) and Laird (2006).
The fundamental pre-condition for self-sealing to occur is the generation of dilatant fractures (mode I or mixed mode I and II fractures, see Fig. 5‑24) in a preceding deformation event. Development of these fractures is locally associated with a change in the fluid pressure and the stress state. Phenomenologically, self-sealing leads to progressive reduction of the fracture aperture, but the underlying processes involve a volumetric expansion of the rock mass in the vicinity of the fracture (or system of fractures). These key processes are of hydromechanical and physicochemical nature and act either (i) mainly at the fracture surface, or (ii) in the (intact) rock behind the fracture wall. In both cases, the main factors controlling the changes to fracture aperture are the changes in total stress, and the changes in fluid pressure in the fracture and the pore pressure in the rock matrix.
Along the fracture surface, changes in fluid pressure can equilibrate rapidly compared to the pore pressure in the rock matrix, especially at very low effective stress. In the case of fluid injection (e.g. injection tests, Section 5.7.4), the fluid pressure acts orthogonally to the fracture surface (i.e. the normal stress) and forces the opening of the fracture. However, in the absence of active injection, pore pressure in the fracture and behind the fracture wall will decrease with dilation associated with fracturing because of the low compressibility of water. Negative pore pressure (suction) could also develop.
At the end of active injection, once fluid pressure decreases, an increase in the effective stress along the fracture occurs and generally results in rapid closure. The response to the effective stress changes depends on the hydromechanical properties of the rock. Clay-rich rocks, and the Opalinus Clay in particular, have a relatively low stiffness and a capacity to deform plastically. The compliance of the material leads to the deformation of asperities along the fracture surface (e.g. contact points), which further favours the decrease in fracture transmissivity. In contrast, the rough surfaces of more competent rocks can sustain much higher stresses without a significant reduction in transmissivity.
In contrast, pore pressure in the adjacent rock behind the fracture wall changes slowly because of the low conductivity (low pressure diffusivity) of the intact Opalinus Clay. A reduction in fluid pressure (or suction increase) in the dilatant fracture generates a gradient in the fluid pressure from the adjacent rock (higher pore pressure) towards the fracture, enhancing the effective stress reduction and therefore the volumetric expansion (Section 5.5.2). At the particle level, mechanisms typical of expansive geomaterials such as clays in contact with water and brine occur. These are the crystalline swelling between clay-particle stacks (water hydration) and double-layer
swelling because of the repulsive electrostatic forces within the quasi-crystals (Laird 2006, Bourg & Ajo-Franklin 2017). These mechanisms also affect the clay minerals of the asperities at the fracture wall, and further contribute to fracture closure (Fang et al. 2017).
Swelling is considered the key process of fault transmissivity reduction over longer timescales of in-situ tests (months to years, e.g. Marschall et al. 2017, Soom et al. 2021). As emphasised in Fig. 5‑23, the expansion is enhanced at low effective stress. Osmotic pressure gradients can further contribute to water movement. In partly saturated conditions, e.g. in the excavation damage zone (EDZ) of the tunnel, suction can also be an important driver from higher saturation rocks (the undisturbed rock mass) to those of lower saturation (the EDZ). Saturation increase also causes effective stress reduction and therefore volumetric expansion of the rock.
Fig. 5‑46:Concept of self-sealing sequence in the Opalinus Clay
Fracture aperture in black, larger clastic grains in white, clay matrix in light grey: (a) Initial condition with existing large fracture in the centre. (b) Fracture closing comes at the expense of material expansion in the vicinity of the fracture where effective stresses can be particularly low (locally "unstressed fracture surface"). This creates new fissures and leads to material disintegration. (c) Further closure can lead to fissuring deeper into the initially undamaged rock, but this will be closed in a similar fashion. Drawing based on a synchrotron X-ray micro-tomography study of Voltolini & Ajo-Franklin (2020).
Self-sealing of faults and joints is typically accompanied by the formation of smaller subsidiary fractures in the intact rock (e.g. Seiphoori 2019a, Wenning et al. 2021b, Voltolini & Ajo-Franklin 2020, Di Donna et al. 2019, Crisci 2019) to compensate for the initial fracture porosity (Fig. 5‑46). This is because of the locally unconstrained surfaces along the fracture, which can lead to swelling beyond the elastic limit and induce new fractures. Hence, the main fracture closes, but the fractured zone grows into the intact rock. Smaller fractures then close in similar fashion and self-sealing is complete when a hydromechanical and hydrochemical equilibrium is achieved in the near-field of the fracture. Hence, the initial fracture aperture is compensated by disintegration of the rock in a wider zone, and finally a homogenisation and local increase in "matrix" porosity compared to the undisturbed porosity. However, a local increase in matrix porosity equates to only very small or negligible changes in the overall transport properties (Section 5.7.5).
Additional mechanisms can further contribute to the sealing of fractures, but their contribution is subordinate to those described above. These involve "creep" defined as secondary compression (i.e. volumetric deformation with time at constant effective stress; Favero et al. 2016, Crisci et al. 2021) or the precipitation of secondary minerals ("veins", Section 5.7.6).