Opening of fractures induced by shear dilation can be a significant source of fracture permeability change. In this study, the zones of fracture shear slip were examined through three-dimensional thermo-mechanical analysis of a nuclear waste repository model using the finite element method. Stress evolutions in selected locations revealed the main mechanisms of the generation of thermal stress important for fracture shear slip. The implications are that fractures of different orientations are vulnerable to shear slip at various locations throughout the lifespan of a geological repository. Stress paths obtained from the thermo-mechanical analysis were used as stress boundary conditions in order to investigate the effect of stress change on permeability. DFN-DEM (Discrete Fracture Network - Discrete Element Method) analysis showed that normal deformation dominated fracture closure/opening in four models and shear dilation dominated in the remaining two models. In the latter two models, modest permeability increases up to a factor of four were observed during thermal loading history. Permeability changes caused by shear dilation were not recovered after cooling of the repository, which was in contrast with the recovery of permeability changes for models in which normal fracture closure dominates.
Various studies have shown that shear dilation of fracture can be a source of significant fluid flow in fractured rock. A field investigation by Barton et al. (1995) supports the contention that critically stressed fractures are the ones that carry the major portion of the fluid flow and this finding was demonstrated numerically by Min et al. (2004a). While characterizing permeability is one of the most important tasks in determining the feasibility of geological repository for nuclear waste, it is important to recognize that the fluid-carrying ability of underground rock, i.e., its permeability, is a dynamic variable over the lifespan of a geological repository (Rutqvist and Stephansson, 2003). Excavation of the repository tunnel and the deposition holes and the thermal stress generated from the heat-generating nuclear waste are the main sources for the alteration of the stress state, and hence the permeability, of rocks around repository. While the effect of excavation is expected to develop in the near-field, which is defined as within a few times diameters of the repository tunnel and the deposition holes, the influence of thermal stress will reach mid- and far-field which can be several hundred meters from the periphery of the repository.
The objectives of the current study are:
to examine the contribution of thermal stress to the shear slip of fractures in mid- and far-field around the repository,
to investigate the effect of the evolution of stress states on the permeability of repository settings, and
to identify the shear slip potential through the entire lifespan of a KBS-3 type of a deep geological repository.
This study is composed of three steps as shown in Figure 1. In the first step, three-dimensional thermomechanical analysis is conducted to obtain the full stress history in the far-field around the repository.
