Pretty Porous

Flow and deformation in fractured poroelastic media

Fractures are features which are commonly found in geological materials, and they can have a strong impact on their hydraulic and mechanical properties. For example, highly conductive fractures in a low-permeable rock can act as preferential flowpaths along which rapid fluid flow can occur. Besides that, the fractures provide interfacial areas for the transfer of mass and/or heat between the fractures and the surrounding rock. Several geotechnical engineering applications make use of this fact, as for example geothermal energy or unconventional shale gas production techniques.

The motivation for the development of the model presented here originates from a project related to underground radioactive waste storage. One approach is storing the waste inside tunnel systems that are excavated within low-permeable clay formations, which act as barriers for flow and transport of potentially radioactive components over large time scales. Transport away from the emplacement tunnels could be driven by the pressure increase that is expected to occur due to the release of hydrogen gas as a consequence of the anaerobic corrosion of the metal canisters. Besides this, the excavation of the tunnels leads to the creation of fractures in the near vicinity of the emplacement tunnels.

The scientific question that arises is how the fractures present in the surroundings of the tunnels influence the hydraulic properties of the clay rock. Clay rock is relatively soft, which is why the current hypothesis is that the dilation of the present fractures can help to reduce the pressure build-up inside the emplacement tunnels. To this end, experimental studies on cylindrical rock samples taken from the surrounding clay rock are envisaged, with which the dilation of the fractures as function of the pressure increase is to be quantified.

The model that is presented here was designed to provide a tool with the help of which the experimental results can be better interpreted, and which allows for studying the hydraulic properties of a number of synthetically generated rock samples and fracture networks. In experimental studies, this would involve very large technical and financial efforts. The model considers a poroelastic rock matrix, that is, the interaction between the flow through and the deformation of the rock sample is taken into account, and the rock is described by means of a linear elastic material law. Besides this, flow along the fractures is considered, where the fractures are modeled as two-dimensional planes as the fracture apertures are typically very small in comparison the extent of the samples. The aperture is then a variable defined on the fracture planes and is a function of the deformation of the medium. This way, the influence of the deformation on the hydraulic properties of the fractures, and in turn on the hydraulic properties of the entire sample, is captured.

Credits: University of Stuttgart / Dennis Gläser