Modelling of flow and transport processes in fractured media

Nowadays, hard-rock systems are used in multiple ways, e.g as radioactive repositories, „geothermal reactors“ and so forth, posing especially safety questions regarding the spreading of a contaminant plume (e.g. landfill leachate, agricultural furnish into groundwater). What is the travel time of the plume? Is the groundwater safe? Is there a need to remediate? In order to answer these important questions, research has on the one hand to characterize the natural attributes of the bedrock in more detail, while on the other hand, there is a need for a better understanding of flow and transport processes in fractured-matrix systems.

Discontinuities (shear zones, fissures etc.) significantly influence the flow and transport processes in a hard-rock aquifer. The open cracks direct the flow along their pathways independently of the main flow direction, causing, for example, secondary flow directions. It is possible to derive the porosity and the permeability from the bedrock characteristics, such as the fracture density, connectivity and aperture. There are two main physical processes acting in fractured-matrix systems. When fractures are filled with clay (sealed), one finite matrix exists, in which flow processes are rather slow. The dominant process in the matrix is the high accumulation of possible contaminants. In contrast, open fractures enhance the flow and transport of a contaminant, leading to higher fluxes than in a heterogeneous porous medium. In comparison to the matrix system, the potential of accumulation is infinitely small. To describe these two different dominant physical processes (specific storage, flow velocity) by a numerical model is quite challenging.

To describe the characteristics of a hard rock by a synthetically generated fractured-matrix aquifer, the fracture-matrix generator FRAC3D was further developed by Silberhorn-Hemminger [2002] and Assteerawatt [2008]. The rock characteristics, which can be obtained by laboratory or field tests, are fitted to geostochastic or stochastic distribution functions. The measured geological attributes are assigned to the program FRAC3D as function parameters, which will then generate a synthetic fractured-matrix system with the same rock characteristics as in nature. Thus, the fracture-matrix generator is the link between nature and the numerical model.

There are several possible ways of modelling the flow and transport process. With FRAC3D, it is easy to use a discrete approach. In this case, the modeler has to know the exact location of all fractures. Usually, this approach is applied only to small model domains, due to the high computing capacities required. Another possibility for describing flow and transport is by a double-porosity model. Fractures and matrix are viewed as single continua. The interface between the two continua is usually approximated by a linear transfer term (Lang [1995]). A further approach to modelling flow and transport processes in a fractured porous medium is the application of the model „Multiple Interacting Continua“ (MINC). The matrix blocks (porous media) are only connected by the fracture matrix system. Each matrix block consists of an arbitrary sub-quantity of continua. The influence of a fracture on the porous media decreases with distance to a (sub-) continuum.