Detailed questions cannot always be dealt with efficiently and economically in a large-scale principle model. The size of the model results in long computing times and the discretisation is often not fine enough.

One solution is to separate the area of interest as a sub-area and only then consider it in more detail. The partial model is coupled to the overall model by specifying boundary conditions, usually mass flows. The advantages of this approach can be summarised in the following points:

• Great flexibility in sub-area delimitation

• Mesh changes and the variation of parameters can be handled efficiently.

In order to minimise the influence of the model adjustments inside the sub-area on the boundary conditions, the boundaries of the sub-area should be defined far away from the area under investigation. The following criteria should also be taken into account when defining the boundary:

Creation of fractures in a 3D model