: DFN – Discrete Fracture Network – is primarily a modeling framework for fractured geological systems that aims to integrate field data into simulations of flow and/or deformation. It is complementary to, or competing with, continuum methods with both advantages of easily integrating the statistical properties of fracture networks, and of not assuming any homogenization scale. The core element is the DFN conceptual model, which makes a functional link between data from different sources, prior knowledge and medium models. We discuss some fundamental issues about this conceptual model, namely (i) the upscaling of small-scale measurements to site-scale relationships, (ii) intrinsic variability versus geological determinism, (iii) the way to incorporate a priori knowledge, (iv) the transformation of a statistical description into a medium model, (v) the critical characteristics (length scales, scaling laws or physical properties) of fractures for a given DFN application. The main product of the DFN conceptual model is medium models, whose role is to extrapolate/interpolate data with a faithful representation of the geological system. The way in which fracture correlations are taken into account, or not, in the generation process plays an important control on the connectivity and flow properties of medium models.

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