We present a 3D network model of fractures that allows a systematic
study of the interplay and relative importance of the two key factors determining
the character of flow and transport in the system. These factors are
geometry of the network and the aperture variability of its elements. Empirical
functional relationship is obtained that quantifies the dependence of the effective
hydraulic conductivity on aperture variability and on the network
structure. This leads to an explanation for the field-length dependence of
permeability observed in fractured and heterogeneous porous formations. Transport
through the network is investigated and relation between aperture variability
and the Peclet number is established.