FLUID PERMEABILITY OF DEFORMABLE FRACTURE NETWORKS

We consider the problem of defining the fracture permeability tensor f or each grid block in a rock mass from maps of natural fractures. For this purpose we implement a statistical model of cracked rock develope d by M. Oda, where the permeability tensor is related to the crack geo metry via a volume average of the contribution from each crack in the population. In this model, tectonic stress is implicitly coupled to fl uid flow through an assumed relationship between crack aperture and no rmal stress across the crack. We have included three enhancements to t he basic model. (1) A realistic model of crack closure under stress ha s been added along with the provision to apply tectonic stresses to th e fracture system in any orientation. The application of compressive s tress results in fracture closure, and consequently, a reduction in pe rmeability. (2) The fracture permeability can be linearly superimposed onto an arbitrary anisotropic matrix permeability. (3) The fracture s urfaces are allowed to slide under the application of shear stress, ca using fractures to dilate and result in a permeability increase. Throu gh two examples we demonstrate that significant changes in permeabilit y magnitudes and orientations are possible when tectonic stress is app lied to fracture systems.