Abstract

Fluid flow in fractured rock is controlled by flow paths in a network of discrete pre-existing fractures. The same can be said for fracture networks induced by loading or unloading. In both cases, or in the combined case, the determination or evolving modification of an equivalent permeability tensor is necessary for up-scaling the equivalent continuum hydro-mechanical properties of a fractured rock. The evolution of fracture aperture during shearing and the associated dilation process control the permeability of single fractures and fracture networks. In order to understand this process, a number of rotated Discrete Fracture Networks is generated with different mean fracture orientations. Permeability tensors are calculated, from distinct element hydro-mechanically coupled models, when initial fracture apertures are correlated with fracture trace length in 2D models at different stress ratios. A non-linear behavior between normal stress-normal displacements of fractures is adopted so that both scale and stress-dependent normal stiffness of the fractures can be considered. The results show that overall permeability continued to decrease, with the increase of stress ratio, when zero dilation is assigned to the fractures. However, when the dilation mechanism of the fractures is modeled, equivalent permeability decreases with increasing stress magnitudes when stress ratio is not large enough to initiate shear dilation processes in a fracture. In this case, normal closure of the fractures is the dominating mechanism for decreasing the overall permeability of models. When stress ratios increased up to k=3 or higher, most of the fractures will experience shear dilation with increasing overall flow rates of models. Flow rate distribution at different stress ratios show that a small number of large fractures with large initial aperture values remains conductive in all stress conditions and control the permeability and flow pattern of the models significantly. With increasing stress ratios, it becomes more and more difficult to establish an equivalent permeability tensor, compared with the non-stressed model. For models with non-zero values of fracture dilation angle, the existence of permeability tensor is not affected by changing in the magnitude of dilation angle when small stress ratio is applied. However, when stress ratio increases up to k=3 or higher, the values of dilation angles lead to additional contributions from shear dilations of fractures which become the main reason for increasing of permeability, compared to the models with lower dilation angle.

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