Relationship between the geometrical and structural properties of layered fractured rocks and their effective permeability tensor. A simulation study

SummaryThe influence of geometrical and structural properties of layered fractured rocks on their permeability tensor is investigated for an elementary volume at the meter scale. Various synthetic networks were generated with a three-dimensional model that incorporates a pseudo-random process controlled by mechanical rules of fracturing. The permeability is supposed to be a function of the discontinuities aperture in an impermeable matrix. Steady state, saturated laminar flow is assumed within discontinuities of constant aperture (parallel joint walls). For 81 networks associated to different sets of numerical parameters, the geometrical and structural properties are computed from 200 statistically equivalent simulations and averaged to asses the mean geometrical and structural properties. In a similar way, the mean permeability tensor associated to each network is calculated. Then, the effect of the geometrical and structural properties of the network on the permeability tensor values is studied. We show that some geometrical and structural properties may have non-negligible effects: The bedding perpendicular joint spacing and the bedding plane stopping capacity can induce more than one order of magnitude change in the permeability values, while other investigated properties (mean and standard deviation of the bedding perpendicular joints length distribution) have a very small influence. These results were compared with the permeability tensor inferred from one analytical model which provided permeability values of the same order of magnitude as those inferred from the numerical model. This model can thus be considered within the hierarchical procedure proposed to characterize rock mass permeability tensors in the field.