Physical Modelling of Stress-dependent Permeability in Fractured Rocks

This paper presents the results of laboratory experiments conducted to study the impact of stress on fracture deformation and permeability of fractured rocks. The physical models (laboratory specimens) consisted of steel cubes simulating a rock mass containing three sets of orthogonal fractures. The laboratory specimens were subjected to two or three cycles of hydrostatic loading/unloading followed by the measurement of displacement and permeability. The results show a considerable difference in both deformation and permeability trends between the first loading and the subsequent loading/unloading cycles. However, the micrographs of the contact surfaces taken before and after the tests show that the standard deviation of asperity heights of measured surfaces are affected very little by the loadings. This implies that both deformation and permeability are rather controlled by the highest surface asperities which cannot be picked up by the conventional roughness characterization technique. We found that the dependence of flow rate on mechanical aperture follows a power law with the exponent n smaller or larger than three depending upon the loading stage. Initially, when the maximum height of the asperities is high, the exponent is slightly smaller than 3. The first loading, however, flattens these asperities. After that, the third loading and unloading yielded the exponent of around 4. Due to the roughness of contact surfaces, the flow route is no longer straight but tortuous resulting in flow length increase.