Laboratory measurement of orthogonally anisotropic and low permeability of a crystalline rock

Crystalline rocks such as granitic rocks are characteristic of anisotropic and low permeability. It is due to the distribution and connectivity of preexisting microcracks. In this study, hydraulic properties (hydraulic conductivity and specific storage) of granite was measured by transient pulse permeability test and then the relationship between the properties and microcrack distribution was analyzed based on stereology. The results showed that the hydraulic properties of the granite are strongly controlled by the geometry and distribution of microcracks.

Laboratory measurements of hydraulic properties of low-permeability rocks need to be carefully conducted for precision. In general, transient pulse method suits low-permeability materials. Remarks on measuring hydraulic constants of rocks in laboratory are given and a modified analytical solution of the transient pulse method in consideration of the constituents of the test system are also introduced in the presentation.

3-D distribution of microcracks in crystalline rocks is difficult to be visualized even if micro-focus X-ray CT scanner is used. Therefore, the 3-D distribution of microcracks was reconstructed using microphotographs taken in three mutually perpendicular surfaces. Once the geometry and distribution of microcracks are estimated, the hydraulic conductivity tensor of the rock can be calculated.