[H-RE13] New Developments in Engineering Geology

[HRE13-03] The importance of microcracking in crustal rocks for fluid migration pathways: applications to geological disposal of radioactive waste, geothermal exploration and CCS

*Toru Takeshita¹ (1.Adviser, Land Infrastructure Division, Pacific Consultants Co., Ltd.)

Keywords:microcracks in the crustal rocks, fluid migration pathways, geological disposal of radioactive waste, geothermal exploration, carbon dioxide capture and storage (CCS), universal stage attached to an optical microscope

In the new development of applied geology in recent years, basic researches on geological disposal of radioactive waste, geothermal exploration and carbon dioxide capture and storage (CCS) are notable. In these researches, a targeted important underground process is the migration of crustal fluid (i.e. water and carbon dioxide), which is either liquid or gas. For example, in case of geological disposal of radioactive waste, if radioactive nuclides leak from the artificial barriers (canister + overpack + buffer material), these dissolve into water in rocks (natural barriers) and migrate through it. The radioactive nuclides migrate through water, which occurs along grain boundaries or microcracks. Here, only if water channels are connected and water can migrate driven by hydraulic gradient, the radioactive nuclides can migrate relatively fast, but if not, water diffusion in water is negligibly slow. For example, in the excavation damaged zone (EDZ) which will be generated by the geological disposal of radioactive waste, since the coefficient of permeability is much higher than those in the surrounding rocks, the radioactive nuclides migrate through the EDZ much faster. In this case, along the EDZ the advective transfer is two orders of magnitude faster than the diffusive one (Bianchi et al., 2015). Generally, the structures which show high coefficients of permeability leading to the high-speed migration of radioactive nuclides, and therefore could be concerned for the geological disposal sites of radioactive waste are fault damage zones (e.g. Mitchell and Faulkner, 2009). Here, those structures such as minor faults, fractures and deformation bands which densely develop nearly parallel to the main fault (e.g. Trabi et al., 2009) could cause the increase of the coefficient of permeability. However, even if fault damage zones exist near the proposed radioactive waste geological disposal sites, radioactive nuclides would not directly move into the water conduction layer, but they originally move through the surrounding rocks (matrices) of low conductivity, and then are finally brough into it (e.g. fig. 1 of Tsang et al., 2015). Here, since it can be inferred that water-filled microcracks play an important role in water migration in the matrices, the orientation distribution and density of microcracks, and their relationship to the coefficients of permeability will be a future important research topic in the basic researches of radioactive waste geological disposal. Similarly, maximum attention must be paid to the fact that crustal fluids migrate along fractures and microfractures in rocks for geothermal exploration and carbon dioxide capture and storage (CCS). Finally, in this presentation, I will explain how we can analyze the preferred orientation of microcracks and related microstructures using a universal stage attached to an optical microscope.