[H-RE12] New Developments in Engineering Geology

[HRE12-P02] The importance of microcracking in crustal rocks for fluid migration: relation to radioactive waste disposal, geothermal exploration and carbon dioxide capture and storage

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

In the new development of applied geology in recent years, basic researches on radioactive waste geological disposal, 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 radioactive waste geological disposal, if radioactive nuclides leak from the artificial barriers (canister, overpack and buffer material), these dissolve into water in rocks (natural barriers) and migrate through it. In case that we use crystalline rocks such as granite as the natural barriers, 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, 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 radioactive waste geological disposal, since the coefficient of permeability is much higher than those in the surrounding rocks, 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 radioactive waste geological disposal sites 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.
Also, maximum attention must be paid to the fact that crustal fluids migrate along fractures in rocks for geothermal exploration and carbon dioxide capture and storage (CCS). For example, it has been known that in the famous Kakkonda geothermal area, Iwate Prefecture, the drilling was conducted through the brittle-ductile transition, and in the brittle region hydrothermal convection is occurring in the brittle region (e.g. Saishu et al., 2014). Hydrothermal convection is dispensable to extract the geothermal energy indicating that water-filled fractures are connected to the surface of the Earth. Therefore, the special distribution of fractures including that of microcracks is important for geothermal exploration. In case of CCS, the leak of highly pressurized carbon dioxide during the underground storage is a serious problem. Here, the detection of the special distribution of fractures is similarly very important.