Estimation of upscaled hydraulic conductivity profiles along a borehole in low permeability crystalline rock using discrete fracture network models

In Japan, the Mizunami Underground Research Laboratory (MIU) project is being run by the Japan Atomic Energy Agency to establish comprehensive techniques for investigation, analysis and assessment of the deep geological environment with different spatial scales in fractured crystalline rock. The project is implemented in three overlapping phases: Surface-based investigation (Phase I), Construction of shafts and galleries (Phase II) and Operation of research galleries (Phase III). In this study, the following issues are considered to achieve the above-mentioned goals of the project and to establish methodologies of investigation, modeling and interpretation for evaluating the groundwater flow characteristics efficiently: - to newly develop the groundwater flow modeling methodologies considering the hydraulic heterogeneity due to the water-conducting features (WCFs) in the fractured rock - to establish the methodologies to refine the groundwater flow model based on the information in the phased approach of the project (Phase I to III) The modeling and analysis considers hydraulic heterogeneity on the block scale (block dimension in the order of several tens of meters). The data from the deep borehole investigations during the Phase I have been applied for upscaling of transmissivity profiles into equivalent hydraulic conductivities. Profiles of the equivalent hydraulic conductivity can be applied to groundwater flow models based on an equivalent porous medium approach that are used for hydrogeological characterization at over the several kilometers scale. A combined interpretation of core observations, BTV (Borehole Television) and fluid logging has been used to identify and to classify WCFs among observed fractures in three deep boreholes. Transmissivity profiles of WCFs were determined by the interpretation of fluid logging and the results of short interval packer tests. An equivalent hydraulic conductivity for the sub-divided block is calculated using a hydraulic discrete fracture network (hydroDFN) model based on the information. Long interval packer test results have been used to evaluate uncertainty in the calculated equivalent hydraulic conductivity. Groundwater flow analyses using developed hydroDFN model are also carried out to assess groundwater flow conditions around galleries.