[H-CG21] Nuclear Energy and Geoscience

[HCG21-04] Permeability measurement of borehole core samples using a probe permeameter and application to regional hydrogeological modeling

*Taiki Kubo¹, Jacek Scibek, Katsuaki Koike¹ (1.Graduate School of Engineering, Kyoto University)

Keywords:Permeability, Toki granite, geological disposal , Mizunami area

To evaluate the safety of geological disposal of radioactive waste, it is important to understand the regional hydraulic structure in underground. The permeability of crystalline rock is dominated by fracture system. However, when the stability on the very long-time scale is considered, properties of the rock matrix and alteration parts also contribute to hydraulic structure. In this study, we develop a borehole core measurement technique with a probe permeability test instrument (permeameter) and propose its application to regional hydrogeological modeling. The permeameter is suitable for extracting changes in permeability in rock matrices and small alteration zones, which are difficult to measure by hydraulic borehole tests. We selected the Mizunami area in Gifu Prefecture as a case study site. Many geological investigations have been implemented in this site by JAEA to understand the deep underground properties. Kubo et al. (2013, 2019) proposed the methods of 3D fracture modeling (GEOFRAC) and hydraulic parameters estimation. In this presentation, additional data and statistical analysis results are added to them by measuring permeabilities of more than 100 boring core samples, and a new composite geological model which integrates them is proposed. From the measurement results, the spatial variation of hydraulic conductivity obtained by hydraulic tests was improved and revealed more in detail. In addition, rock properties and their distribution were characterized by the detailed observations of the core samples, which has a significant effect on the permeability besides fractures. Permeameter measurements revealed that a region of granite with many voids, termed the episyenites zone, had high permeability. It is considered that such areas can form a different flow pattern than the groundwater flow system dominated by a fracture system. Finally, a groundwater flow simulation was performed using a hydrogeological model constructed by integrating the observed data with the GEOFRAC model. The results were used to interpret the flow system around the major faults in the study area.

Acknowledgment:
This work was supported by the Ministry of Economy, Trade and Industry (METI) of Japan.

Kubo, T., Koike, K., Liu, C., Kurihara, A., Matsuoka, T., 2013. 3D hydraulic conductivity modeling of fractured granitic body using geostatistical techniques and its application to regional groundwater flow analysis. Journal of Geography 122, 139-158. https://doi.org/10.5026/jgeography.2012ap02 (Japanese with English abstract)
Kubo, T., Matsuda, N., Kashiwaya, K., Koike, K., Ishibashi, M., Tsuruta, T., Matsuoka, T., Sasao, E., Lanyon, G.W., 2019. Characterizing the permeability of drillhole core samples of Toki granite, Central Japan to identify factors influencing rock-matrix permeability. Eng. Geol. 259, 105163. https://doi.org/10.1016/J.ENGGEO.2019.105163