Scale effect and heterogeneity of hydraulic conductivity of sedimentary rocks at Horonobe URL site

Abstract

The Japan Atomic Energy Agency has been developing the Horonobe Underground Research Laboratory in sedimentary rock at Horonobe, on Hokkaido, Japan, as part of the research and development program on geological disposal of high-level radioactive waste. Fundamental parts of the geological and hydrogeological investigations in the project include BHTV and EMI logging, fluid logging, pressure build-up tests and hydraulic packer tests in eleven 470-m to 1020-m deep boreholes, supplemented by laboratory permeability tests on core samples from the sedimentary formations. The results show that hydraulic conductivity varies significantly and that fractures/faults have a close relationship to the heterogeneities in hydraulic conductivity. This paper discusses the heterogeneities in hydraulic conductivity from the perspective of scale effect, to characterize and model the hydrogeological environment. It was determined from the relationship between hydraulic conductivities and test interval lengths that the heterogeneity of hydraulic conductivity in the Wakkanai Formation decreased with increase of test interval length. Considering this observation, the following concept for hydrogeological modeling dependent on the scale is proposed; the Wakkanai Formation should be modeled as a fractured medium at scales ranging from 10-m to 200-m and as a homogeneous medium at scales over 200-m. The Koetoi Formation, the other formation of interest and the intact rock in both formations could be modeled as homogeneous media.