Assessment of Hydraulic Connectivity of Fractures in Mudstones by Single‐Borehole Investigations

Abstract

AbstractAssessing the hydraulic connectivity of fractures by single‐borehole investigations is crucial to radioactive waste disposal but is still a challenge as such connectivity is difficult to measure directly. This study presents geological, hydrological, hydrochemical, and rock‐mechanical data for three faulted/fractured mudstones (the Koetoi, Wakkanai, and Palfris Formations) and proposes a new methodology for assessing the hydraulic connectivity of fractures. The methodology consists of three steps: (a) dividing the formation into two domains with a ductility index (DI) of >2 and <2 (DI is defined as the effective mean stress normalized to the tensile strength of intact rock), (b) estimating the hydraulic connectivity of fractures by analyzing pressure change obtained by packer tests and geological interpretation, and (c) verifying the estimation using pore pressure and water chemistry/geochemistry. The first step is necessary because the failure mode of damage‐zone fractures in fault zones can differ between the DI > 2 and DI < 2 domains, which may lead to significant differences in the hydraulic connectivity of fractures. During the second step, potential domains in which the hydraulic connectivity of fractures is limited are identified where upward trends, characterized by slopes of ∼0.5–1.0, are observed during the middle to late period of elapsed time on log‐log plots of pressure derivatives. Although the third step can be performed in various ways, this study employs the observation of pressure anomalies and the detection of young external water. Analyses of the three formations demonstrate the applicability and reliability of the proposed methodology.