Abstract:

In this study, we proposed a fabric-enriched Continuum Damage Mechanics model to investigate the coupled influence of damage and healing on the mechanical and transport properties of salt rock. In order to infer the form of fabric tensors, we carried out creep tests on granular salt assemblies under constant temperature and humidity conditions and used micro-computed tomography for microstructure characterization. Using microscope imaging and micro-CT scanning, we analyzed the probability distributions of crack radius, void areas and crack spacing and used them as a basis to derive macroscopic evolution laws. A stress path comprising a tensile loading, a compressive unloading, a creep-healing stage, and a reloading was simulated. As expected, stiffness decreases (respectively increases) and permeability increases (respectively decreases) upon damage (respectively healing). Results also highlight the increased efficiency of healing with temperature. The micro-macro relationships established by statistical image analysis also provide the evolution of microstructure descriptors during the test. Simulations show that permeability changes are controlled by changes in crack connectivity, which dominate changes of porosity. The proposed framework is expected to improve the fundamental understanding of coupled processes that govern microstructure changes and subsequent variations of stiffness and permeability in salt rock, which will allow the assessment of the long-term performance of geological storage facilities.

Introduction

The very low permeability of salt rock is its principal advantage for nuclear waste disposals and high-pressure gas storage. During secondary creep, dislocation glide produces isochoric deformation. Geometric incompatibilities between grains result in grain pile-ups, stress concentrations, and crack propagation. The coalescence of cracks can increase permeability by several orders of magnitude, which is very critical to the long-term performance of geological storage facilities in salt.

Many approaches have been proposed to predict the permeability in rocks. A large number of models are based on Kozeny-Carman relationship to link permeability with porosity. However, experimental results indicate that when micro-cracks propagate in salt, permeability increases significantly, whereas the change in porosity is moderate (e.g., Gloyna and Reynolds, 1961; Peach, 1990). Guéguen and Dienes (1989) first introduced the percolation concept and proposed pipe and crack models to estimate permeability in rock materials. A geometrical model based on the Bethe Lattice was then proposed to improve this crack model (Peach and Spiers, 1996). Oda et al. (2002) used a fabric tensor to deduce the variations of the anisotropic permeability tensor during damage propagation in granite. Researchers have also made efforts in incorporating microstructure parameters such as the tortuosity and the specific pore surface area into the classical Kozeny-Carman model (e.g., Chan et al., 2001). However, most of these frameworks do not take into account the evolution of mechanical properties, especially under the coupled influence of damage and healing.

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