The Impact of the Anisotropy of Shale Creep on the Long-Term Stress Evolution of a Geological Nuclear Waste Repository
Authors: Sasaki T. , Rutqvist J. 
Published: September 2023

ABSTRACT
Shale is considered a suitable host rock for geological nuclear waste repositories due to its low permeability and tendency to reduce stress concentrations via creep. However, such a claim has not been carefully examined from the perspective of shale anisotropy caused by its bedding. It is thus crucial to examine the effect of shale anisotropy on the long-term performance of geological nuclear waste repositories. In this study, we assessed the effect of mechanical (i.e., elastic and creep) anisotropy of shale on the long-term stress evolution of a hypothetical geological repository. A new constitutive model for anisotropic shale creep was developed based on the power-law creep model and the anisotropic plasticity theory. The developed anisotropic creep model was implemented in the TOUGH-FLAC simulator and a thermo-hydromechanically (THM) coupled simulation was carried out for the long-term (&gt;10,000 years) performance assessment of the geological repository. We examined three different shale formation scenarios: (i) isotropic elasticity & creep, (ii) anisotropic elasticity & isotropic creep, and (iii) anisotropic elasticity & creep. Results show that the stresses in the repository were not significantly affected by the simulated anisotropic shale elasticity and/or creep, suggesting that isotropic elasticity & creep would sufficiently capture the long-term mechanical behavior of geological nuclear waste repositories.

INTRODUCTION
It is known that the behavior of geological nuclear waste repositories is highly thermo-hydromechanically (THM) coupled and it is thus crucial to incorporate each of the THM processes in the performance assessment of the repository. Although the mechanical process is sometimes considered time-independent as rock deformation would not evolve under constant stress conditions, they can creep (i.e., deform viscoelastically or viscoplastically) over the timescale considered for the geological disposal (&gt; 10,000 years) and hence creep can affect its long-term performance. Meanwhile, shale is considered a suitable host rock for the geological disposal due to its low permeability, and it is characteristic of shale to possess inherent anisotropy due to the presence of bedding planes. However, it remains uncertain if/how such inherent anisotropy affects the creep behavior of shale and the performance of a shale repository. Without understanding the anisotropy effect, it would not be possible to carry out robust performance assessment of shale repositories, hence it is critical to examine such an anisotropy effect. In this study, a THM coupled numerical modelling of a hypothetical geological repository was carried out to assess the effect of elastic and creep anisotropy of shale on the stress evolution of the repository for over 10,000 years. A new constitutive model for anisotropic creep was developed based on the power-law creep model and the anisotropic plasticity theory. The model parameters were calibrated against laboratory creep test results on shale with varied loading angle to the bedding plane. In order to assess the effect of elastic and creep anisotropy, three different shale cases were simulated: (i) isotropic elasticity and creep, (ii) anisotropic elasticity and isotropic creep, and (iii) anisotropic elasticity and creep. The results of these simulation cases along with the detail of the numerical model and anisotropic creep model are provided in the following sections.