Hydro-Mechanical Analysis of Packer Test in Opalinus Clay: Implications for Formation Pressure Estimation

Opalinus Clay is a shale formation that has been chosen as a host rock for the construction of a radioactive waste repository in Switzerland. This work presents an advanced hydro-mechanical (HM) analysis of a hydraulic packer test conducted in the Opalinus Clay shale section of a deep borehole drilled for repository site characterization. The analysis aims to enhance the interpretation of packer test results by evaluating the effects of mechanical processes on hydraulic conductivity and formation pressure estimation, which are crucial for assessing the long-term safety of potential nuclear waste repositories.

The approach involves developing a unified 3D HM coupled model using finite elements that integrates all stages of the packer test, accounting for vertical anisotropy in permeability and mechanical properties. The study incorporated two stages: (i) Poroelastic modelling and (ii) Elastoplastic analysis.

First, the Opalinus Clay  is modelled as a linear-elastic anisotropic material. The model considers the stress release due to borehole opening, the total stress applied by the drilling mud, and the pore pressure increment resulting from the drilling fluid pressure. A parametric analysis is performed, focusing on hydraulic parameters, varying horizontal hydraulic conductivity and formation pressure to evaluate their impact on test results.

For the second stage, the elastoplastic model eADP (Madaschi et al. 2023) is used to evaluate the effects of non-linear stress-strain behaviour and the formation of damage zones on test outcomes. The model incorporates elasto-plastic anisotropy and reproduces the quasi-brittle behaviour that characterizes shales. The constitutive parameters have been calibrated with an extensive dataset of laboratory mechanical tests on Opalinus Clay sourced from the sites under investigation by Nagra (Crisci et al. 2024).

The results indicate that while the interpreted value of permeability using pure hydraulic models can be considered robust, the formation pressure is affected by mechanical perturbations. The elastoplastic analysis reveals that the development of a damage zone around the borehole can significantly influence the pressure build-up during the test. The modelling results assist in quantifying the effects of borehole disturbance on the hydraulic head derived from packer tests and allow to better reconcile those with results from long-term monitoring data.

References:

Crisci, E., Giger, S.B., Laloui, L., Ferrari, A., Ewy, R., Stankovic, R., Stenebråten, J., Halvorsen, K., Soldal, M., 2024. Insights from an extensive triaxial testing campaign on a shale for comparative site characterization of a deep geological repository. Geomechanics for Energy and the Environment 38, 100508. https://doi.org/10.1016/j.gete.2023.100508.

Madaschi, A., J., Leuthold, L., Cantieni, L., Laloui (2023). Comparative numerical calculations in the context of tunnel design for nuclear waste repositories in Opalinus Clay. Proceedings 10th European Conference on Numerical Methods in Geotechnical Engineering. London. Zdravković L, Konte S, Taborda DMG, Tsiampousi A (eds).