Towards a better assessment of hydraulically induced damage in geo-engineering applications – HydroGeoDam

Predicting occurrence of hydraulically induced damage in geological systems constitutes a major challenge in subsurface engineering. For instance, geo-resources completion, underground storage management or even building and maintenance of construction sites can all be affected by the progressive development of cracks due to circulation of fluid in the host medium. These highly coupled Hydro-Mechanical (HM) processes which originate at different length scales, from the crack scale (centimeter) to the fracture or fault scale (meter or decimeter), constitute prominent issues in geo-engineering that are not yet fully understood. The overall goal of this project is thus to improve our understanding of the HM processes taking place in the subsurface for a better assessment of the associated risks and to propose non-intrusive methods to help assess these processes. The underlying idea is to benefit from such understanding for an enhancement of subsurface resource management, especially in terms of environmental protection, and risk mitigation of underground facilities. If the phenomena involved in hydraulically induced damage processes are reasonably well understood at the laboratory scale, considerable difficulties remain in the interpretation of in-situ behaviours where several features come into play (e.g., geological structures, material and stress heterogeneity, anisotropy). Our u<nderstanding of the processes involved is often restrained by the limitations of currently available numerical models to reproduce such phenomena, especially when they take place at a larger scale. There is, therefore, a critical need to build more capable numerical models (fully coupled, multi-scale…) to gain better insights into these complex phenomena and thus improve our predictive capabilities towards resources management and risk mitigation. In order to better characterize cracking mechanisms and to tackle the more general problem of upscaling from the laboratory scale to the engineering scale, HM experiments will be performed on samples of different sizes, from the millimetric scale to the metric scale. Monitoring techniques and fully coupled inversion methods will be developed and fracture initiation and propagation investigated in terms of fluid flow rate, fluid viscosity and stress conditions. Multi-scale numerical models able to describe and predict these HM processes will be developed and their predictions evaluated with regards to the experiments. Different but complementary numerical approaches based on the Smooth Particle Method (SPH), the Discrete Element Method (DEM) and the eXtended Finite Element Method (XFEM) will be used at the different scales. The numerical approaches will also be confronted to in-situ experiments performed at the Tournemire Underground Research Laboratory (URL) to investigate the relevance of upscaling procedures and the forecasting capability of the models. Finally, the uncertainty due to the inherent heterogeneity of geological systems will be investigated to identify and evaluate the model sensitivity.