Fault Stability Perturbation by Thermal Pressurization and Stress Transfer Around a Deep Geological Repository in a Clay Formation

Abstract

The increase of temperature of a low‐permeability, fluid‐saturated media may trigger significant thermal pressurization, where the expansion of pore fluid cannot be accommodated by the thermal expansion of the pore space. With the aid of a coupled thermohydromechanical numerical simulator, we investigate the possible impact of thermal pressurization during the life of a deep geological repository (DGR) for high‐level radioactive waste, characterized by the emplacement of radioactive material‐filled canisters in a series of parallel tunnels, excavated in a low‐permeability clay formation. We represent the fault as a planar structure embedded in a thermoporoelastic material and shear activation evaluated by a strain‐softening Mohr‐Coulomb failure criterion. The results show that stress changes caused by temperature and thermal pressurization of a rock mass around the emplacement tunnels may trigger a slip event on a fault plane in proximity of the geological disposal site: Rupture nucleates at depth, hundreds of meters below the DGR. Stress transfer plays a key role, while a direct hydraulic connection between the repository and the nucleation zone is not necessary in order to trigger rupture. A low stress ratio may favor the occurrence of slip up to a distance of 600 m of the fault from the outermost tunnel. These results highlight the need of investigating hydromechanical properties and local stress conditions at depth to characterize the geomechanical response of weak planes located in the surroundings of a high‐level radioactive waste repository and to provide sufficient knowledge for the safe development of the DGR site.

TOUGH3-FLAC3D: a modeling approach for parallel computing of fluid flow and geomechanics

“…11ef). These results are valid for steeply dipping faults and are well in agreement with a recent 2D study on fault reactivation during disposal of nuclear waste at depth [46]. The removal of some bottlenecks, thanks to the use of binary files rather than ASCII, and the removal of save/restore operations for FLAC3D, largely helped in improving the performances of the coupled approach.…”

Chemo‐Mechanical Coupling in Fractured Shale With Water and Hydrocarbon Flow

“…Clay content also affects the strengths of faults (Bakker et al., 2019; Hyndman et al., 1997; Ikari et al., 2009) and contributes to fluid diffusion‐sorption processes that result in expansion and/or contraction of the clay‐bearing rock (Huang et al., 1986). The potential to generate permeability by induced damage continues to raise concerns regarding the deployment of large‐scale geologic CO 2 storage (Cappa & Rutqvist, 2011; Zoback & Gorelick, 2012) and of nuclear waste repositories (Urpi et al., 2019), where shale and mudstone form the main seal system that prevents the escape of fluids. Yet, field evidence shows that it is technically difficult to sustain hydrocarbon production from shale reservoirs, requiring the use of “proppants” (Hou et al., 2020).…”

Modelling the water injection induced fault slip and its application to in-situ stress estimation

“…Q is the sum of flow rates [m 3 /s] into the flow element from all surrounding contacts and ∆𝑡𝑡 is the time step. V, ∆𝑉𝑉 and 𝑉𝑉 𝑚𝑚 are respectively current flow element volume [m 3 ], volume change in the current step and the average volume.…”

“…Fault reactivation or shearing of fractures can be an important issue related to the safety of deep geological disposal for radioactive waste [1,2,3]. Elevated risk may arise when fault reactivation is triggered, leading to an increase in permeability and a potential flow path for transport of radionuclides, if released from waste package.…”