Revealing hydro-mechanical properties of a natural fracture in low-permeable shale

Shales are considered to be sealing units for geological carbon storage and suitable host rocks for nuclear waste repositories due to their low permeability. However, the presence of fractures within these formations can significantly alter their flow, transport, and deformation behavior, which is central to the safe implementation of underground storage projects. Fractures not only increase the overall permeability of the rock but also contribute to its anisotropic behavior. This study focuses on characterizing the hydro-mechanical response of a natural fracture to aqueous fluid injection within a shaly specimen of Opalinus Clay. Laboratory experiments were performed to measure the mechanical and flow properties of intact and fractured rock specimens. Subsequently, a three-dimensional (3D) numerical model of water injection into the fractured specimen was developed. This model explicitly accounts for fracture geometry with strain-dependent aperture changes based on the cubic law assumption. Experimental measurements indicate that the fractured shale exhibits permeability up to two orders of magnitude higher than that of the intact counterpart. However, the simulations reveal that fracture permeability locally spans up to eight orders of magnitude. This significant change in permeability affects fluid flow within the rock specimen. The numerical model best reproduces the experimental results for a normal stiffness of the natural fracture of 18.7 MPa/mm at effective mean stresses below 15 MPa, and of 187.2 MPa/mm at higher confinements. This outcome highlights the critical importance of defining the hydro-mechanical parameters of fractures under realistic effective stress conditions with far-reaching implications for secure underground storage.