Speaker
Description
Fractured geomaterials exhibit strongly anisotropic mechanical behaviour and complicated hydro-mechanical (HM) interactions driven by the activation of pre-existing structures such as weakness planes, faults, and fractures in clay-rich materials and the associated permeability evolution. The potential of fault activation in low-permeable clay shales is a major concern for a wide range of geo-energy applications, including ensuring integrity of geologic sequestration sites, understanding and optimising hydraulic stimulation and production in shale gas reservoirs, and the safety assessment of nuclear waste disposal. A critical issue arises in the disposal of nuclear waste when the permeable flow paths through the initially impermeable host rock barrier are developed.
In this contribution, we present an implementation of a multi-surface plasticity model with a non-orthogonal ubiquitous joint formulation based on the Coulomb yield criterion. The model is applied to investigate the potential creation of permeable flow paths through an initially impermeable or low-permeability host rock barrier in the presence of arbitrarily-oriented weakness planes. The HM behaviour of clay rock is addressed using an embedded fracture permeability model, which represents the hydraulic effects of fractures or faults within a continuum porous medium without explicitly meshing them as discrete lower-dimensional elements. This permeability model is derived from cubic-law flow in finite-thickness elements and is formulated as a function of fracture aperture and shear strain induced by the onset of plastic deformation. The model formulates an anisotropic permeability tensor to account for preferential flow aligned with fractures orientations.
The multi-surface plasticity constitutive model, incorporating a tension cut-off formulation within a transversely-isotropic elasticity framework, is implemented in MFront, a code-generation tool dedicated to material modelling, and its coupling with the HM process through an embedded fracture permeability model is performed using the OpenGeoSys solver. The multi-surface approach comprises the Mohr-Coulomb yield criterion, representing plastic behaviour of the host rock matrix, and the Coulomb yield criterion, describing plastic behaviour along fractures formed by three intersecting sets of joints, with at least one set oriented at an oblique angle within the host rock.
The tension cut-off formulation is implemented through the plastic model based on the hyperbolic approximation to the physically distinct yield criteria.
The implemented coupled model is applicable to a variety of real-world geo-engineering and geo-energy applications including fault reactivation analysis induced by fluid injection, assessment of caprock integrity for CO$_2$ and geological hydrogen storage, nuclear waste disposal and environmental safety, slope stability, stability of underground excavations, and tunnel constructions.
Model verification is performed through a series of representative numerical examples and comparisons with available analytical solutions from the literature. The model is subsequently applied to fault reactivation and underground excavation problems.
| Country | Germany |
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