Large poroelasto-plastic deformations due to radially outward fluid injection

Abstract: Flow-induced failure of granular materials is relevant to a broad range of geomechanical applications. Plasticity, which is the inherent failure mechanism of most granular materials, enables large deformations that can invalidate linearised models. Motivated by fluid injection into a borehole, we develop a steady-state model for the large deformation of a thick-walled, partially-permeable, elastic--perfectly-plastic annulus with a pressurised inner cavity. We account for pre-existing compressive stresses, as would be present in the subsurface, by subtracting a compressed initial state from our solutions to provide the additional disturbance due to fluid injection. We also introduce a simple parameter that allows for a smooth transition from an impermeable material (i.e., subject to mechanical loading at the inner wall) to a fully permeable material (i.e., subject to an internal pore-pressure gradient), which would be relevant to coated boreholes and very-low-permeability materials. We focus on the difference between poroelastic and poroelasto-plastic deformations, the role of kinematic and constitutive nonlinearity, and the transition from impermeable to fully permeable. We find that plasticity can enable much larger deformations while predicting much smaller stresses. The former makes model choice increasingly important in the plastic region, while the elastic region remains insensitive to these changes. We also find that, for a fixed total stress at the inner wall, materials experience larger deformations and generally larger stresses as they transition from impermeable to fully permeable.