Full Field Strain Measurements Uncover the Influence of External Far-Field Stresses on Fracturing During Cavity Expansion in Anisotropic Sandstone

Failure of porous granular rocks by fluid overpressure encompasses both natural and artificial phenomena. To better comprehend the mechanisms behind the formation and propagation of the fractures leading to failure, we subject multi-axially confined Vosges sandstone specimens to cavity expansion until sample failure. The specimens are prevented from invasion of pressurizing fluid by an internal soft jacket covering the borehole. Interlaced radial and orthoradial deformation bands appear around the cylindrical injection hole during initial fluid pressure build-up; at later stages, tensile cracks appear from the boundary of the hole. The cracks are dominantly mode I, and propagate mostly in straight line as pressure is increased, thus following their initial trajectories, apart from some instances of crack coalescence. In the experiments with radial symmetry of loading (equal far-field stresses), crack propagation does not show preferential orientation (opposite cracks or star-shaped cracks) and the fractures propagate with a slight angle to the bedding planes toward the sample boundary. In the experiments with asymmetric loading, the emancipating cracks initiate near the top and bottom of the borehole wall (region of largest tensile stress). For a particular experiment where the applied axial load is parallel to the bedding plane of the specimen, cracks propagate initially with a slight angle to the bedding planes, and then, en echelon cracks parallel to the bedding are observed during later deformation stages. The experimental results indicate that initial crack patterns are mainly controlled by the deviatoric stress state, while at much later deformation stages, crack propagation is mostly influenced by material anisotropy. The observations with equal far-field stresses also suggest that the number of propagating cracks influences the breakdown pressure: the more cracks there are, the higher the pressure peak in the hole, which is consistent with theoretical analyses of fracture mechanics.