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This paper investigates the main parameters that affect the propagation of a fluid driven-fracture in a poroelastic medium. The fracture results from the pumping of an incompressible Newtonian viscous fluid at the fracture inlet, and the flow in the fracture is modelled by the lubrication theory. Rock deformation is assumed as porous-elastic. Leak-off in the host rock is considered to account for the diffusion effects in the surrounding formation. The propagation criterion is of the cohesive type. Finite element analysis was performed to compute the fracturing pressure and fracture dimensions as a function of the time and length. It was found that higher pressures are needed to extend a fracture in a poroelastic medium than in an elastic medium, and the created profiles of poroelastic fracture are wider. It was found that grain compressibility plays a minor role and does not result any significant difference in the fluid pressures and fracture dimensions. Wider fracture profiles are obtained with higher injection rates. The fluid pressures and the fracture apertures are larger in the case of a high permeability formation. [ABSTRACT FROM AUTHOR]

In this paper the writers examine a borehole failure model that is based on fracture mechanics and layer buckling theories and compare its predictions with experimental data. The model assumes that the main failure mechanism of borehole collapse takes place in the form of (preexisting or formed) layers buckling. The model introduces a combination of fracture mechanics parameters with length dimension that scales the size of the holes, thus allowing for size effect predictions. The writers review the model and compare its predictions with available experimental data of hollow cylinder tests on weak rocks. The writers found for the model predictions and for the experimental data a strong correlation between hollow cylinder strength normalized by the rock strength and hole size normalized by the square of the ratio of fracture toughness over tensile strength. These findings can be used for constructing efficient mechanical models for predicting or interpreting failure of boreholes and perforations in mining and petroleum engineering. [ABSTRACT FROM AUTHOR]

We present results of a parametric study on shearband formation in geomaterials, examining the influence of material dilatancy on shearband properties. The study is based on a new higher order continuum theory, called gradient elastoplasticity, developed to model localization of deformation. We present results for two problems with different initial stress fields: The biaxial test and the hollow cylinder test. We show that nonassociativity promotes bifurcation at lower deformation, leading to localized deformation and more abrupt final failure. Both the inclination and thickness of the calculated shearbands decrease with increasing nonassociativity. Gradient elastoplasticity is capable of reproducing the experimentally observed localized failure modes, including details such as reorientation of shearband close to a free boundary. [ABSTRACT FROM AUTHOR]