BOREHOLE YIELD AND HYDRAULIC FRACTURE INITIATION IN POORLY CONSOLIDATED ROCK STRATA .2. PERMEABLE MEDIA

The effective stress distributions around a plane strain circular opening in granular elastoplastic saturated permeable media are calculated. Plastic zone propagation is estimated by using a Mohr-Coulomb strain-weakening criterion, coupled with a steady-state pore pressure distribution. Focusing on permeability changes around injection wells and considering factors such as cohesive strength, injection pressure and in situ stress, we investigate the effective stress evolution around the opening. The relation between the in situ stress and the hydraulic fracture initiation pressure is also investigated for a pre-yielded saturated media. It is concluded that the breakdown pressure cannot be used for estimating in situ stresses in such circumstances.