The role of the intermediate principal stress on the direction of damage zone during hydraulic stimulation

The Mohr-Coulomb criterion is widely used to describe rock failure and the orientation of failure planes with respect to the stress field. However, in many cases, the seismic clouds created during hydraulic stimulations do not follow the orientations predicted by the Mohr-Coulomb criterion. In this paper we model rock deformation using continuum damage mechanics and calibrate the parameters using the stress-strains curve and failure of laboratory rock samples (KTB amphibolite and Westerly granite) under true-triaxial loading. We then present results of numerical simulations of the evolving fault structure associated with fluid injection. We simulate the effects of σ2 on failure conditions and failure plane orientation initiated and driven by fluid injected in the center of the simulated rock volume. A pore pressure rise at the injection point decreases the volumetric strain while keeping the deviatoric strain unchanged. Therefore the failure process starts close to the injection well, where the pore pressure is the highest, and propagates away from the well as more fluid reaches the tip of the damage zone. The final geometry of the damage zone demonstrates that the orientation of faulting is strongly affected by σ2. The angle between σ1 and the damage zone changes from 35° at σ2= σ3 to almost 0° at σ2= σ1. At σ2= σ3 (σx=σy), cylindrical symmetry allows many possible conjugate directions and poly-modal fault patterns with orthorhombic symmetry comprising clusters of four sets of shear failure planes. The Mohr-Coulomb criterion is shown to be valid only at σ2= σ3. Although not exactly following the Mohr-Coulomb criterion, when σ2 is less than halfway between σ1 and σ3, it can still be considered as a good approximation. When σ2 is more than halfway between σ1 and σ3, deformation is more similar to mode-I type failure geometry. Yet, the direction of the damage zone meanders and is not always parallel to σ2. The curvature of the meanders decreases with increasing σ2. The mechanism that creates the geometry related σ2 is related to the effect σ2 has on the proximity to failure.