Off-Fault Shear Failure Potential Enhanced by High-Stiff/Low-Permeable Damage Zone During Fluid Injection in Porous Reservoirs

rohmer, j. ; nguyen, t.k. ; torabi, a.;

Several studies have focused on the role of damage zone (DZ) on the hydromechanical behaviour of faults by assuming a fractured DZ (i.e. low stiffness/high permeability). Yet, this vision may not be valid in all geological settings, in particular, in high-porosity reservoirs as targeted by several underground exploitations. We investigate the impact of a high-stiff/low permeable DZ on the shear reactivation of a blind, undetectable normal fault (1 kmlong,≤10m offset), with a 0.5 m thick low-porosity/permeability fault core during fluid injection into a
high-porosity reservoir. The spatial distribution of effective properties (elastic moduli, Biot’s coefficients and permeability) of DZ including deformation bands (DB; elliptic inclusions) and intact rock were derived using upscaling analytical expressions. The influence of DZ on
the hydromechanical behaviour of the fault zone was numerically explored using 2-D plane strain finite-element simulations within the framework of fully saturated isothermal porous media by accounting for an orthotropic elastic rheology. The numerical results showed that
the presence of DB plays a protective role by reducing the potential for shear reactivation inside the fault core. On the other hand, they favour shear failure in the vicinity of the fault core (off-fault damage) by accelerating the decrease of the minimum principal effective stress
while limiting the decrease of the maximum one. This behaviour is strongly enhanced by the fault-parallel DZ effective stiffness, but limited by the combined effect of fault-normal DZ effective permeability and of the Biot’s coefficients. This can have implications for the location and size of aftershocks during fault reactivation.