Formation of damage zone and seismic velocity variations during hydraulic stimulation: numerical modelling and field observations

During hydraulic stimulations, a complex interaction is observed between the injected flux and pressure, number and magnitude of induced seismic events, and changes in seismic velocities. In this paper, we model formation and propagation of damage zones and seismicity patterns induced by wellbore fluid injection. The model includes the coupling of poroelastic deformation and groundwater flow with damage evolution (weakening and healing) and its effect on the elastic and hydrologic parameters of crystalline rocks. Results show that three subsequent interactions occur during stimulation. (1) Injected flux-pressure interaction: typically, after a flux increase, the wellbore pressure also rises to satisfy the flux conditions. Thereafter, the elevated pore pressure triggers damage accumulation and seismic activity, that is, accompanied by permeability increase. As a result, wellbore pressure decreases retaining the target injected flux. (2) Wellbore pressure-seismicity interaction: damage processes create an elongated damage zone in the direction close to the main principal stress. The rocks within the damage zone go through partial healing and remain in a medium damage state. Damage that originates around the injection well propagates within the damage zone away from the well, raising the damage state of the already damaged rocks, and is followed by compaction and fast partial healing back to a medium damage state. This 'damage wave' behaviour is associated with the injected flux changes only in early stages while fracture's height (h) is larger than its length (l). The ratio h/l controls the deformation process that is responsible for several key features of the damage zone. (3) Stress- and damage-induced variations of the seismic P-wave velocities (V-p). V-p gradually decreases as damage is accumulated and increases after rock failure as the shear stress is released and healing and compaction are dominant. Typically, V-p decreases within the damage zone and increases in most regions outside the damage zone. After a 'damage wave' that is originated at the well, V-p rises back and may exceeds its initial values. Similar transient variations of the elastic parameters and the effects of h/l are observed at the Soultz-sous-Forets Enhanced Geothermal System (EGS) records of induced seismicity during hydraulic injection.