Seismicity and hydromechanical behavior of a fractured porous rock under a high pressure fluid injection

We performed high-pressure water injections in boreholes set in a fractured porous limestone at 250 m-depth in the LSBB URL in order to investigate the coupling relationships between induced seismicity and the rock permeability changes. The seismic events and tilts of the free wall of the gallery were monitored at different locations by two 3-component accelerometers and two 2D tiltmeters located from 1.5 to 2 m from the injection chamber. Changes in fluid pressure and flow rate were simultaneously monitored in the chamber. A damage-induced permeability increase of a factor of 2.2 occurred after a 10 seconds long swarm of complex seismic events caused by a pressure peak injection of 3.5 × 106 Pa and a sudden increase in the injected flow rate from 0 to to 5.3 × 10−5 m3 ⋅ s−1. The various angular directions observed on both tiltmeters and 3D accelerometric components indicate that the source of the deformation remains complex and evolves over the time of fluid pressure diffusion in the fractures. At the beginning, signals appear dominated by the source geometry, the rock mass boundary and stress conditions. After the damage and during the pressure decay, tangential slipping of the fracture behavior dominates stress relaxation in the medium. Finally, tilt monitoring coupled with seismo-acoustic measurements present a promising way to quantitatively estimate the relationships between the changes in fractured rocks permeability and seismicity induced by fluid pressurization