Fracture interaction mechanisms and reactivation of natural discontinuities under fluid pressurization conditions can represent critical issues in risk assessment of caprock integrity. A field injection test, carried out in a damage fault zone at the decameter scale, i.e., mesoscale, has been studied using a distinct element model. Given the complex structural nature of the damage fault zone hydraulically loaded, the contribution of fracture sets on the bulk permeability has been investigated. It has been shown that their orientation for a given in situ stress field plays a major role. Based on these results, a simpler model with a fluid-driven fracture intersecting a second fracture has been set up to perform a sensitivity analysis. It is in presence of a minimum differential stress value with a minimum angle with the maximum principal stress that the second fracture could be both, hydraulically and mechanically reactivated. Results also showed that in the vicinity of the fluid-driven fracture, a natural fracture will offer contrasted hydromechanical responses on each side of the intersection depending on the stress conditions and its orientation with respect to the stress field. In this case, we show that a hydromechanical decoupling can occur along the same plane. These results provide insights into fracture-controlled permeability of fault zones depending on the properties of the fractures and their hydromechanical interactions for a given in situ stress field.

Fluid pressure plays an important role in the stability of tectonic faults. However, the in situ mechanical response of faults to fluid pressure variations is still poorly known. To address this question, we performed a fluid injection experiment in a fault zone in shales while monitoring fault movements at the injection source and seismic velocity variations from a near-distance (<10 m) monitoring network. We measured and located the P and S wave velocity perturbations in and around the fault using repetitive active sources. We observed that seismic velocity perturbations dramatically increase above 1.5 MPa of injection pressure. This is consistent with an increase of fluid flow associated with an aseismic dilatant shearing of the fault as shown by numerical modeling. We find that seismic velocity changes are sensitive to both fault opening by fluid invasion and effective stress variations and can be an efficient measurement for monitoring fluid-driven aseismic deformations of faults.

Clay formations are present in reservoirs and earthquake faults, but questions remain on their mechanical behavior, as they can vary from ductile (aseismic) to brittle (seismic). An experiment, at a scale of 10 m, aims to reactivate a natural fault by fluid pressure in shale materials. The injection area was surrounded by a dense monitoring network comprising pressure, deformation, and seismicity sensors, in a well-characterized geological setting. Thirty-two microseismic events were recorded during several injection phases in five different locations within the fault zone. Their computed magnitude ranged between −4.3 and −3.7. Their spatiotemporal distribution, compared with the measured displacement at the injection points, shows that most of the deformation induced by the injection is aseismic. Whether the seismicity is controlled by the fault architecture, mineralogy of fracture filling, fluid, and/or stress state is then discussed. The fault damage zone architecture and mineralogy are of crucial importance, as seismic slip mainly localizes on the sealed-with-calcite fractures which predominate in the fault damage zone. As no seismicity is observed in the close vicinity of the injection areas, the presence of fluid seems to prevent seismic slips. The fault core acts as an impermeable hydraulic barrier that favors fluid confinement and pressurization. Therefore, the seismic behavior seems to be strongly sensitive to the structural heterogeneity (including permeability) of the fault zone, which leads to a heterogeneous stress response to the pressurized volume.