Coupled seismo-hydromechanical monitoring of inelastic effects on injection-induced fracture permeability

Abstract

We present in situ measurements of fluid pressure, deformation and seismicity in natural fractures together with coupled hydromechanical simulations. We conducted a step-rate water injection (~3.5MPa and 1200 s) to induce the local pressurization of a critically stressed fractured carbonate reservoir layer located at 250m-depth in the Low Noise Underground Laboratory (LSBB), southern France. An observed factor-of-3 increase in the fracture permeability was associated with the injection-induced fluid pressure increase and about 100 triggered seismic events. Both normal opening (a few microns) of the fluid-injected fracture and the associated tilt (<1 micro-radian) of the fracture near field displayed inelastic behavior highlighting an irreversible fracture shear and dilatant failure, amounting to about 1/3–1/2 of the maximum measured deformations.Using a plane-strain finite-difference coupled hydromechanical model, our calculation shows that tensile failure first occurred in the injection zone and then shear failure spread along fractures into the surrounding unsaturated rock through stress transfer from the injection zone. The most striking result of these model simulations is that the mechanical weakening of the fractures in the near field induced a 2–5×105Pa release of the normal stress across the fluid-injected fracture that provoked fracture slip and increase in permeability. A geological exploration of the fracture zone after the experiment showed that no major failure had occurred, and we therefore relate these strength and permeability variations to the slight reactivation (~microns) of pre-existing fractures.