Fluid migration in faulted shale rocks: channeling below active faulting threshold

Abstract

During fluid injection tests performed inside the clay-rich Tournemire fault zone, the monitored fault displacement exhibited limited amplitudes whereas a sharp increase of the permeability occurred as the fluid pressure reached a threshold value. The injected fluid channeled through the fractured zone or at the interface between the host rock and the fault core over decameter distances without triggering large irreversible deformation of the fault zone. To assess the underlying hydro-mechanical mechanisms of this sudden increase of permeability, a numerical model was set up to simulate an injection test inside a fracture. When the fluid pressure outweighs the mechanical contribution of the heterogeneities along the fracture planes, the sudden increase of permeability appears. The existence of the channeling process, occurring below a critical fluid pressure at the fracture scale, could explain why it is possible to generate fault leakage inside a non-active fault zone crossing shale rocks. Based on the in-situ and laboratory monitoring, an analytical expression based on TPHM (Two-Part Hooke’s Model) can reproduce the stress-dependent permeability evolution. For low values of the effective stress, a simple cubic law formulation does not reproduce correctly the sharp increase of permeability identified once the Fracture Opening Pressure (FOP) is reached.