Influence of heterogeneous fractured fault damage zones on shear failure onset during fluid injection

Abstract

Fault stability analysis is traditionally performed by assimilating fault systems to surfaces. Yet, faults are complex and heterogeneous geological systems, whose compartmentalized architecture generally corresponds to an inner core (FC) of small thickness (i.e. principal fault plane) surrounded by outer, often fractured damage zones (DZ). Depending on the fractures’ network characteristics, the latter compartment can be related to complex spatial distribution of hydro-poro-elastic properties, which can strongly influence the shear failure tendency of the fault zone during massive injection of fluid into reservoirs. Using the upscaled DZ properties derived from outcrop surveys at Cirques de Navacelles (South of France), we investigate this issue using coupled hydromechanical simulations in the framework of fully saturated orthotropic elastic porous media. By comparing the shear failure tendency for the heterogeneous DZ cases to the ones with homogeneous DZ, we highlight that: 1. Whatever the stress regime (extensional or compressional), the maximum injection pressure is greater in the heterogeneous cases; 2. Under extensional regime, the presence of the DZ limits the development of shear failure tendency in the center of the first DZ compartments directly adjacent to FC, whereas shear appears to rapidly develop along the whole reservoir thickness for the homogeneous case; 3. Under compressional regime, the presence of the DZ enhances the localization of shear failure along FC-DZ interface, whereas shear failure preferably develops in the injection zone in the homogeneous case.