How pore pressure changes affect stress and strain in fault zones with complex internal structure-Insights from hydromechanical modelling

Faults not only affect the hydraulic regime and the local stress state, but are also prone to reactivation by pore pressure changes, potentially leading to induced seismicity, among others. In the present study, a generic finite element model with a detailed fault zone description including damage zones and a fault core with highly strained rock and fault core lenses of host rock is used to study the stress and strain evolution due to pore pressure changes. Special focus is on the temporal evolution of the effective stresses within the fault zone and the strain partitioning between its various subunits. The coupled hydromechanical simulations comprise five different scenarios covering pore pressure increase due to fluid injection and pore pressure decrease due to fluid production for permeable and semipermeable fault zones as well as different distances between the fault core lenses. Modelling results show that the variable hydromechanical properties of the different fault zone rocks lead to spatially and temporarily highly heterogeneous stress and strain patterns within the fault zone and its surrounding. In particular, substantial stress rotations of up to 45 degrees and differently in the various subunits of the fault zone are found in response to pore pressure changes. Plastic straining as an indication for potential fault reactivation is observed particularly in the injection scenarios. Models provide a template for modifications with respect to fault zone architecture and hydromechanical properties. They can also serve as starting point to define bulk fault zone properties for reservoir-scale simulations which cannot capture the complexities of a fault zone regarding geometry and material distribution.