Coupled continuum modeling of fracture reactivation and induced seismicity during enhanced geothermal operations

We developed a coupled code to obtain a better understanding of the role of pore pressure changes in causing fracture reactivation and seismicity during EGS. We implemented constitutive models for fractures in a continuum approach, which is advantageous because of the ease of integration in existing geomechanical codes (FLAC3D), the speed of the calculations and the flexibility of the fracture representation. For modeling the mechanical behavior of the fracture zone the softening ubiquitous joints model was used with random strength properties for the fracture zone. We implemented a hyperbolic deformation with effective stress for the reversible tensile fracture opening, and a linear relationship between plastic shear strain and irreversible fracture opening. The effective permeability of the fracture network was described by a cubic dependence on the fracture aperture. We created a model inspired on the Soultz-1a-Forêts GPK3 injection well, intersected by a dominant fracture zone. Due to injection of water, the model reproduced reactivation of the fracture zone and we observed the growth of a zone with large directional permeability. The model was used to perform a sensitivity analysis on parameters like injection rates, in-situ stress regime, fracture strength, and frictional weakening. This allowed us to evaluate the trends of their impact on fracture reactivation, including reactivated area, seismic moment and moment magnitudes.