3D thermo-poroelastic analysis of fracture network deformation and induced micro-seismicity in enhanced geothermal systems

This study considers three-dimensional (3D) analyses of a fracture network in an enhanced geothermal system (EGS) with special emphasis on the role of coupled thermo-hydro-mechanical processes and fractures mechanical interactions. The behavior of the system is modeled by coupling a thermo-poroelastic displacement discontinuity (DD) method (for fracture opening and ride, fluid and heat diffusion in the reservoir matrix) with a finite element method for the fluid and heat convection and conduction inside the fractures. The nonlinear characteristics of the fracture deformation in the normal (change of fracture status from joint fracture to hydraulic fracture) and shear deformation (change of fracture status from stick to slip) are taken into account. The resulting method is then used to simulate relatively short term injection/extraction processes into/from a synthetic fracture network consisting of a major fracture intersected by a set of smaller natural fractures. Injection/extraction into/from the fracture network results in gradual shearing of the fractures that impact the thermo-hydro-mechanical characteristics of the fracture system. It is also shown that the early micro-seismic events are associated with fracture slip on connected fractures due to thermal perturbation. Also, continued injection leads to stress intensity conditions favorable for fracture propagation in shear and tensile modes which could increase reservoir surface area and further contribute to seismicity.