Three-dimensional poroelastic modeling of injection induced permeability enhancement and microseismicity

• A combined boundary element/finite element numerical method is developed.
• It considers the effects of stress shadow, fluid diffusion and fracture opening and ride.
• The response of a hydraulic fracture/natural fracture network to injection is investigated.
• Simulations illustrate injection pressure variation with network deformation and permeability change.
• The simulations illustrate the potential for induced micro-seismicity due to slip on natural fractures.

A combined boundary element/finite element numerical method is employed to investigate the response of a hydraulic fracture/natural fracture network to injection. The model is three-dimensional and poroelastic. It considers the effects of fracture interactions, fluid diffusion into the matrix and the natural fractures, fracture opening and shear dilation. The natural fractures behavior is modeled using non-linear deformation in both shear and normal directions. For the shear mode, the slip-weakening model is used to simulate the post-shear failure behavior of the fractures. An example simulation is carried out to study the response of a fracture network to injection. The calculated injection pressure profile is used to ascertain the deformation response of the fracture network and its permeability enhancement. The simulation results clearly illustrate the potential for induced microseismicity due to permanent shear slip on natural fractures surrounding a hydraulically driven major fracture. Although the slip on the natural fractures can potentially contribute to seismic activity, its contribution to permeability enhancement depends on whether or not they propagate to connect to form a “hydraulically” connected network with the main fracture. When the natural fracture system is not connected to the main flow path, the MEQ information may overestimate the so-called stimulated reservoir volume.