Simulation of micro-seismicity in response to injection/production in large-scale fracture networks using the fast multipole displacement discontinuity method (FMDDM)

Analysis of fluid flow, permeability, and micro-seismicity in large fracture networks are essential for economical production from engineered geothermal and petroleum reservoirs. In this work, the fluid pressure change, as a result of injection and production inside a stochastic, deformable, non-propagating fracture network is modeled using the fast multipole displacement discontinuity method combined with the finite difference method. The former is employed to simulate the response of the fracture network to changes in the pressure due to fluid injection. In addition to pressure changes inside the fracture network, changes in fracture status with respect to normal (“joint” versus hydraulic fracture) and shear (stick versus slip) deformation, dilation, and friction coefficient are considered. Rate-and-state friction model and the Mohr-Coulomb criterion are used to model slip and potential micro-earthquakes and their seismic moments. Simulation results show that increase in pressure inside a fracture network leads to permanent slip and nucleation of micro-earthquakes which are mostly influenced by the orientation of the fractures and the magnitude of fluid pressure and the injection rate. Injecting the same volume of water over a longer time period decreases the number and magnitude of seismic events.