Fully coupled hydro–mechanical controls on non-diffusive seismicity triggering front driven by hydraulic fracturing

The spatio–temporal evolution of fluid-injection-induced seismicity is often bounded by a triggering front that expands away from the injection point in space and time. For some injection scenarios, the triggering front is thought to be directly linked to pore pressure diffusion, but in the case of hydraulic fracturing, the stress interaction of the growing tensile fracture with natural joints may be more significant. In order to explore the concept of a triggering front in this context, we use a fully coupled hydro–mechanical finite-discrete element (FDEM) approach to simulate microseismicity induced by hydraulic fracture growth. The medium contains a network of randomly oriented pre-existing fractures that are activated based on the Mohr–Coulomb failure criterion. As expected, the primary triggering front is defined by the envelope of microseismicity that tracks the hydraulic fracture, although more distal events are triggered by mechanical stress changes beyond the bounds of the triggering front. However, these distal events are approximately synchronous with initiation of the hydraulic fracture and are attributed to far-field elastic perturbations associated with the stress wave spread in the medium. A field example indicates that patterns of seismicity that emerge from our simulations have characteristics similar to observed microseismicity during hydraulic fracturing.