Modeling injection-induced seismicity through calculation of radiated seismic energy

Seismicity induced by the injection of fluid into the fractured ground is one of the most challenging issues facing geothermal and deep wastewater disposal industries. This paper introduces an energy-based numerical methodology to study roles of fluid injection in triggering rupture (seismic slip) along preexisting faults. The methodology is developed in the Universal Distinct Element Code (UDEC) using its quasi-static and dynamic schemes and calculates the total seismic energy radiated by a rupture when more energy is made available in the system than can be stored or consumed. As an example of the application of the developed methodology, we study effects of fluid injection on rupture dynamics by pressurizing a single fault surrounded by impermeable rock, representing a simplified analogy for the injection process in deep wastewater disposal and geothermal activities. We discuss effects of raising the fluid pressure on initiating rupture over well-oriented (or critically loaded) and misoriented faults. Results show that fluid injection can trigger a rupture along both well-oriented and misoriented faults although the notion of seismicity may be observed along the well-oriented fault as early as the beginning of the injection process. The well-oriented fault generates higher seismic energy magnitude as more energy is available for rupture due to the higher peak shear stress and stress drop on the fault. Making simplifying assumptions, this study also found that fluid can be injected under a high-pressure increment before and after the fault initial activation while the radiated seismic energy remains relatively insignificant. However, gradually increasing the fluid pressure at the onset of rupture reduces the radiated seismic energy by 30%. Comparing the seismic moment and radiated seismic energy for each event reveals that while radiated seismic energy varies between different values of pressure increment, the calculated seismic moment stays constant, showing the possible ineffectiveness of the seismic moment in representing the intensity of injection-induced ruptures.