Seismicity triggered by the injection of fluid into the fractured ground is one of the most pressing, yet difficult to address, rock mechanics issues facing oil, natural gas, and geothermal projects today. Injection-induced seismicity originates from the stress re-distribution and the resultant changes in the strain energy stored in the Earth’s crust following slip along an existing fault and/or failure of the surrounding ground. This paper numerically studies seismicity triggered from the injection of water into a single fault, a simplified analogy for settings characterized by injecting fluid into the fractured reservoir following slip along pre-existing fracture zones. An energy-based analysis is developed for studying these forms of unstable slip events by calculating the kinetic energy released if conditions for seismic slip emerge. In this method, the stress distribution and the relative changes of the energy terms in the system are assessed before, during, and after slip has occurred along the discontinuity. Validation testing is first conducted by simulating a direct shear test using the distinct element code UDEC. A more realistic model of a finite fault is then developed to capture the triggering role of pore pressure inside the fault. The Continuously-Yielding and Mohr-Coulomb elastoplastic joint models are adopted for representing brittle and ductile discontinuity responses, respectively. A range of material and system parameters are selected to cover both stable and unstable slip conditions, where unstable slip leads to a release of kinetic energy (i.e. seismicity) because more energy is released from the system than can be stored or consumed through the failure process. We then study the effects of raising the fluid pressure and initiating slip along the discontinuity. For each model, we calculate the radiated seismic energy from the total kinetic energy released during failure and evaluate the relative magnitudes of instability for the given conditions based on this measure. Changing the initial loading conditions, two cases of loaded fault are studied: well oriented and misoriented fault. The results show that for the case of a well-oriented fault, the rapid increase of fluid pressure within a fault can trigger a self-propagating and unstable slip, even for the case of the ductile fault, although the magnitude of the radiated seismic energy increases dramatically with more brittle failure characteristics assigned to the discontinuity. For the case of misoriented faults, not critically loaded, there is a pore pressure threshold under which the rate of increasing injection pressure does not trigger seismicity. These results can partially help adjust the injection pressure for maintaining the seismicity level in an injection site below a certain range through controlling the down-hole injection pressure.

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