Fracture Activation and Induced Seismicity During Long-Term Heat Production in Fractured Geothermal Reservoirs

We study fracture activation and induced seismicity during the long-term heat production of fractured geothermal reservoirs. The fracture system is modeled by the discrete fracture network approach that realistically represents the distribution and behavior of natural fractures in the system. Using a novel fully coupled thermal-hydro-mechanical model, we systematically investigate the interplay among in situ stresses, injection pressure and temperature on the spatio-temporal evolution of heat production-induced seismicity. Our results show that in situ stress state exerts a fundamental background control on the seismicity occurrence, while injection-induced fluid pressurization and thermal perturbation act as two competing triggering factors. Induced seismic events occur mainly in the near field of the injection well and along the main streamlines of fluid flow through the reservoir. Under a low differential stress condition, fracture activation is dominated by late-stage thermal drawdown, such that the injection temperature controls the timing, magnitude and number of seismic events. However, under a high differential stress condition, the effect of early-stage pressurization dominates with critically stressed fractures activated once or twice. The fracture activation is sensitive to the variation of pressure gradient, whilst the injection temperature attempts to influence the magnitude but in general not the timing and number of thermal drawdown-induced secondary seismic events. Furthermore, these anthropogenic parameters (i.e. injection pressure and temperature) strongly affect the long-term thermal energy output of the reservoir. Our research findings have important implications for the mitigation of seismic hazards by optimizing production strategy design during heat extraction from deep geothermal reservoirs. Highlights The interplay among in situ stresses, injection pressure and temperature on long-term heat production-induced seismicity is investigated. The seismicity triggering mechanism by simultaneous, transient propagations of hydrodynamic and thermal fronts is quantified. The spatial pattern of recurrent seismicity driven by thermal front migration in a critically stressed fracture system is analyzed.