A coupled continuum model representing thermo-hydro-mechanical behaviors is applied to follow the evolution of induced seismicity within a prototypical enhanced geothermal system (EGS) reservoir. The model is applied to the potential Newberry EGS field (USA) by assuming fracture sizes of 10 to 1200 m. Models are classified by their conceptualization of the fractured reservoir geometry as networks of discrete fractures and with equivalent fractured media as fillin. The THMC model is applied to a doublet injector-producer to explore the spatial and temporal triggering of seismicity for varied fracture network geometries both shallow (2000m) and at depth (2750m). The magnitude of the resulting seismic events is in the range-2 to +1.9. The largest event size (~1.9) corresponds to the largest fracture size (~1200m) within the reservoir. The rate of hydraulic and thermal transport has a considerable influence on the amount, location, and timing of failure, and ultimately, on the event rate. The event rate is highest when the fracture density is highest (0.9m-1) and the initial stresses highest (at depth). In all cases, the a-value decreases and the b-value increases with time. The b-value is largest (~1.34) for the highest fracture density and the highest stress regime. Thermal energy recovered during production is also greatest at depth and for the highest density of fractures.
We use a continuum model of reservoir evolution to explore the interaction of coupled thermal, hydraulic and chemical processes that influence the evolution of seismicity within a fractured reservoir from stimulation to production. Events occur from energy release on seeded fractures enabling moment magnitude, frequency and spatial distribution to be determined with time. Event magnitudes vary in the range À2 to +2 with the largest event size (~2) corresponding to the largest fracture size (~500 m) and a prescribed stress drop of 9 MPa. Modelled b-values (~0.6-0.7) also correspond to observations (~0.7-0.8) for response in the Cooper Basin (Australia). We track the hydrodynamic and thermal fronts to define causality in the triggering of seismicity. The hydrodynamic front moves twice as fast as the thermal front and envelops the triggered seismicity at early time (days to month)with higher flow rates correlating with larger magnitude events. For later time (month to years), thermal drawdown and potentially chemical influences principally trigger the seismicity, but result in a reduction in both the number of events and their magnitudes.
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