Heat Transfer in Enhanced Geothermal Systems: Thermal-Hydro-Mechanical Coupled Modeling

Massive water injection during the stimulation and production stages in geothermal reservoirs can induce a strong coupled response. Fracture properties may be significantly altered due to fluid transport, heat transfer, and chemical reactions. We explored the impact of fluid circulation rates on the heterogeneity of thermal drawdown that may develop within the reservoir and its potential impact on the timing and magnitude of induced seismicity. The following study develops a dimensionless semi-analytical model that incorporates the reservoir scale, fracture spacing, and injection mass flow rate to determine thresholds for the evolution of uniform or shock-front distributions of thermal drawdown within the rock comprising the reservoir. The semi-analytical model is derived based on the balance of heat conduction within the fractured medium and the Warren-Root fracture model. We define two bounding modes of fluid production from the reservoir. For injection at a given temperature, these bounding modes relate to either low or high relative flow rates. At low relative dimensionless flow rates, the pressure pulse travels slowly, the pressure-driven changes in effective stress are muted, but thermal drawdown propagates through the reservoir as a distinct front. This results in the lowest likelihood of pressure-triggered events but the largest likelihood of late-stage thermally triggered events. Conversely, at high relative nondimensional flow rates, the propagating pressure pulse is larger and migrates more quickly through the reservoir but the thermal drawdown is uniform across the reservoir without the presence of a distinct thermal front, and is less capable of triggering late-stage seismicity. We evaluate the uniformity of thermal drawdown as a function of a dimensionless flow rate that scales with fracture spacing, injection rate, and the distance between the injector and the target point. This dimensionless scaling facilitates design for an optimum flow rate value to yield both significant heat recovery and longevity of geothermal reservoirs while minimizing associated induced seismicity.