Numerical Simulation of Enhanced Geothermal System With Complex Fracture Networks

Geothermal resource is one of the most promising renewable energies, which is successfully accessed by the enhanced geothermal system (EGS) characterized by a complex fracture network. Therefore, complex fracture networks with hydraulic and natural fractures of different fracture orientations and intensities are established in this study. The natural fractures are randomly distributed in the geothermal reservoir. The working fluid flowing and heat exchange in the geothermal reservoir are simulated by coupling model, considering the effects of temperature variation on the density, viscosity, thermal conductivity and specific heat capacity of working fluid, and the permeability of reservoir matrix. Effects of natural fracture orientation and intensity, position of injection and production wells, and geothermal reservoir permeability on the heat production behavior are analyzed. The simulation reveals that the low-temperature region in the geothermal reservoir could breakthrough along natural fractures. When the direction of injection and production wells is consistent with the orientation range of natural fractures, the reservoir exploitation ratio and the heat extraction rate are the largest, but the temperature stability time of the produced fluid is the shortest and the cooling rate is the fastest. Increasing the intensity of natural fractures is helpful to increase the heat extraction rate, but the temperature of production wells decreases more easily. Increasing the distance perpendicular to the hydraulic fracture can better improve the performance of production wells. High reservoir permeability is not conducive to the stability of production fluid temperature, but can greatly increase the exploitation ratio of the geothermal reservoir.