Complex fracture network, Deep geothermal energy, Enhanced geothermal system, Hydro-thermal coupling, Heat transfer of fluid flow, Energy conservation, TJ163.26-163.5, Renewable energy sources, TJ807-830

Alternate-temperature loading can cause cyclic thermal stress; therefore complex artificial fracture networks are formed in hot dry rock owing to thermal expansion and contraction under alternating thermal loading. In this study, the heat recovery potential of hot dry rock with a complex fracture network under alternating temperature loading was simulated numerically based on the hydrothermal coupling algorithm and compared with that of hot dry rock with a single fracture. The results show that the complex fractures formed under alternate thermal loading have better heat recovery performance than a single fracture, and the heat recovery performance of a complex fracture network is closely related to the intersection angle of fractures in the network. The temperature at the fracture outlet increases with an increase in the intersection angle between the branch fractures and the main fracture. At the angle of 30° between the branch fractures and main fracture, the average temperatures at the fracture outlet were 0.031% and 0.025% higher than those at the angles of 90° and 60°, respectively. Simultaneously with the water-injection, the temperature in the branch fractures gradually increased with an increase in the angle of the crossing fractures. When the angle between the branch and main fractures was 90°, the temperature fields on the two sides of the branch fracture were symmetrical. When the angle was 60° or 30°, the temperature fields on the two sides of the branch fracture were asymmetrical. The results of this study provide theoretical and technical support for the extraction of deep geothermal energy.