Hydraulic fracturing is a crucial technique for the extraction of geothermal energy from hot dry rock reservoirs. However, the development of such reservoirs faces significant challenges due to the high in-situ stress and strong elastic-plastic behavior of these rocks, which often result in simplified fracture geometries and subsequent low heat extraction efficiency. To address this issue, a novel reservoir treatment method based on thermal expansion and contraction principles is proposed. By applying alternating heating-cooling treatments to the reservoir, cyclic thermal stress is generated within the rock to enhance the complexity of post-fracturing fracture networks. To investigate the resultant hydraulic fracture propagation under alternate-temperature loading, a custom-developed thick-walled cylinder expansion fracturing device was employed to study the fracture propagation mechanisms in hot dry rock samples under cyclic thermal loading. The fracture network complexity was characterized by the fractal dimension method. Experimental results demonstrated that alternate thermal load cycling significantly enhances the fracture network complexity compared to conventional single-phase heat treatment. The maximum improvement in fractal dimension (3.86% increase) was observed at 500 ℃. Under alternating temperature loads, the upper surface fractures predominantly exhibited bilateral symmetric structures. At 600 ℃, a substantial increase in branched fractures and rock debris near boreholes occurred, indicating that alternating temperature loads significantly enhance the complexity of engineered fracture networks in hot dry rock. These findings suggest that incorporating thermal cycling into hydraulic fracturing processes can significantly improve the fracture network complexity, thereby enhancing the efficiency of heat extraction from hot dry rock reservoirs.

The effective plugging of artificial fractures is key to the success of temporary plugging and diverting fracturing technology, which is one of the most promising ways to improve the heat recovery efficiency of hot dry rock. At present, how temporary plugging agents plug artificial fractures under high temperature remains unclear. In this paper, by establishing an improved experimental system for the evaluation of temporary plugging performance at high temperature, we clarified the effects of high temperature, injection rate, and fracture width on the pressure response and plugging efficiency of the fracture. The results revealed that the temporary plugging process of artificial fractures in hot dry rock can be divided into four main stages: the initial stage of temporary plugging, the bridging stage of the particles, the plugging formation stage, and the high-pressure dense plugging stage. As the temperature increases, the distribution distance of the temporary plugging agent, the number of pressure fluctuations, and the time required for crack plugging increases. Particularly, when the temperature increases by 100 ℃, the complete plugging time increases by 90.7%.