Influence of fracture network characteristics of hot dry rock on heat extraction performance: A numerical simulation study

Considering the many hot dry rock (HDR) fields in China, efficient development of geothermal energy is an effective means to achieve carbon neutrality and net zero carbon emissions. Enhancing the heat exchange efficiency of the geothermal reservoir fracture network is an effective method for realizing the beneficial development of HDR resources. To analyze the influence of the characteristics of fracture network on the heat recovery performance of HDR, this study examines the coupling relationships among various physical fields and develops a thermal-fluid-solid multi-field coupling numerical simulation method that incorporates the effects of low-temperature and evolution of rock matrix permeability. Furthermore, the established model was verified using the thermal-coupled theoretical solution, experiment, and monitoring data from the Hijiori field. Based on these findings, a THM coupled injection-production heat transfer model, considering the characteristics of the fracture network in the Gonghe Basin of Qinghai Province, was developed. The influence of fracture network on geothermal development was then systematically analyzed. The results indicate that increased number of fractures and wider fracture spacing are more conducive to improving heat extraction performance. The dominant channel reduces heat transfer efficiency in surrounding fractures. Besides, when the width of the dominant channel is more than 2 mm, there is a double increase in the ratio of the dominant channel width to the width of fractures in the heat transfer zone. Overall, the impact of the dominant channel is evident after low-temperature fluid injection over time. The characteristics of the fracture network, especially the dominant channels, are critical for designing geothermal exploitation strategies in HDR systems.