Abstract: Hot dry rock (HDR) is a major carrier of deep geothermal energy, characterized by its vast reserves, extensive geographical distribution, and low-carbon clean attributes. It represents a key direction for renewable energy development under the framework of the dual carbon goals. To investigate the impact of complex fracture structures and injection-extraction schemes on the heat extraction efficiency of HDR reservoirs, this study developed a large-scale artificial rock sample with prefabricated fractures and established a thermo-hydro-mechanical (THM) coupled injection-extraction heat exchange experimental system. Fracture frameworks were constructed using paper and proppant particles, and artificial rock samples measuring 400 mm×400 mm×400 mm were fabricated through a mold casting technique. A series of heat exchange experiments was systematically executed under varying combinations of fracture geometries (long straight fracture, deflected fracture, branched fracture) and injection-extraction schemes (single injection-single extraction, single injection-dual extraction). The experimental outcomes indicate that fracture geometry exerts a limited effect on the temperature field, whereas seepage flow plays a predominant role. The injection-extraction scheme significantly affects heat exchange performance; when the injection rate escalates from 11 mL/min to 50 mL/min, the cooling rate of the produced fluid rises from 0.6  °C/10  min to 10.2  °C/10  min. This study reveals the critical roles of flow distribution and the stability of seepage pathways in short-term heat extraction capacity, providing experimental evidence and theoretical support for fracture design and optimization of injection-extraction parameters in enhanced geothermal system (EGS).

Key words: hot dry rock (HDR), enhanced geothermal system (EGS), prefabricated fractures, injection-extraction experiment, seepage capacity, heat extraction efficiency