Experimental research on the permeability of fractured-subsequently-filled granite under high temperature-high pressure and the application to HDR geothermal mining

To reveal the physical characteristics of fractured-subsequently-filled granite and to guide hot dry rock (HDR) mining, this paper studied the permeability at 500 °C and 1000 m hydrostatic pressure of the parent rock (type Ⅰ granite), hydrothermal fluid backfill (type Ⅱ granite), cementation interface between the backfill and the parent rock laterally positioned through the specimen (type III granite), and cementation interface between the backfill and the parent rock longitudinally positioned through the specimen (type IV granite). The results show that the threshold temperature of the permeability changes for the four types of granite (i.e., types I, II, III, and IV) are 300 °C, 200 °C, 300 °C and 250 °C, respectively. From the threshold temperature to the maximum test temperature, the permeability of type I, II, III and IV granite increases exponentially by 1, 2, 2 and 3 orders of magnitude, reaching 10−6 D, 10−4 D, 10−5 D and 10−4 D, respectively. Micro-observation results of the micro-structures show that thermal cracking of the heterogeneous rock mass is the main reason for the increase in permeability of type I and III granite; the low strength and deteriorated mechanical properties caused by dissolution are the main reasons for the permeability of type II granite to significantly exceed that of type I and III granite; the extreme heterogeneity and the deteriorated mechanical properties of the backfill together indicate that type IV granite has the best permeability. During reservoir construction by hydraulic fracturing in fractured-subsequently-filled granite, the flow of the fracturing water along the direction perpendicular to the cementation interface is limited. However, when the reservoir temperature exceeds 250 °C, the backfill strengthened by the cementation near the cementation interface restores the weak-plane structure characteristics; when the reservoir temperature exceeds 400 °C, the thermal cracking fractures near the cementation interface interconnect with each other along the cementation interface to form a permeability channel. Therefore, the fracturing water in the backfill can easily flow along the cementation interface. Eventually, reservoir construction in fractured-subsequently-filled granite can greatly reduce engineering costs, increase reservoir water-rock heat exchange areas and improve heat exchange efficiency, which provides a new technical and theoretical approach for deep HDR geothermal exploitation.