Effect of fracture backfill on the hydraulic fracturing behaviour of deep granite under high-temperature and high-pressure conditions: Implications for hot dry rock geothermal mining

Hot dry rock (HDR), a renewable source of geothermal energy, has the potential to meet the growing demand for energy harvested with low atmospheric CO2 emissions. The development of large-scale, high-connectivity geothermal reservoirs through hydraulic fracturing is crucial for efficient HDR exploitation. In this study, hydraulic fracturing tests, combined with acoustic emission (AE) monitoring, were conducted on three types of fractured-and-subsequently-filled granite specimens, namely those with fracture backfill orientations parallel, vertical or oblique to the horizontal plane. Experiments were performed under high temperatures (room temperature to 400 °C) and high pressures using a true-triaxial hydraulic fracturing test system. Fracturing characteristics, including breakdown pressures, fracturing patterns and fracture conductivity, were analysed. The results suggest that thermally induced fractures around backfills during hydraulic fracturing reduce breakdown pressure. These fractures also create enhanced-permeability regions, promote fracture network formation within the backfills, and influence the primary fracture propagation direction. These findings highlight a viable approach for the large-scale, commercial exploitation of HDR geothermal energy.