Fractal fracture toughness measurements of heat-treated granite using hydraulic fracturing under different injection flow rates

In geothermal energy extraction research, fractal fracture toughness measurements of high-temperature rocks during hydraulic fracturing have yet to be investigated but are crucial for hot dry rock reservoir reconstruction. In this paper, real-time high-temperature hydraulic fracturing tests at different injection flow rates were performed using thick-walled hollow cubic granite specimens with pre-crack. By introducing fractal theory, the fractal dimension was deployed to describe the tortuous morphology of the hydraulic fracture surface before establishing a fracture toughness theoretical model under fractal fracture. Acoustic emission events were monitored during fracturing, the effect of temperature and rate on the microstructure of the hydraulic fracture surface was examined using scanning electron microscopy (SEM) after fracturing. Finally, the effect of temperature and rate on hydraulic fracture characteristics and the correlation between fractal and fracture toughness were discussed. Experimental results showed that the fractal fracture toughness decreased with increasing specimen temperature and decreasing injection flow rate. As with other fracture characteristics such as breakdown pressure, fractal dimension, and microstructure, it was not governed by either temperature or rate alone, but was facilitated by the coupling of both factors, which could be explained by the thermal shock of hightemperature rock exposed to fracturing fluids. The results of fractal fracture toughness were greater than linear fracture toughness, which is attributed to greater fracture energy consumption due to mineral grains separation and micro-crack development associated with fractal fracture compared to linear fracture. This also indicated that fractal fracture toughness conformed favourably to the essence of rock fracture. These results can provide novel insights on the fracture mechanism of hydraulically fractured thermal reservoir at different injection flow rates.