Fractal characteristics and acoustic early warning index of rock with thermal storage potential under fatigue fracturing

Fatigue fracturing technology is of great significance to improve the efficiency of geothermal energy extraction and reduce the risk of induced earthquakes. In this study, granite with thermal storage potential was selected as the sample, and carried out conventional fracturing (monotonic pressurization) and fatigue fracturing (pulsating pressurization) tests. First, the influence of fracturing mode on the fractal characteristics of the samples was analyzed. The fractal dimension and 3D roughness of the samples were calculated, and the distribution characteristics of morphology parameters on the sample surface were analyzed. Then, the differences in the microcrack evolution process under different fracturing modes were compared based on the acoustic emission (AE) frequency spectrum characteristics and energy. Finally, this study used the function F to analyze the time series of AE events and established an acoustic early warning index. Our laboratory work has shown that as the temperature increases, the strength and fracture toughness of the sample under the coupling effect of high temperature-fatigue load decrease nonlinearly, the fractal dimension and 3D roughness increase linearly, and the maximum cumulative AE energy increases exponentially. The height difference of the sample surface increases, the normality of the slope angle distribution weakens, the grayscale gradient increases, and the texture information increases. The peak frequency signals of the samples under conventional fracturing and fatigue fracturing are mainly concentrated in 100 kHz–400 kHz. Compared with conventional fracturing, the energy release of the sample under fatigue fracturing increased by about 10 %, and the fracture toughness decreased by about 10 %. Under fatigue fracturing, the number of cracks generated in the sample is greater, the size is larger, and the growth time is longer. The fracture morphology of the sample is also more complex and rougher. Fatigue fracturing is superior to conventional fracturing in promoting energy dissipation inside rocks and low stress-induced crack growth.