Experimental Comparison of Mechanical Properties and Fractal Characteristics of Geothermal Reservoir Rocks after Different Cooling Treatments

In the process of geothermal well drilling, the high-temperature rock in the reservoirs is cooled by different cooling media, which leads to the change of mechanical properties, affecting the rock-breaking efficiency of the bit. Sandstone and granite were taken as the research objects and heated to 400°C, 600°C, and 800°C, then were cooled by natural, water, and liquid nitrogen (LN2) cooling. Ultrasonic detector tests, the split Hopkinson pressure bar (SHPB) experiments, and scanning electron microscope (SEM) tests were conducted. The effects of heating temperature and cooling methods on the dynamic mechanical properties were quantified and analyzed. Thermal shock damage evolution and the damage mechanism of high-temperature rock under different cooling ways were discussed. The fractal method was applied to the analysis of fragment distribution, to obtain the corresponding relationship between fractal dimension and heat treatment temperature and cooling methods. The research results showed that thermal and cooling treatment has a significant effect on the mechanical properties of rock. With the increase of the treatment temperature, the dynamic peak stress decreased, and the peak strain increased. The temperature sensitivity of sandstone to temperature was higher than that of granite, while sandstone was more significantly affected than granite in terms of the sensitivity of the cooling method. Fractal dimension could be used to analyze the fragmentation of high-temperature rock cooled in different ways after impact. The experimental results were expected to provide an adequate theoretical basis for the development and utilization of geothermal resources