Abstract: To improve the fracturing effect of hard rock, a high pressure foam fracturing device is proposed. The device uses high-speed piston to impact foam, applying loads in the borehole to achieve the fracturing of hard rock. Based on the theory of fluid mechanics and conservation of energy, the dynamic model of the device is established. The relationship between the dynamic parameters and structural parameters of the device is analyzed. The influence of the device structural parameters on the fracturing pressure is explored. Besides, the energy evolution rule of the piston's stamping process is revealed. The high pressure foam fracturing test system is built to verify the feasibility of the system and the rationality of theoretical analysis. The results show that after 200 mm displacement, the fracturing pressure of the fracturing piston can reach 80 MPa. The diameter of the piston rod and the area of the oil drain port have a striking influence on the fracturing pressure. The influence of the diameter of the buffer piston on the fracturing pressure is weak, while its influence on the retarding speed is more significant. When the diameter of the buffer piston increases from 70 mm to 90 mm, the buffering speed decreased from 4.8 m/s to 1.5 m/s, and the buffering effect increased by 3.2 times. Through experiment, the effective breaking of rock sample is realized. The experimental results of the fracturing pressure agree well with the theoretical calculation. The research provides a theoretical basis for the improvement of hard rock fracturing performance, also provides a theoretical reference for the research of fracturing devices with other high-pressure medium.

Key words: hard rock fragmentation, high pressure foam fracturing, dynamic model, fracturing pressure