Dynamic characteristics of anisotropic shale and rock-breaking efficiency of the axe-shaped tooth under different impact load-bedding angles

Percussion drilling technology can be used to increase the rate of penetration in deep shale reservoirs, but the interaction mechanism among impact loads, drilling teeth and rock has not been sufficiently investigated. For this reason, shales with different bedding angles are used to carry out impact compression and tensile experiments as well as the rock-breaking experiments by single axe-shaped tooth, the variation of dynamic strengths, rock failure characteristics, fractal dimensions, and tensile/compression ratios under different load-bedding angles ( α ) are investigated. Then, the three-dimensional scanning device is used to measure the penetration depth and rock-breaking volume under different load-bedding angles. The results show that with the increase of load-bedding angle (0°–90°), the compressive strength decreases and then increases, with the lowest strength at α = 45° and the highest strength at α = 0°; the tensile strength decreases and then increases, with the lowest strength near α = 30° and the highest strength at α = 90°. With the growing impact rate, the effect of load-bedding angle on dynamic compressive strength decreases, and the effect on dynamic tensile strength becomes more significant. When the impact velocity is high (≥8.0 m/s), the tensile-compressive ratio first decreases and then increases, and both reach a minimum at a load-bedding angle of 30° and a maximum at 60°. With the increasing of the load-bedding angle, the depth of tooth penetration increases and then decreases, and the highest depth of tooth penetration and the highest energy absorption efficiency are achieved at α = 45°; the width of the impact pit increases and then decreases, and the maximum width value is achieved at α = 30°, with the smallest value of the specific work value of the rock-breaking. The results have significant reference value for improving the rock-breaking efficiency of percussion drilling in deep anisotropic formations.