Simulation Experimental Study on Evolution Law of Cumulative Damage in Coal under Shocks by Multiple High Pressure Air Blasting.

High pressure air blasting (HPAB) is a new type of waterless physical expansion method for enhancing coalbed methane extraction from low-permeability coal reservoirs. In order to explore the influence of shocks by multiple HPAB on the cumulative damage evolution of coal, shocks by multiple HPAB experiments were conducted using simulated coal samples and a self-developed HPAB device. During the experimental testing process, The wave velocity of coal samples under shocks by multiple HPAB was tested using an ultrasonic tester, and the time-domain signal of the ultrasonic wave was converted into a frequency-domain signal using Fourier transform theory. the evolution of acoustic waves and damage morphological characteristics of coal with increasing HPAB times was analyzed. Additionally,The evolution process of the three-dimensional pore and fracture structure of coal samples under Shocks by multiple HPAB was studied from a microscopic perspective using CT scanning and 3D reconstruction technology. The experimental results indicate that (1) With increasing shock by HPAB times, the damage in the area near the blasthole increased linearly, while the damage in the middle and far zones exhibited a slow growth during the first first-third shock by HPAB and a rapid growth during the third-fifth shock by HPAB. The damage in the near blasthole decreased rapidly with increasing distance from the blasthole. Compared with the near blasthole, the cumulative degree of coal damage in the middle and far zones is relatively small. (2) With increasing shock by HPAB times, The time-domain signal of near-field ultrasound changes from a regular waveform to a disordered and distorted waveform, and the amplitude shows a gradually decreasing trend. The main frequency band shifts from high-frequency signals to mid low frequency signals, and the main frequency band signal decreases from 48.48 kHz before HPAB to 4.88 kHz after the fifth shock by HPAB. (3) Compared with before HPAB, After the fifth shock by HPAB, the number of pores increased by 29.27%, the maximum equivalent radius of pore increased by 16.27%, the number of throats increased by 25.28 times, the maximum equivalent radius of throats increased by 55.08%, and maximum channel length of throat increased by 25.44%. With increasing shock by HPAB times, the microcracks inside the coal evolve from closed and semiclosed pores to open pores, forming a well-developed three-dimensional pore fracture network channel. The research results aim to provide theoretical and experimental basis for the study of cumulative fatigue damage and mechanical performance degradation effects of coal under shocks by multiple HPAB.