Damage Characteristics and Laws of Micro-crack of Underwater Electric Pulse Fracturing Coal-rock Mass

The purpose of this study is to study the mechanism of crack initiation, the damage characteristics and the propagation law of high-voltage pulse hydraulic fracturing coal so as to evaluate the fracturing effect of coal-rock mass. Based on the mechanism of high-voltage discharge fracturing in water, crack fracture and propagation, the numerical calculation and experiment of high-voltage pulse discharge (3kV, 5kV) based on a water pressure (3MPa) were carried out. The CT scanning technique and PCAS image processing program were used to study the crack initiation and propagation of coal-rock mass. From the results, it is shown that under the action of high-voltage pulse discharge in water, the crack of coal-rock mass is dominated by the I-II composite crack and its damage type is mainly the tensile-shear damage; The periodic oscillation of the shock wave in the water causes the stress at the tip of the coal body to have an oscillating variation. The crack growth rate is related to the Rayleigh wave speed, and the growth of the crack length is unstable; The crack propagation and extension in the coal-rock mass are related to the preset angle of crack, the peak pressure of water shock wave and the direction of the maximum principal stress. Specifically, with the same voltage, the larger the pre-crack angle, the smaller the crack length, and the larger the crack width, area, and cracking rate; With the same crack angle, the higher the discharge voltage, the larger the cracking rate, length, width and area of crack. Compared with the 3 kV high-voltage discharge, the crack propagation pattern is more complicated under the condition of 5 kV discharge. The more the number of cracks, the larger the crack length, width, area and cracking rate, and the better the fracturing effect of coal-rock mass.本研究旨在探究高压脉冲水力压裂煤层的起裂机制、损伤特性与扩展规律，以此评价煤岩体的压裂效果。基于水中高压放电压裂、裂纹断裂及扩展的作用机制，本研究开展了基于水压（3MPa）的高压脉冲放电（3kV、5kV）数值计算与实验研究。采用CT扫描技术（CT scanning technique）与PCAS图像处理程序（PCAS image processing program），对煤岩体的裂纹起裂与扩展过程进行分析。研究结果表明：在水中高压脉冲放电作用下，煤岩体裂纹以I-II复合型裂纹为主，损伤类型以拉剪复合损伤为主；水中冲击波的周期性振荡使得煤体尖端应力呈现振荡变化特征。裂纹扩展速率与瑞利波速（Rayleigh wave speed）相关，且裂纹长度的增长过程并不稳定；煤岩体内部的裂纹扩展与延伸，与预设裂纹角度、水冲击波峰值压力及最大主应力方向密切相关。具体而言，在放电电压相同的工况下，预设裂纹角度越大，裂纹长度越小，而裂纹宽度、面积与开裂率则越大；在预设裂纹角度相同的工况下，放电电压越高，裂纹的开裂率、长度、宽度与面积均越大。与3kV高压放电工况相比，5kV放电工况下的裂纹扩展模式更为复杂，裂纹数量更多，裂纹长度、宽度、面积与开裂率均更大，煤岩体的压裂效果也更优异。