瓦斯抽采钻孔孔周裂隙演化及等效裂纹宽度试验研究

Experimental study on evolution of circumferential crack and equivalent crack width of gas drilling borehole

为探究瓦斯抽采钻孔孔周裂隙扩展特征,取不同含水率的含孔方形试样,开展在渐进性破坏过程中数字散斑相关测量,获得单轴压缩下试样表面裂纹扩展过程中应变场的演化特征;采用RSM-SY7声波仪获取试样在破坏过程中的透射波形、波速和时间,根据声波透射过程中的时差计算公式,计算出试样在破坏过程中的等效裂纹宽度,揭示含水率对含孔试样裂纹扩展规律的影响。结果表明:(1)在试样渐进性破坏过程中,随着含水率的增大裂隙扩展明显减少,峰值应力σp的离散性显著降低;(2)超声波检测结果显示,饱水状态下首波到达时间较干燥状态下延迟8~10μs,且声波能量在时间域上衰减较快;(3)对试样破坏过程中等效裂纹宽度和应力水平试验曲线的拟合,得出等效裂纹宽度随应力水平的增高呈现指数级增长,可以对裂纹扩展参数进行定量化描述;裂隙扩展速率随含水率的增高显著降低,其中控制含水组的等效裂纹宽度在0.003~0.004mm范围,干燥试样的等效裂纹宽度在0.012~0.013 mm范围。

In order to explore the characteristics of crack propagation around the gas drilling borehole,the uniaxial compression test of the pore samples under different water conditions was carried out,and the evolution characteristics of the strain field in the surface crack propagation process of the samples during compression was measured by the digital speckle correlation measurement.The transmission waveform,the wave velocity and time during compression was measured by RSM-SY7 ultrasonic system.According to the calculation formula of time difference during the acoustic wave transmission process,the equivalent crack width of the sample during the failure process is calculated,and the influence of water content on the crack propagation rule of the sample with holes is revealed.The results show that:(1)In the progressive failure process of the sample,the crack propagation decreases obviously with the increase of water content,and the dispersion of peak stressσp decreases obviously.(2)The ultrasonic testing results show that the arrival time of the first wave in the saturated state is delayed by 8–10μs compared with that in the dry state,and the acoustic wave energy decays rapidly in the time domain.(3)Fitting the test curve of equivalent crack width and stress level in the failure process of the sample,it is concluded that the equivalent crack width increases exponentially with the increase of stress level,which can quantitatively describe the crack propagation parameters.The crack propagation rate decreases significantly with the increase of water content,with the equivalent crack width of the control water content group being between 0.003 mm and 0.004 mm,and the equivalent crack width of the dry sample being between 0.012 mm and 0.013 mm.