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

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 stress σp decreases obviously. (2) The ultrasonic testing results show that the arrival time of the first wave in the saturated state is delayed by 8-10 μs 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. © 2019, Science Press. All right reserved.