224 / 2018-07-20 21:08:33

The characteristics of microseismic waveforms induced by hydraulic fracturing of coal seam in underground coal mines

coal mine; coal seam; hydraulic fracturing; microseismic monitoring; microseismic waveform characteristics

矿井瓦斯与煤与瓦斯突出 > 煤矿瓦斯

The highly accurate microseismic (MS) data acquisition apparatus and sensitive sensors are employed to monitor the fractures induced by the hydraulic fracturing (HF) of coal seam in underground coal mines. The field test of MS monitoring for coal seam HF is conducted in Xieqiao Coal Mine, and lots of effective and weak MS waveforms induced by coal seam HF are successfully recorded during this test. Some useful methods such as the short term averaging/long term averaging (STA/LTA) method, wavelet transformation, Hilbert-Huang transformation, the envelope analysis and curve fitting method help to make comprehensive insights into the amplitude-frequency, time-frequency, the duration and the peak following attenuation characteristics of MS waveforms induced by coal seam HF. The characteristics of time-frequency distribution, energy distribution of frequency band, the duration and the peak following attenuation of mining, blast and coal seam HF induced MS waveforms are statistically studied. The results show that the frequency of coal seam HF induced MS waveforms mainly focus on 70~100 Hz. This range occupies more than 50% of the total energy of the waveforms. Most of the waveforms last less than 1.0 s, and the duration time fluctuates around 0.5 s. The attenuation after peak is gentle with the attenuation coefficients less than 0.6, much smaller than those of mining and blast induced MS waveforms. These characteristics, the peak frequency, duration time and peak following attenuation, are very sensitive to distinguish these three typical kinds of induced MS waveforms. The further study shows that the principles of fractures’ propagation influenced by the pressurized water and the backfill effect of pores by large volume of pressurized water injection and the pressurized water itself are the key factors for the characteristics of hydraulic fracturing induced MS waveforms. The conclusions are theoretically beneficial to the weak MS signals and events automatic pick-out, the MS location for the hydraulic fractures and the initiation and propagation principles of fractures induced by coal seam HF in underground coal mines.