| Coal-rock hydraulic fracturing technology is widely used in coal mine and has achieved good technical and economic results.It has become an important technical guarantee for safe and efficient production of coal mine,but it does not have effective monitoring means.Micro-seismic/acoustic emission theories and technologies are used to study acoustic emission(AE)activity in coal-rock hydraulic fracturing.Characteristics of AE waveforms induced by coal-rock fracture are studied by doing hydraulic fracturing experiments,using time-frequency analysis,Hilbert-Huang transformation,double difference localization and MATLAB programming;A kind of AE events detection method suitable for coal-rock hydraulic fracturing experiments is developed;The localization of AE events induced by hydraulic fracturing experiment is done to study the dynamic formation of fracture.The main phenomena of test block rupture are the peak of water pressure,the peak acoustic emission energy and the seepage on the surface of samples.The average cracking water pressures of limestone and coal are 18.5 MPa and 8.75 MPa,respectively.The peak rupture time of limestone and coal is 600-700 s.The peak rupture time of limestones is generally less than that of coals.The analysis of background noise in the experiment by wavelet transformation shows that the background noise in the experiment process does not affect the reception of AE signal by AE monitoring system;Hilbert-Huang transformation can process AE signal of coal-rock fracture and give the characteristics of AE waveform,such as amplitude,arrival time,duration,attenuation ratio and dominant frequency.The amplitude of AE waveform in hydraulic fracturing is smaller than that in uniaxial compression test;And the duration of AE waveforms in uniaxial compression test is mainly in the range of 0.2 ms to 0.8 ms.The duration of AE waveforms in hydraulic fracturing experiment is mainly between 1.1-1.3 ms,in which the duration of AE waveform of limestone is generally longer than that in coal,and the duration of AE waveform in limestone decreases with the increase of compressional pressure.When the attenuation ratio of AE waveform in uniaxial compression experiment is between 0.32 and 0.4,20% of AE waveforms' energy increases slowly and drops rapidly.When the attenuation ratio is 0.74-0.88,40% of AE waveforms' energy shows the characteristics of fast rising and slow falling.The attenuation ratio of waveforms in hydraulic fracturing experiment is 94% or 78%.Waveforms' energy shows the characteristics of fast rising and slow falling.Water explains that attenuation ratio of acoustic emission waveforms in hydraulic fracturing of limestone and coal is larger than that in uniaxial compression test;The dominant frequency of AE waveforms in uniaxial compression test is about 178 kHz,and the dominant frequency of AE waveforms in limestone is larger than that in coal.The dominant frequency range of AE waveforms in hydraulic fracturing experiment is 160-170 kHz.With the increase of confining pressure,the dominant frequency of AE waveform in hydraulic fracturing experiment limestone decreases.The semblance number of hydraulic fracturing AE signal obtained by automatic identification method ranges from 0.06 to 0.16.Through the peak value of semblance number,nine groups of effective AE events can be detected in 1 s.Comparing the results of automatic AE event recognition method for hydraulic fracturing with those of artificial identification and long-short window method under different threshold conditions,the results show that the automatic AE event recognition method for hydraulic fracturing is flexible,efficient and less affected by signal-to-noise ratio of waveform and has less missed pick-ups and higher accuracy.The results of AE locsalization show that compressional pressure loading can cause small energy micro-fracture in the surrounding and edge of the sample;Water injection fracturing is divided into initial stage,middle stage and late stage of fracturing.In the initial stage,the upper region of the sample is fractured but the energy is small.The middle stage of fracturing is the main stage of coal's fracturing in this experiment,and crack mainly occurs around the sealing packer.After the occurrence of the main fracture,the coal and stone blocks are basically completely fractured,and the fracture in late stage will extend to the outside of the sample along the direction of the main fracture;The final fracture produced by the hydraulic fracturing is not strictly parallel to the maximum principal stress or perpendicular to the minimum principal stress: sampling of the sample under natural conditions is random.Or processing accuracy and the impact of processing on the integrity of the sample will lead to incomplete uniform stress in the fracturing process,and uneven stress state will lead to irregular cracks in the test block. |
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