| In China, coal is the most important energy resource. It plays a dominant role in China's energy structure, and it provide the power to promote economic development. With the increasing demand of resources and the diminishing of the upper-layer coal resource, the coalmine industry has changed its spearhead towards the deeper-layer mining. According to statistics, the mining depths of the coalmines are currently marching at a speed of 10m-12 m per year, so it brings a significant challenge to the mining safety. With the increasing depths of coalmines, so does the ground stress of the coal layer. The increase of the latter diminishes the permeability of coal seam, the content of gas increases tremendously. Especially the increase in number of coal seam that characterizes by its high soft, high ground stress, high gas content and low permeability brings great difficulties to the coalmine mining industry. Gas drainage is the most effective method to decrease gas content of coal seam, and it plays a key role to ensure the safety of coalmine. Disappointingly, the majority of China's coal seams are the low permeability ones, which exists considerably large difficulties to the gas drainage. The current methods of enhancing permeability such as hydraulic fracturing, hydraulic hole are still largely restricted on its outcome due to each of its restricting conditions. Thus, these technologies are still not to be promoted. As a result, great emphasize ought to put on researching and inventing permeability-enhancing technology, which not only is easy to operate but also produces considerable energy. Based on the above content, a new method of enhancing permeability of coal seam by using high-voltage electrical pulse is put forward in the paper and it has a great theoretical significance and utility value.Based on the application of high voltage pulse in oil production and mineral processing, the characteristics of the coal are analysed in detail in the paper. In this paper, the experimental system of high-voltage electrical pulse on coal is established, the fracture characteristics of different strength samples under different parameters of high-voltage electrical pulses are analysed, the deformation characteristics, the fracture and the AE characteristics of different strength samples are researched. Furthermore, the mechanism of high-voltage electrical pulse on coal is proposed, the application method of the technology is also proposed, and the develop methods of the equipment of the in-field technology is pointed out which takes on a great significance in increasing permeability of coal seam. The main research results are as follows:(1) The high strength specimens are made by adjusting the proportion of cement and sand into one in three, one in four and one in five. At the same time, using cement and sand as main materials, and gypsum, mica, perlite powder as auxiliary materials to make soft coal, medium hard coal and hard coal samples. In addition, some coal samples are also made. After testing the uniaxial compressive strength, poison ratio and elasticity modulus of the cement specimens, the result has indicated that the physical characteristics of the cement specimens are similar to the ones of similar coal samples. Moreover, the strain gages are buried in samples to prepare for the future require of testing the strain character.(2) Experimental system of high voltage electrical pulse on coal is established. The system includes high voltage power supply, energy storage capacitor, electrode and some other parts including trigger switch, ignition switch and so on. The stress- strain test system and AE system are established as well. The process is the following: the power frequency current is changed into medium frequency current of 600 volt and 1000 hertz through medium frequency inverter, and then changed into high voltage current through silicon pile, the high voltage current is then transferred to electrode through trigger switch and igniting switch. When the magnitude of voltage is greater than threshold, the electrodes will discharge to produce shock wave to fracture coal samples.(3) With the established experimental system of the high-voltage electrical pulse on coal, the fracture characteristics of the three different strength cement samples, 1:3, 1:4, 1:5 are tested under conditions of different parameter. As shown in the results, the high strength specimen of proportion 1:4 are completely pulverized after one pulsing under the voltage of 13 kV and the capacitance of 40?F, the high strength specimen of proportion 1:3 appears big cracks after pulsing eight times when the voltage is 11 kV and the capacitor is 20?F, the high strength specimen of proportion 1:5 appears obvious cracks after pulsing 12 times when the voltage is 11 kV and the capacitor is 10?F. In order to research the interior deformation of samples after every pulse, the appropriate parameters using in the experiment are set to be 11 kV and 10?F.(4) The small-scale experimental system of high-voltage electrical pulse is established. The blasting vibration is used to test speeds in different direction under different energy. The result shows that the speed in z direction is at its maximum, followed by y direction, and is at its minimum in x direction. The speed is increasing with the increasing energy and the decreasing distance.(5) The method of FFT is used to analyse the shock wave signals to obtain its frequency. According to the research, the frequency under different energy mainly concentrates on the range of 0-500 Hz. Specifically, the energy frequency of 11 kilovolt and 14 kilovolt concentrates within 200 Hz, and 300 Hz respectively. After the EEMD decompose of shock wave signals under different voltage, twelve IMF components and residual component are obtained. The results show that the frequency from IMF_h2 to IMF_h6 is advantageous for x-direction signals, IMF_h3 to IMF_h6 for y-direction, and IMF_h3 to IMF_h4 for z-direction. Also, with the increasing energy, the energy of the range of IMF_h4 frequency increases as well.(6) The high strength cement samples are tested under the high-voltage electrical pulse system with parameters of 11 kV and 10?F. As for the 1# high-strength samples, the top of the sample starts to appear small cracks after six pulses, and obvious cracks after 17 pulses. As for the 3# high-strength samples, small cracks started to show after seven pulses, and big cracks after twelve pulses. For the high-strength coal sample, signs of obvious cracks showed after 14 pulses, and for the soft coal sample, cracks after 12 times. The results demonstrate that with the increasing strength, the required energy increases, therefore, so does the times of pulsing. Also, the cracks always start from the top of samples. Moreover, the results show that after pulsing many times, the samples always appear some symmetric main cracks and some small cracks along the main crack. For the high-strength cement samples, the process of producing cracks can be divided into preparation stage, slow development stage, rapid development stage and failure stage. On the other hand, for lower strength samples, cracks appear directly on the surface after multiple pulses.(7) As for 2# high strength sample, when the gap already exists on the sample, obvious cracks will be shown after thirty pulses. As for medium-strength sample, the sample appears obvious cracks on the sample with the premise that the sample has its feeble plane. The results show that the sealing quality has a great deal influence on the propagation of shock wave of high-voltage electrical pulse and the feeble plane can help the appearance of cracks.(8) The deformation signals of samples are analyzed using FFT and EEMD. The result shows that the signals of 50 Hz and its multiple frequencies are the interference signals. After filtering the interference signals, the real deformation curves are obtained. The results show that in the process of pulsing, the interior deformation of samples changes irregularly, and the transverse strain increases, longitudinal strain decreases with the strength of samples decreasing. The strain results show that the shock wave attenuating more rapidly with the strength of samples decreasing.(9) The AE is used to analyze the fracture character of samples. The results show that the accumulated curves of ring counts can be divided to three stages, slow rising stage, rapid rising stage and intense activity stage, and the energy of bottom of samples is greater than the top.(10) Through the experiment of small scale raw coal, after pulsing many times, the fractal dimension and permeability are greater than before, the results show that the original fractures are extended and new fractures are produced. The big scale briquette are used to simulate soft coal, the top of soft coal appears many cracks, and the longitudinal length of cracks is less than hard coal. And the strain values are less than hard coal.(11) In the process of high-voltage electrical pulse, the liquid-electric effect, cavitation effect and heat effect are main action forms, and the experimental results show that high-voltage electrical pulse can produce cracks through shear action and mechanical vibration. In the process, the strength of coal will be decreased after many times pulsing, and water will produce water-wedge effect to produce more cracks. At the same time, mechanical vibration can break the obstruction in the fracture to increase the flow ability of gas.(12) Based on the use of the technology in the oil field, the application modes of the technology are proposed, and the modes are divided in two kinds, enhance permeability on the ground and underground. The procedures of enhancing permeability on the ground are water injection, lower the equipment into water, hoist equipment after pulsing many times and clear the hole. The procedures of enhancing permeability underground are drill hole, clear hole, water injection, place equipment, heal hole, pull equipment after pulsing many times and clear hole.(13) Because the existence of gas is different from oil, so the high voltage electrical pulse equipment have to be developed according to coal mine, such as heal the electrodes, add the blast-proof shell to other, provide water environment for electrode through water pipe or water bag, and develop high-performance cable. So the equipment package should be developed to apply in field, and the results should be evaluated comprehensively. |
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