Research On The Electro-hydraulic Characterstics And Coal Crack Effect Of Underwater High Voleage Discharge

Underwater high voltage pulse discharge technology as an engineering technology are widely and maturely used in the field of electric spark source, electro-hydraulic forming, sewage treatment, oil exploitation, etc. The basic principle and working characteristics of it affects its application in the practical engineering effect. At present there are many researches on the underwater high voltage pulse discharge technology, but it is according to the summary of experience in their respective fields of engineering and it is difficult to be widely promoted and applied to CBM well fracturing. In this paper, according to coal seam crack anti-reflection engineering background, and based on the mature theoretical experience of the traditional hydraulic fracturing technology and the underwater high voltage pulse discharge technology in other areas, the paper has put forward the drilling water injection high voltage pulse discharge crack theory, which is integrating hydraulic fracturing theory with high voltage pulse discharge theory together. The experiment of underwater high voltage pulsed discharge effect and the trail of coal seam crack effect was completed in the laboratory, which was carried out under different water pressure and discharge voltage. This paper achieved some results and preliminary demonstrates the feasibility of underwater high voltage pulse discharge technology in the aspect of CBM extraction, which provides theoretical guidance for later engineering application.The main research contents and results of this dissertation include:(1)Based on the basic theory of underwater high voltage pulsed discharge, the basic characteristics of the water shock wave in electro-hydraulic effect are analyzed theoretically, and the energy calculation formula and the pressure attenuation formula of the water shock wave are obtained. Theoretical analysis is made on the crack mechanism of the electro-hydraulic effect on the coal seam, and the influence of the breakdown delay and hydrostatic pressure on the pulse fracturing effect is studied. After analysis, it is considered that the longer breakdown delay time is, the peak pressure of water shock wave will reduce, and the fracturing effect becomes worse; the shorter breakdown delay time is, the peak pressure of water shock wave increasing, and fracturing effect becomes better; with the increasing of the hydrostatic pressure, it is conducive to the formation of longer bedding fractures, but it will reduce the peak pressure of water shock wave, and impact the coal seam fracturing effect. Therefore, there is an optimal value of hydrostatic pressure in each set of discharge parameters, which makes the best effect of coal cracking.(2)In the experimental conditions of discharge voltage 7kV~13kV and hydrostatic pressure 0MPa~12MPa, the experiment of electro-hydraulic effect of pulse discharge are carried out under different hydrostatic pressure and discharge voltage. Through analyzing the obtained experimental data, we can draw the following conclusions: with the discharge voltage increases, the breakdown delay is reduced, the shock wave peak pressure will increase; with increasing hydrostatic pressure, the breakdown delay increases, the peak pressure of the shock wave will first rise high then decreases.(3)Under the condition of 3MPa hydrostatic pressure, the test is carried out under different discharge voltage of underwater high voltage discharge respectively on four specimens. Using the ultrasonic testing method to analyze the coal fracturing effect while different electro-hydraulic effect happens, the results show: under the action of electro-hydraulic effect, new impact fracture mainly generated in coal existing original cracks, making the original cracks extended; with the enhancement of electro-hydraulic effect, the number of fracture are increased, expanding the pulse fracturing area, ultimately makes interconnected between the cracks, covering the whole of the specimen through the cracks.In this paper, the electro-hydraulic effect and coal crack effect of underwater high voltage discharge were studied from the perspective of theory and experiment. Initially established the underwater high voltage discharge technology is feasible in coal bed methane extraction applications, and determined the suitable discharge parameters for practical engineering. The paper obtained the general rules of the coal crack effect by pulse discharge, and for underwater high voltage discharge technology in coal seam fracturing application provides a certain theoretical basis and method of operation.