Research On Time-domain Characteristic In Water Shock-wave Based On Hydro-electric Effect And Its Application In Coal Permeability Enhancement

Coal bed gas, also known as methane, which is a kind of clean energy, as well as arch-criminal of safety accidents, such as underground explosions, and greenhouse gas effects. Reasonable utilization of coal bed methane can not only reduce the incidence rate of coal mine safety accidents and air pollution, but also provide the necessary energy for the productions and lives of mankind. However, more than 95% of the coal seam in the high gas coal mine in our country has the defects of poor permeability and soft coal quality, which leads to the problem of easy blocked drill and low extraction rate. Therefore, increasing the fracture degree of gas coal and rock mass is a difficult problem to be solved in coal seam gas drainage.In recent years, the technique of high pressure pulse discharge in coal seam drilling is a new method for the coupling of static and dynamic loads,which is now attracting much more attention among scholars and engineering technicians. Its main principle is using electro-hydraulic effect produced by pulsed high-voltage discharge to form a high pressure pulsed water shock, under which irregular and large range of penetrating crack will be produced in coal seam, so as to achieve the purpose to increase coal seam permeability.As a kind of special dynamic load, the time domain characteristics of the pulse water shock wave(including front time, the slope of the rising,etc.) has a profound influence on the dynamic propagation of the crack in the coal body. Different water shock characteristics in time domain represent the different velocities of cracks absorbing energy from outside. However, under the limitation of its material properties, there is also a limit value as to the velocities of cracks releasing energy in the crack propagation process in coal body. Under the game of these two energy rates, the mode of crack propagation in coal body also varies.By controlling the discharge voltage and hydrostatic pressure, this study has obtained several different front time and rising edge slope of waveform in water shock that generated by high voltage electrical pulse in water. This article emphatically studied the change laws of front time and rising edge slope leaded by the changes of discharge voltage and hydrostatic pressure and its influence on the effect of coal permeability enhancement. The main achievements of the study are as follows.(1) The change laws of front time and rising edge slope leaded by the changes of discharge voltage and hydrostatic pressure were analyzed based on the relative knowledge of shock wave and mechanism of water shock generated by high voltage electrical pulse in water. Theoretical calculation formulas about front time and rising edge slope related to discharge voltage and hydrostatic pressure were obtained.(2) Controlling variable method was chose in the experiment of high voltage electrical pulse in water. The results show that when hydrostatic pressure is constant, with the increases of discharge voltage, the front time decreases while the rising edge slope increases; on the contrary, when discharge voltage is constant, with the increases of hydrostatic pressure, the front time increases while the rising edge slope decreases. The results of experiment are consistent with theoretical calculation formulas.(3) After the analysis based on the basic knowledge of crack extension under impact load, we found that the front time of water shock could influence the crack extension in coal through influence the stress intensity factor in crack tip. Theoretical calculation formulas about stress intensity factor involves water shock front time were obtained through the introduction of front time to stress intensity factor in crack tip.(4) At last, how the front time influences the crack extension was studied with the help of RFPA-Dynamic, which is a numerical analysis software. The simulation result shows that different front time have influences on both stress wave propagation in coal and acoustic emission phenomena. And the pattern of crack extension also differs from another because of the difference of front time. Shorter cracks were generated under shorter front time while longer front time is beneficial to the extension of crack.In a word, the research is expected to provide theoretical and experimental basis for the application of methane extraction technique in borehole of coal seam based on high voltage electrical pulse & hydraulic fracturing.