Study On Mechanism Of Coal Bed Methane Desorption And Seepage Under Sonic Vibrating Of Cavitation Water Jets

With the increasing depth of coal mining,the intrinsic properties of coal seams (micro-porosity, low permeability and high adsorption) become more apparent. It's difficult to execute gas drainage in the process of coal-gas dual-energy mining, which often causes ethane explosion, environment pollution and energy wasting. So far, many techniques have been studied to improve gas drainage, such as caving, hydraulic fracture, CO2 injection and physical field excitation. Sonic vibrating method is deems as promising, it can increase the methane drainage rate efficiently; however, it is hard to further promote for it's immature technical process. Thanks to the technology of water jet with high pressure, this problem can be solved effectively, because cavitation water jet in the situation of submergence can produce noise when bubble collapse, and this noise can promote methane desorption and seepage. No research has been reported about how sonic vibrating of cavitation water jet promotes methane desorption and seepage although this technology has been widely adopted in coal seams slotting.In the paper, the mechanism of cavitation, spectral characteristics of cavitation noise, propagation model and heat effect are researched by lab experiment, theoretic analysis and numerical simulation, then how cavitation promotes the methane desorption and seepage is studied via experiment.Finally, the mechanism of gas desorption and seepage under sonic vibrating of cavitation water jets and field test is studied theoretically., some conclusions are arrived as follows.①The growth of bubble cloud includes swelling and compressing, reinflating and recompressing, collapsing and falling in the cavitation water jets. Cavitation develops on the boundary layer between throat and diffusion section of Convergent-divergent nozzle. In the submerged suspended-particulate, cavitation develops on both sides of suspended-solid at vertical speed orientation. The result of numerical simulation is close to that of experimental method: The cavitation bubble changes with pump pressure and confining pressure, larger the pump pressure is, cavitation develops more obviously; and suitable confining pressure could produce the perfect bubble.②The noise of bubble collapse can be described by single index model, double exponential model or pulse sequence model, but the pulse sequence model can better express the characteristics of cavitation noise waves. Propagation of noise will raise coal temperature and promote methane desorption and seepage.③The experiment of vibration of cavitation shows that the frequency band of cavatation vibration is wide, 2000-10000Hz, when bubble collapse, it means the size of bubble is variable. Additionally, there exists concentrated bubble diameter at the frequency from 5400 to 6800Hz. Vibration frequency of the more concentrated impact on the coal seam gas desorption flow is more significant.④The experiments show that cavitation can effectively promote the methane desorption and seepage. Compared with the situation of no cavitation, desorption time of coal gas shortens by 16.4% and desorption quantity increases by 22.6% when cavitation number is 0.0038.⑤Adsorption barrier is decided by macromolecular structure of coal and components of gas, the deeper adsorption barrier is, the more energy is needed when desorption, and absorption mount is larger. The energy of desorption is provided by collisions of gas molecules or temperature rising. And it must be larger than wasted energy when absorption if methane molecules escapes from coal. The mechanism of gas desorption and seepage under sonic vibrating of cavitation water jets through the effects of mechanical vibration, thermal effects and the effects of hydraulic fracturing and damage to the promotion of coal seam gas desorption and seepage is studied theoretically. Imposed cavitation sonic vibrating, mechanical vibration effects reduce the coal skeleton stress, fracturing damage effect to improve the porosity of coal, coal bed system temperature for the coal seam gas desorption, together strengthen desorption and seepage of coal seam gas.⑥Field tests show that using high pressure water jet cutting drilling and cavitation sonic vibrating, the highest single hole concentration of gas extraction increased by 27.3 times, the highest pure gas drainage rate by 15 times. Under the same conditions with the occurrence of coal projects crosscut than expected time to reduce 45.27%. Reduce the number of pre-drilling holes 30.5%, 33.7% reduction of drilling footage.Three Main innovation points of the dissertation are:①The first time a high-pressure water jet cavitation visualization experiment of the entire process to determine the pump pressure, confining pressure on the cavitation effects, reveal the mechanism of bubble cloud breeding, development, and collapse and shaped features, is carried out.②Sonic vibrating of cavitation water jet is firstly moved to promote methane desorption, seepage and methane drainage rate, using self-developed cavitation sonic vibrating for coal gas desorption flow experimental equipment, and the relationship model between cavitation water jet parameters and coal gas desorption flow is set up.③The cavitation noise spectrum optimization model, attenuation law, noise wave energy equation and model of mechanical vibration induced crack damage is established, and the mechanism of sonic vibrating of cavitation water jet to promote methane desorption and seepage is revealed. It is the fundamental basic of theory and experiment for its engineering application.