| Safe and effective co-mining of coal and gas in low-permeability gas-rich coal seams of a deep mine is a major problem imposing restrictions on sustainable development of China's coal industry.To solve permeability of low-permeability coal seams with high gas in complex geological condition at great depth scientifically,a liquid carbon dioxide phase transition induced fracturing and antireflection technology was put forward in this paper.Based on a research method combining theoretical analysis,experimental investigation,numerical simulation and field industrial comparison test,a rather complete theoretical and application system has been established for this technology.Regarding main research content and new viewpoints gained,they are as follows.(1)Not only was liquid carbon dioxide gas explosion as a phase transition induced fracturing and antireflection analyzed in theory,but the evolution process for carbon dioxide in gas storage chamber of a fracturing device to boil and expand resulting in vapor explosion during gas explosion as a phase transition stage was interpreted.Moreover,coal mass crack area formation process together with the relevant partition patterns,cracking conditions and fracture development law,which are all caused by gas explosion,was revealed.As a result,computational formulas for fracture circle of coal masses where liquid carbon dioxide gas explosion as a phase transition took place were established by means of theoretical derivation.(2)Based on an independently designed and constructed physical experiment platform,time/position-varying point shaped charge blasting pressure evolution properties of liquid carbon dioxide were experimentally explored.It has been found that pressure peak directly facing the explosive notch was 244MPa and the corresponding pressure rising time is about 1.2ms.Pressure duration fitting functions of linearly ascending stage and logarithmically decreasing section were expressed in Pg= 201940t and Pg=-22.591n(t-t0)+15.84 respectively.Meanwhile,pressure peaks at places 300mm,600mm and 900mm away from the explosive notch of fracturing device related were 60MPa,22.42MPa and 21.37MPa,which correspond to pressure rising time of 15.13ms,15.42ms and 15.60ms separately.With the increase in such a distance to the explosive notch,gas pressure peaks rapidly went down in the first place and then declined slowly and steadily.On the whole,they presented a change law of quadratic parabola.(3)According to a physical and mathematical model of boiling,expanding and explosion venting processes in gas storage chamber of a liquid carbon dioxide fracturing device established,phase transition induced gas explosion evolution process in this chamber was probed into by virtue of numerical simulation.In addition to computing and analyzing the evolvement of related pressure field,temperature field,flow field and gas-liquid ratio,hydrokinetic mechanism of coupling between the pressure and phase transition induced boiling in the gas storage chamber was investigated to set forth two-phase flow characteristics in the course of explosion venting in the chamber.(4)By means of numerical simulation,liquid carbon dioxide gas explosion as a phase transition induced fracturing and antireflection effects were studied specific to low-permeability coal seams with high gas in the case that different influencing factors were adopted.According to results,influence range of gas explosion induced fracturing tends to expand in line with a linear relation expressed in R=3.26L+0.446,when precracking length rises.As ground stress climbs,such an influence range exponentially reduces in a non-linear way;and,quantitative relation between them can be written into an equation R = 3.096e-0.06?.Under circumstances of coal mass strength increase and decrease,influence radii of their plastic zones remain basically the same and mechanical strength of the coal mass itself exerts little influence on the scope covered by fracturing induced by gas explosion.With the increase in gas pressure,influence range of gas explosion induced fracturing coal mass has a tendency to expand.Function expression between the influence radius of gas explosion induced fracturing and gas pressure variation is R = 0.041 Pg + 0.743.(5)Principle,working mechanism and characteristics of the liquid carbon dioxide induced fracturing and antireflection technology were systematically stated.Five major systems were developed for auxiliary equipment of liquid carbon dioxide induced fracturing and antireflection and a measuring method was also proposed specific to the gas zone to be emitted in roadway of stope face and the range covered by liquid carbon dioxide phase transition induced fracturing and antireflection,so as to realize the purpose of fracturing and antireflection application effect judgment.(6)Through actual stope face measurement based on a tracer gas method,antireflection radius of the liquid carbon dioxide gas explosion as a phase transition induced fracturing coal seam was determined to be 2m.Due to gas explosion,gas emission quantity during boring of the coal seam with its influence range goes up by 4-8 times;however.gas emission attenuation coefficient reduces by 0.76-0.93 times.(7)As for heading face of the roadway,it has been judged according to gas content and coal drilling cutting parsing indexes that pre-drainage time of roadway tunneling decreases to 15 or 16 days from originally 30 days thanks to liquid carbon dioxide gas explosion as a phase transition.Contrast experiment shows that initial gas emission quantities of boring in hectometer along the gathered-energy direction and the direction without energy gathering of a carbon dioxide induced fracturing device differ by 1.7 times.(8)Comparative experiments of different antireflection techniques selected for the stope face demonstrate that influence range of antireflection measures on the coal mass can be arranged as follows:hydraulic fracturing deep-hole shaped charge blasting>liquid carbon dioxide gas explosion as a phase transition.With regard to the order of the maximum mash gas concentration during gas drainage borehole,it is liquid carbon dioxide gas explosion as a phase transition>deep-hole shaped charge blasting>hydraulic fracturing.Finally,it can be ranked as liquid carbon dioxide gas explosion as a phase transition>hydraulic fracturing>deep-hole shaped charge blasting.Meanwhile,benefits,efficiency,effects and safety indexes of diverse antireflection technologies in their field implementation procedures were also quantified by such contrast experiments. |
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