| Chinese geological conditions are complex.Gas content in coal seam is high and gas extraction in low permeability coal seam is difficult,which causes the frequent occurrence of mine gas disasters.With the extension of mining depth,the stress,permeability and gas content of coal seam are increasing,which poses an increasingly serious threat to the safety of coal mine workers and production efficiency.The study and implementation of increasing permeability technology for low permeability coal seam is an effective way to improve the efficiency of gas extraction and solve coal mine gas disaster.Based on damage mechanics,fracture mechanics,fluid mechanics and other basic theories,this paper combined physical and mechanical properties and permeability characteristics of coal seam.The method of combining theoretical analysis,numerical simulation and underground industry experiment was adopted to make thorough research on the principle of liquid CO2 explosion in coal seam,blasting crack propagation law and blasting effect,action mechanism of displacement,dilution,diffusion,permeability improved expansion of gas-injection,etc.Finally,it reached the purpose of increasing coal seam and promoting gas extraction.Major studies include the following aspects.(1)Liquid CO2 blasting technologyThe principle and characteristic of liquid CO2 blasting technology were studied,which provide references for antireflection in coal seam,roadway excavation,coal drop and roof caving in working face,and other work.The TNT equivalent of liquid CO2 blasting was 180g calculated by the computational model of compressed air and steam vessel explosion for container MZL200-1180/50.The change law of pressure time-process of high pressure gaseous CO2 in the main pipe of blasting device in the process of liquid CO2 blasting was monitored and the liquid CO2 blasting process was analyzed.(2)Crack propagation law of coal seam in the process of liquid CO2 blastingCombining stress wave theory,coal rock mechanics,blasting theory,damage mechanics and fracture mechanics theory,liquid CO2 blasting and antireflection mechanism was studied.The guiding role of control holes on blasting fissures was analyzed.And the generation and propagation law of pulverizing area,radial fracture,tangential fracture and circumferential crack on extending cracks in the process of liquid CO2 blasting was studied.(3)Numerical simulation study on liquid CO2 blasting of coal seamEstablishing a finite differential constitutive model of FLAC3D,liquid CO2 blasting in different ground stress,having control holes and having no control holes,different blasting device spacing and different hole spacing were simulated.The results show that when the ground stress is 10 MPa,the effective influencing radius of single hole blasting is 4 m in the direction of vent hole and 6 m in the normal direction of that in the horizontal bedding.When the spacing of blasting device is determined as 5m and the blasthole spacing is determined as 7.5m,multi hole continuous blasting can achieve the ideal blasting effect.The stress and displacement change curves of coal with different distances from blasting position were monitored,the results indicate that the permeability of coal seam shows negative exponential increase relation with the distance from boreholeThe COMSOL numerical model of single hole extraction with different permeability is established.The results show that the effective influencing radius of borehole extraction before and after the blasting is respectively 55 days and 10days.4)Industry experimental study on liquid CO2 blasting in coal seamQitaihe mine area was selected to conduct industry experiment on increasing coal seam permeability by liquid CO2 blasting.The effective influence radius and change law of permeability of each test site were investigated,and gas extraction parameters before and after the blasting were monitored.It can be concluded that the effective influencing radius of liquid CO2 blasting in coal seam decreases linearly with the increase of ground stress.After the blasting,the attenuation coefficient of borehole natural gas flow is reduced to 0.0476d-1 by 0.7253d-1,gas permeability coefficient of coal seam is increased by 17.49-22.76 times,extraction concentration increases by 3.16 times,extraction pure quantity increases by 5.75 times and extraction mixed flow increases by 1.71 times.(5)Technology of liquid CO2 blasting improving permeability and gas injection for promoting gas extractionThe mechanism of permeability improved expansion of gas injection into coal seam was studied.The effective influencing radius of gas-injection extraction was adopted to demonstrate effects of gas-injection extraction in different injection pressures.COMSOL numerical simulation and underground test of gas-injection extraction were conducted.The results indicate that when the effective influencing radius of gas extraction reaches 3.5m,gas extraction in coal seam with low pressure of 0.5 MPa needs 37d,and gas injection extraction in coal seam with high pressure of 3 MPa needs 20d.After the blasting,gas injection extraction with low pressure of 0.5 MP continues for 20d,the effective influencing radius can reach 3.5m.It indicates that liquid CO2 blasting improving permeability and gas injection all have good effects on promoting gas extraction.Synthesizing feasibility,costs and effect of promoting gas extraction,liquid CO2 blasting improving permeability and gas injection for promoting gas extraction was selected as an ideal promoting gas extraction technology scheme,which was applied in 205 working face of Tingnan coal mine.The results show that gas extraction concentration averagely increased by 2.57 times,extraction pure quantity increases by 8.92 times,and gas extraction mixture increased by 3.43 times.Technology of liquid CO2 blasting improving permeability and gas injection for promoting gas extraction in low permeability coal seam has been studied,which has practical significance to reduce blasting cost and to improve antireflective effect and gas extraction efficiency.It also provides references for the implementation and popularization of low-cost permeability improvement of coal seam and promotion of gas extraction technology. |
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