Research On Pneumatic Punching And Unloading Technology Of Soft Coalbed Methane

With the increasing intensive production and mining depth of coal in China,the gas management work of deep coal resources is also facing a severe test.Liangbei coal mine II 1 coal seam is a soft and low permeability coal seam,through the layer drilling hydraulic punching method of gas control engineering volume,construction time is long,the effect is not ideal,seriously affect the normal succession of coal mine production.This thesis adopts theoretical analysis,laboratory experiments and numerical simulation methods to optimize Liangbei gas pre-pumping,and proposes pneumatic punching unloading pressure and increasing permeability technology for soft and low permeability coal seams,which solves the problems of spraying,collapsing and poor slag discharge in the original combing and punching process,and increases the hole making efficiency of cavity drilling by more than 40%,and verifies the reliability of this technology system through field data,which is of great significance to the mentioning of soft and low permeability coal seams.The reliability of this technology system has been verified by field data,which is of great significance to the treatment of loose and low permeability coal seams.The main conclusions of this thesis are as follows:（1）The laboratory experimental results show that the moisture,ash and volatile matter of coal body of Liangbei coal mine II1 seam are 1.24%,21.25%and 17.24%respectively,which are poor and lean coal with medium to high degree of metamorphism.The geological structure is complex,and the coal body is seriously damaged under the tectonic action.The measured maximum gas pressure of the mine is 2.75 MPa,the maximum gas content is 12.46 m³/t,the initial velocity of gas discharge is 16.9 mm Hg,the solidity coefficient is 0.18,the coal is soft,the adsorption constant a value is 22.39 m³/t,and the adsorption capacity is medium.The test data indicate that this coal seam has a strong risk of protrusion,and the method of unloading and increasing permeability can be adopted to improve the permeability and diffusion capacity of the coal seam and enhance the gas extraction efficiency of the coal seam.（2）Based on the stress-strain theory and Moore-Coulomb strength criterion,the stress distribution and evolution law around the hole after pneumatic cavitation were analyzed.After pneumatic cavitation,the pressure relief range of coal body will increase with the increase of hole radius,and the corresponding model was established and simulated by FLAC^3D.When the cavitation radius increases from 0.4 m to 0.6 m,the effective pressure relief radius of the borehole increases from 2.4 m to 3.2 m,and the corresponding damage volume of the plastic zone increases from 104.64 m³ to220.08 m³,which makes the permeability coefficient of the coal seam within the pressure relief range increase significantly.（3）Based on the simplified full stress-strain curve of the coal body,the concepts of equivalent plastic strain and sudden change coefficient of permeability were introduced,and the permeability evolution model of the gas-bearing coal body in the elastic stage under the consideration of effective stress was constructed,and the equations of coal seam gas inventory,matrix gas diffusion equation and fracture gas seepage equation were constructed,and the gas pressure distribution around the borehole was analyzed by COMSOL numerical simulation software The gas pressure distribution around the borehole and the evolution of permeability were analyzed by COMSOL numerical simulation software,which showed that the effective radius of gas in the borehole increased with the extension of extraction time.（4）Combined with the problems in the field construction,the construction process was improved by combining pneumatic cavitation and hydraulic punching cavitation through the wind-water conversion device,which not only solved the problems of spraying,collapsing and poor slag discharge in the original construction process,but also solved the problem of insufficient power of pneumatic punching cavitation in hard coal seams.At the same time,the effective extraction radius of the drill hole was measured by the flow method,and the drill hole design was optimized based on the measured effective radius,which not only avoided the cavitation blank zone,but also reduced the repeated construction of the drill hole.（5）Pneumatic punching and cavitation was applied in the bottom pumping lane of the 32041 working face,and the efficiency of pneumatic punching and cavitation was improved by more than 40%compared with the original technology.Compared with ordinary drilling,the gas extraction concentration of single hole increased by 5 times and the pure volume of gas extraction increased by nearly 7 times,and all the gas treatment in the test drilling section reached the standard,which not only improved the effect of gas extraction,but also shortened the time cost of gas treatment.The thesis has 48 Figures,13 tables and 100 references.