| China is a major producer and consumer of coal resources,and the coal industry is an important industry related to national lifeblood and energy security.Coal still accounts for more than half of China’s primary energy structure.Coal is a strong support for China’s rapid economic development.Coal seam gas content is high,the original coal seam gas extraction is difficult,coal seam pressure relief drainage and coal reservoir fracture reconstruction must be considered in China’s coal mines.Made seam coal seam pressure relief is an effective way to solve the low permeability coal seam extraction,as unloading hydraulic punching holes form anti-reflection effect,numerical simulation research,the basis of the research is less,and the hole shape and seepage characteristics of coal is effective for optimizing drilling parameters,determine borehole radius of extraction and water under the condition of gas extraction from coal standard has important significance.In view of the above problems,this paper takes the shape of hydraulic punching hole of soft coal seam drilling as the research object,and analyzes the force characteristics of drilling hole.On this basis,the Bergmark-Ross equation,which is the best mathematical equation to describe the shape of the discharge body,was combined with the PKN model to construct the trajectory equation of coal and rock particles during hydraulic punching,and was plotted in MATLAB.The shape of the pores is ellipsoid with different three axes.In Xin ’an coal mine,the field observation of hole shape was carried out,and the photos before and after water punching were obtained,which were then drawn in CAD as the section drawing of hydraulic punching,which was found to be nearly consistent with the image presented in MATLAB.Considering the adsorption expansion deformation,the compression deformation of pore pressure and the external in-situ stress of coal,the mathematical model of porosity of gas-bearing coal is established.The dynamic evolution mathematical model of permeability of gas-bearing coal is established based on the Kozeny-Carman equation and the variation of pore volume and skeleton deformation of coal.COMSOL was used to simulate the hydraulic punching process of drilling through strata in Xinan Mine,and the evolution law of permeability changing with external water was simulated under different pore shapes with pumping time of 90 d.The results showed that: With the extension of extraction time,the decreasing area of gas pressure becomes larger and larger.In space,the gas pressure increases gradually with the increase of distance from drilling hole,until it approaches the original gas pressure.Permeability decreases with the increase of water content.According to the stress distribution of Xinan Mine,gas pre-drainage simulation was carried out in COMSOL through drilling hole,and the pressure relief range and extraction radius were compared under the conditions of two kinds of hole morphology.The test showed that: Compared with cylindrical holes,the extraction radius,desorption surface area and effective extraction volume of equivalent ellipsoid holes are 0.83,0.81 and 1.17 times respectively.According to the conclusion of numerical simulation,the layout of boreholes is optimized.The vertical distance between coal stop point and adjacent boreholes in coal seam is no more than 3.8m,and the spacing between adjacent rows is5.0m.It can avoid the blank zone of pumping and contribute to the safe and efficient production of mine.There are 34 figures,8 tables and 82 references. |
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