| At present,China’s energy forms are tense,and the traditional coal energy is still in an alternative position.However,in coal mining and production activities,various types and forms of disasters have a great impact on coal production,staff life and health and even the economic development of the whole society.Among all kinds of coal mining disasters,coal and gas outburst accidents do great harm and are the biggest hidden danger to property and personnel safety.The permeability of coal seams in China is mostly low,and it is difficult for gas to relieve pressure by itself.Therefore,coal and gas outburst disasters are more likely to occur.At present,the mainstream response is to extract gas in advance by embedding gas boreholes to extract gas in advance,so as to achieve the effect of gas pressure relief.However,in the actual production activities,we often encounter the problems of poor gas pre extraction effect and long extraction time.In order to improve the extraction effect,this paper optimizes and explores the layout of group holes in low permeability coal seams based on COMSOL simulation software.Aiming at the gas drainage technology in coal production activities,this paper explores the following aspects:（1）The fluid solid coupling mathematical model of gas seepage is establishedBased on Darcy’s law,the gas seepage control equation is established through mass conservation equation,Langmuir equation and ideal gas state equation.According to the stress equation,geometric equation and constitutive equation,the governing equation of rock mass deformation is deduced;Combined with the above contents,the mathematical model is established,and finally the fluid solid coupling model of gas migration in coal body is obtained,and the definite solution can be obtained.（2）Numerical simulation of gas drilling in group holesEstablish the simplified model of group hole drilling,that is,the double hole model,and explore the influence of various parameters of gas drilling in the group hole model and the conditions of coal seam on the effective extraction radius.The conclusion reveals that under the group hole mode,the borehole diameter is positively correlated with the effective drainage radius,and its limiting conditions are construction difficulty and hole forming rate.When the borehole spacing is 2m,the drainage negative pressure is 14kpa,and the permeability coefficient is 0.2m~2/（MPa~2·d）,the optimal range of gas borehole diameter is80mm～100mm;There is a negative correlation between the drilling hole spacing and the effective extraction radius,and its limiting conditions are the drilling construction volume and economic investment.When the drilling hole diameter is 94mm,the extraction negative pressure is 14KPa,and the permeability coefficient is 0.2m~2/（MPa~2·d）,the best gas drilling hole spacing is 2m～3m;The negative pressure of pumping production is positively correlated with the effective pumping radius,which is unrestricted,but the benefit of pumping efficiency brought by increasing the negative pressure of pumping production is very limited.Therefore,a large negative pressure should be set within a reasonable range.Under the conditions of drilling hole diameter of 94mm,hole spacing of 2m and permeability coefficient of 0.2m~2/（MPa~2·d）,the best negative pressure of pumping production is 14KPa;The permeability coefficient of coal seam is positively correlated with the effective drainage radius.The permeability coefficient of coal seam has a great impact on gas drainage.Therefore,the technology of increasing permeability should be adopted in low permeability coal seam to ensure the effect of gas drainage.（3）Effect of permeability enhancement technology in lower layer of group hole model on gas migration lawBased on the analysis of in layer antireflection technology and taking hydraulic fracturing antireflection technology as an example,a coal seam model with nonlinear porosity development is established,which is compared with the direct extraction model.The gas migration law of coal seam after pressurization has both similarities and differences from that of original coal seam.The same points are:under a certain drainage time,the closer to the borehole,the lower the gas pressure;At the same position,the gas pressure always decreases with the increase of drainage time.The difference is:at the same extraction time and the same position,the gas pressure of hydraulic fracturing model is always lower than that of direct extraction model;In the hydraulic fracturing model,the gas pressure shows an overall downward trend,and the closer it is to the borehole,the greater the decline rate.In the direct extraction model,due to the low porosity and slow gas flow,it is not enough to fill the low-pressure area immediately.Therefore,the pressure relief effect is obvious only in the area close to the borehole,while the change of gas pressure in the distance around is not obvious.（4）Coupling analysis of multi hole drillingThe single hole model is extended to four hole and nine hole models to represent the drainage of group holes,and the distribution of gas velocity field and pressure under different drainage times are analyzed.It is found that the effect of gas drilling is obvious.Multiple holes can provide greater pressure gradient and produce chain reaction,resulting in the acceleration of gas flow far away from the coal seam,which has a significant impact on the drainage effect,Therefore,in the layout design of group holes,the group holes should not be regarded as a simple superposition of single holes,but the interaction between holes should be considered.（5）Optimal arrangement of group hole drilling and its practical application in a mining faceThe arrangement of group holes is determined by two factors,one is the hole diameter and the other is the arrangement.The arrangement mode can be divided into array arrangement and diamond array arrangement.The difference of the extraction effect of the two arrangement modes is mainly reflected in two aspects:the superposition change of borehole pressure relief range and the effect change of gravity field.Due to the limited thickness of the actual coal seam,the influence of gravity field is small,which is mainly due to the change of extraction effect caused by the superposition mode of pressure relief range between boreholes.Under the condition of the same construction amount and economic investment,The gas drainage effect of diamond array arrangement is slightly better than that of rectangular array arrangement.Combined with the actual situation of the working face of a mine,under the condition of ensuring the same other parameters,the tests are carried out at the hole spacing of 2m,3M and 4m in 7#,8#,9#drilling yards respectively,and the changes of gas pressure are recorded.The results are similar to the numerical simulation curve.Therefore,the simulation has a certain guiding role for engineering practice.The test results show that the gas drainage effect is the best when the hole spacing is 2m.Considering the economic benefits,when the hole spacing is changed from 3m to 2m,the number of holes required is almost doubled,and the increase of drainage effect is limited.Therefore,setting the hole spacing to 3m can take into account the economic benefits,construction volume and gas drainage effect at the same time.Figure[70]table[4]reference[73]... |
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