| In the process of coal mining,various accidents occur frequently.Gas disaster,dust disaster,fire,flood and roof accidents of coal mine in our country is still the "five big disasters",in which,the most prominent one is the gas disaster.There are many highly gassy mines in our country and one of the main restraining factors of safe and efficient exploitation of coal mine is the gas problem.With the gradual depletion of shallow coal resources in China,coal mining is gradually converted into deep mining,the gas problem will become more prominent,and the gas content,the gas emission and emission intensity in the deep coal seam will greatly increase,greatly increasing the difficulty of gas disaster governance and the severity of gas disasters.And gas drainage is a fundamental measure of preventing and controlling of gas accident in our country.In terms of combined gas drainage method and drainage technology in the coal seam and goaf,our country has obtained certain experience and to a certain extent,it can alleviate the problem of gas overrun and local gas accumulation in mining area.But because of failing to find out the variation law of gas flow field and the mechanism of gas migration in drilling and goaf of coal seam,the effect of gas drainage in coal seam and goaf is generally poor,and the drainage rate is low.Considering the problems above,on the basis of laboratory experiments and field observations,dominated by theoretical analysis and numerical simulation,through theoretically analyzing the gas emission law,dynamic change rule and gas migration mechanism of coal seam drilling and goaf,this article build the mathematical model of gas migration in stope under specific mining conditions,develop numerical simulation software for gas migration.By simulation analysis of the laws of gas migration and distribution in coal seam and goaf and based on the actual situation,we can propose the technology scheme of gas drainage in coal seam and goaf.Firstly we analyze the origin and emission law of gas in specific mine.By experimental study on gas desorption law of coal sample in Yuhua Coal Mine,We get the important parameter of coal sample of coal face 2108 in Yuhua Coal Mine such as gas adsorption constant,coal porosity,permeability and so on,and have a better understanding of the gas occurrence situation in coal seam and the physical properties of coal.At the same time,we can also get the key data to researching gas emission in coal seam drilling.In addition we consider the gas diffusion process of abandoned coal in goaf as the desorption process of adsorbed gas in coal,the abandoned coal in goaf as loose accumulation and combination of different sizes of coal particles.Through the experimental study on the gas desorption laws of coal of certain quality under different sizes and pressure,we apply the experimental result to the goaf of coal face 2108 in Yuhua Coal Mine,and get the gas desorption law of residual coal of unit quality in goaf and make a qualitative analysis of gas concentration distribution in goaf.And then to explore gas occurrence and characteristics of gas emission of coal seam 2108 of Yu Hua coal mine,we mensurate the original gas pressure of coal and observe the gas emission velocity when natural gas discharge of drilling and the change of the cumulative flow with the observation time in order to provide the basic parameters for numerical simulation of gas emission in borehole and provide experimental data to verify the numerical model of gas emission.On the basis of the laboratory experiment and field observation,combined with dual porosity characteristics of coal,dual porosity and gas seepage theory of coal body is proposed according to the migration mechanism of gas in coal seam.This theory thinks that the model of gas flow in borehole should include the model of gas flow in matrix and fracture and the coupling relationship between them.On the basis of that,we deduce the dual porosity and gas seepage model of borehole in coal seam,independently develope the computing software named “Analysis software of gas flow in borehole COMESS 2.0” of this model with the aid of computer language,and make the numerical simulation of gas emission by means of the software.Through those,we get the curve of gas emission rate and cumulative gas emission with time and compare them with field measured results.The simulation results show that the gas emission rate and the accumulated gas flow are consistent with the autual measured results as a whole.Then by using the self-developed software to simulate the process of gas emission in borehole under different drainage parameters and get the influence law of different drainage parameters on the effective drainage radius of the borehole.When the drainage time is a constant,the higher the negative drainage pressure is,the bigger the effective drainage radius is,and with the increase of negative drainage pressure,its effect on the effective drainage radius decreases.When the drainage pressure is a constant,the longer the drainage time is,the bigger the effective drainage radius is,and with the increase of the drainage time,the increasing rate of the effective drainage radius decreases.At the same time,with the increase of the drainage pressure,the gas pure quantity increase gradually and the influence of variation of negative pressure on the pure quantity gradually weaken.The relationship between the gas pure quantity of the single borehole and the borehole diameter is linear,which shows that the increase of the borehole diameter will enlarge the gas pure quantity and the effective drainage radius.Under the actual conditions,we can consider increasing the borehole diameter as far as possible.According to the field condition and numerical simulation results of working face 2108 in coal seam 2 #,we design the scheme of gas drainage about this working face.In this scheme,the distance between the drainage holes is 6.5m,the negative pressure is 0.55 MPa,the pre-drainage time is 270 days.According to the determined parameters,we have carried out the parameter layout of bedding drainage borehole in the coal seam.Considering the constantly changing of goaf with the mining of working face,we make a research on gas migration law in goaf.Based on the basic laws of fluid,considering the actual goaf conditions,the two dimensional gas concentration field balance equation in the goaf is established,and the gas concentration field mathematical model in goaf is establish combining with the boundary conditions of goaf under specific mining conditionsBased on the mathematical model of gas migration in goaf,considering the actual characteristics of the goaf in the process of mining,we determined the calculation range of the gas concentration field model and the range of numerical discrete model.According to the finite volume method,the equation of gas pressure field and gas concentration field is established and we obtain linear equations about the node pressure and gas concentration.Considering the characteristic of coefficient matrix of linear equation group,we compress the coefficient matrix,and use the Gauss elimination method to solve the equation group.Based on that,the simulation software of gas migration is developed,and is used to simulate the gas concentration field in goaf under special mining conditions.Taking working face 2108 in Yuhua Coal Mine for example,we use the software of gas numerical calculation self-developed to simulate the distribution laws of multi physics field in goaf under actual advance speed,including gas pressure field,airflow velocity,gas emission velocity and gas concentration field.we discuss the commonly used method of gas drainage in goaf and its advantages and disadvantages,Based on the theoretical and numerical results,we propose closed wall intubation way for gas drainage and make a parameter layout of drainage borehole combining with the actual situation of Yuhua Coal Mine.The innovation of this paper is mainly reflected in the following three points:(1)Based on the experimental study in laboratory and the relevant parameters in the field measurement,combined with dual porosity characteristics of coal,dual porosity and gas seepage theory of coal body is proposed and the dual porosity and gas seepage model of borehole in coal seam is established according to the migration mechanism of gas in coal seam;(2)A numerical simulation software named “Analysis software of gas flow in borehole COMESS 2.0” for gas flow in borehole is independently developed.By using this self-developed software to simulate the process of gas emission in borehole under different drainage parameters and the influence law of different drainage parameters on the effective drainage radius of the borehole is obtained;(3)Based on the basic conservation laws of fluid,the two dimensional gas concentration field balance equation in the goaf is established,and the gas concentration field mathematical model in goaf is establish combining with the boundary conditions of goaf.Based on the finite volume method,the gas pressure field and concentration field in goaf are dispersed,and the linear equations for the gas pressure and the concentration of the node are constructed.Based on that,the simulation software of gas migration in goaf is developed,and is used to simulate the multi physical field in goaf under special mining conditions. |
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