| More than 80%of coal mines in China are mining low-permeability coal seams.Gas extraction technology with low gas permeability is still a difficult technical problem for all countries in the world.In view of the characteristics of low gas concentration in the No.2 coal seam of Wangjialing coal mine,rapid decay of borehole gas emission,and low gas permeability of the coal seam,this paper deeply studies the seepage law of gas extraction in low permeability coal seams and actively carried out industrial tests of various extraction techniques.Based on the basic data related to gas in Wangjialing Coal Mine,this paper simulates the seepage law of gas extraction in the direction of drilling and roadway.Simulation results show that the length of the extended sealing section still causes leakage of air in the roadway through the coal wall into the drilling hole.This leakage provides a source for air seepage into drilling hole and reduces the concentration of gas extraction in the drilling hole.How to avoid this leakage is the key to improving the concentration of gas extraction.By continuously measuring the drilling material volume S and the drill desorption index Δh2 of the drilling hole in the No.2 coal seam,and performing regression analysis on the data,combined with the simulation results,it was determined that the reasonable sealing length of gas extraction in the No.2 coal seam.The reasonable sealing length should be 12m.And improve the traditional method of sealing,After the use of solid-liquid coupling wall-type sealing technology,the pure gas volume for single-hole extraction is increased by about 4 to 6 times,and the extraction concentration is increased by nearly 10 times.The increase of the sealing length and the improvement of the sealing process together effectively reduce the air leakage of the roadway by about 78%.Based on the pressure function theory of the pressure distribution law of the formation gas,a second-order polynomial relationship equation between the gas extraction quantity and the negative pressure of the gas extraction is deduced,and the formula for calculating the gas extraction mixture quantity is derived by combining an orifice plate flow meter and derivation.The relationship between the gas concentration and the negative pressure of gas extraction.In the Wangjialing coal mine 20109 working face,the on-site adjustment of the negative pressure caused changes in extracting concentration and the pure gas volume were investigated and analyzed.The experimental results are basically consistent with the theoretical calculation results of the derived formula.The negative pressure increased by 15～16kPa,the extraction concentration increased by about 10%,and the pure gas volume increased by about 2 times.The theoretical analysis of CO2 phase-change blasting technology was carried out to simulate the stress wave propagation law and displacement deformation law after multi-point blasting in coal.The simulation results show that the permeability of 3m away from the blast site after blasting is 5 times the initial permeability.The CO2 phase change blasting technology was used to increase the permeability of coal around boreholes and the effective radius of impact of CO2 blasting was determined to be 3m.In this thesis,the length of the sealing hole is increased,the sealing process is improved,the suction negative pressure is increased,and the CO2 phase change blasting is used to increase the penetration of a series of techniques to Increase the extraction effect.As a result,the net gas extraction capacity of No.2 coal seam in Wangjialing Coal Mine is increased by about 200 times.This paper proposes a set of comprehensive technologies to increase the extraction effect that use CO2 phase change blasting antireflection and solid-liquid coupling wall-type sealing as the primary means of reducing leakage and increasing permeability,which provide a theoretical basis and solution to the gas extraction problems for the low permeability coal seam in China. |
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