Technological Research On Air Injection To Promote Gas Drainage In High Gas Mine

The coal seam gas reserves are abundant in China,and efficient drainage of coal seam gas is of great importance to protect the environment and prevent gas disasters.However,the generally low permeability of coal seams in most mining areas in China means that it is difficult to achieve good results using conventional gas drainage measures.So,there is an urgent need to seek a new method that can efficiently manage coal seam gas.In recent years,the major research trend is to enhance coal seam gas production technique by injecting gas into the coal seam to boost the flow of coal seam gas and increase the permeability of the coal seam.This paper proposes a new method of underground air injection to improve coal seam gas drainage based on the structural characteristics of coal pore and fissure,the occurrence state of methane in coal and the gas transport mechanism,which is of great significance to improve coal seam gas drainage rate and reduce the chance of underground gas accidents in coal mines.This paper adopts a combination of theoretical research,numerical simulation and field test.Changcun coal mine was selected as the test site,using the pneumatic pressurization system test device with independent intellectual property rights to carry out the engineering application test of air injection for coal seam gas displacement at the head of underground heading face in coal mines,to study the changes of gas flow and gas concentration in the pipeline during the test,and to analyze the enhanced gas drainage technology and its test effect of air injection according to the gas content of coal seam and gas pressure parameter changes;The results of numerical simulation are validated in engineering by field tests using the air injection displacement coal seam gas process system.The main research results of this paper are as follows:(1)According to the basic parameters of coal seam gas and coal seam seepage,diffusion theory,adsorption theory of multi-gas and gas equation of state,established the mathematical model of coal seam gas injection and displacement process,and the change law of coal seam gas pressure and gas content parameters after coal seam air injection were simulated by using Comsol Multiphysics software under different gas injection time.The simulation results show that under single drainage conditions,gas injection can induce a significant reduction in coal seam gas content and gas pressure.In the same time,compared with single drainage,gas injection for drainage can reduce coal seam gas pressure by 6.06～44.89%and gas content by5.81～26.98%.(2)Industrial test of no.3 coal seam underground in Changcun coal mine1)The radius of influence of gas injection displacement in coal seam was tested in the field as well as examined,and the radius of influence of gas injection borehole was determined by testing the change of gas flow.The results indicate that the radius of influence of gas injection displacement is increased with the increasing time of gas injection in the borehole,but the increase is limited.The radius of influence of gas injection in the test coal seam determined in the field is 2m under the condition of gas injection pressure of 1MPa;2)Relevant technological parameters were determined on site,after the implementation of gas injection measures at the head of the digging face,the mixed flow of average drainage after gas injection was 7.21 times of that before gas injection;The concentration of gas decreased to 0.44 times of that before gas injection;The average pure drainage flow is 2.86times of that before gas injection;3)20 days after the implementation of gas injection and drainage measures,during this period,the total amount of gas drainaged was 17251.2m~3,which was 2.59 times higher than before the measures were taken;With the same amount of drainage,it takes only 20 days for pure drainage,but with gas injection measures,only 7.71 days are needed to reach;4)After adopting the process measures of air injection and drainage,the coal seam gas content and gas pressure are significantly reduced,compared with single drainage,the gas content is reduced by 31.85%by air injection for drainage,and gas pressure by 43.24%,showing a significant displacement effect,which is macro evidence of gas displacement by air injection;5)The simulation results using Comsol Multiphysics software are basically consistent with the results obtained in the field,which shows that the mathematical model of air-injected coal seam gas established is correct and can be used as a guide for the engineering practice of gas injection for gas displacement in underground coal seams.