Study On Gas Injection Flooding Methane Replacement To Displacement Conversion Mechanism And Pressure Field Effect

The paper aims at the gas control technology of coal mine gas injection in the underground coal seam to replace methane emission suppression.From the perspective of safety production guidance,the key technical problems such as coal gas injection gas replacement,methane emission reduction mechanism and residual pressure field effect are adopted.The combination of physical similarity simulation,theoretical analysis and field test verification,in-depth study of the quantitative contribution and dominant factors of displacement and displacement during coal injection,and the mechanism of displacement and displacement under coal injection conditions.The mutual relationship reveals the pressure field distribution during coal injection and pressure relief,explain the mechanism of coal gas and gas outburst by underground gas injection to replace coal seam gas.Provide theoretical support for the large-scale promotion and application of coal seam gas injection to prevent of coal and gas outburst technology.The main research contents and results are as follows:(1)The adsorption isothermal experiments of coal on CH4,He,N2,O2,CO2 and Air were carried out on coal samples of №.2 coal seam of Huatai Coal Mine in the Xingong coalfield.The order of strength and weakness is CO2(59.23 m3/t)>CH4(31.68 m3/t)>N2(25.33 m3/t)>Air(23.62 m3/t)>O2(17.44 m3/t).(2)Simulation experiment device and experimental method for coal seam gas injection.Based on pilot experiments and predecessor physics simulations,large-scale(400×300×300mm)coal seam gas injection simulation experiment device was established.According to the actual situation of the underground coal mine,the simulated experimental conditions were set.According to the experimental requirements,it was determined that "key layer pre-pressed coal sample→ vacuuming→ methane adsorption balance→high pressure free methane pressure relief pre-discharge→ gas injection displacement experiment→natural pressure relief after gas injection end" six key steps of the experiment method.(3)Experimental study on flooding methane of coal seam gas injection.Using coal seam gas injection to flood the methane simulation experiment device,the He/N2/CO2/Air flooding coal seam methane experiment was carried out under different gas injection pressure conditions,and the gas injection pressure,gas injection time and injection gas flow rate were monitored and recorded,coal pore pressure,precipitation gas flow and component concentration data.Through comparative analysis,the variation law of gas composition during the experiment with the gas injection time,the law of gas injection and gas flow,the difference between the volume of methane discharged and the volume of gas consumed by the injection,and the influence of gas injection pressure on the displacement effect are obtained.The response characteristics of the source gas and the injected coal and injecting different gases to flood CH4 mechanism.(4)Research on the transformation process and leading role of methane injection mechanism in coal seam gas injection.According to the simulation results of methane gas flooding in coal seams,it is reflected that the gas injection in the coal seam is a dynamic process.The mechanism of different gas injection periods is different,but there are displacement effects and replacement effects,which are throughout the gas injection process.Both effects have a time-sharing dominant effect and gradually transition to each other.Based on the change of CH4 concentration in the gas outlet(100%-50%-end),the gas injection process was divided into three stages to study the conversion process of CH4 mechanism in coal seam gas flooding.Through the whole gas injection process displacement and replacement ratio and precipitation corresponding methane volume analysis,the leading mechanism of coal gas injection flooding methane is obtained.(5)The pressure distribution during coal injection and the residual pressure recovery law after pressure relief.According to the results of physical simulation experiments and numerical simulations,it is verified by field engineering practice that the pore pressure of coal seam increases rapidly from near to far and from time to time,and gradually reaches the peak value.The order of time is CO2>N2>He.Moreover,the maximum pore pressure is less than the gas injection pressure.During the pressure relief process,the pore pressure gradually decreases from near to near with the extension of the pressure relief time until near atmospheric pressure,and the time required for pressure relief is CO2>N2>He.(6)Underground coal injection to eliminate coal and gas outburst mechanism and its guiding significance to technology.Coal seam injection weak adsorption gas(N2 or Air)can reduce the stress on the coal body,increase the coal seam permeability,promote gas drainage,reduce gas internal energy,and achieve the purpose of eliminating coal and gas outburst.From gas source acquisition,economic input,and safety assurance,coal mine underground injection air is the best for flooding coal seam methane.