Experimental Study On CH₄ Seepage Law By N₂ Displacementin Mudstone Of Carboniferous Permian

Taking the Carboniferous Permian coal measure mudstone of Daping mining in Lu’an mining area as the research object,aiming at the low permeability of CH₄ reservoir,through data investigation,sample collection and laboratory test,this paper studies the geological background and physical properties of coal measure mudstone,and carries out the experiments of single component gas adsorption and permeability response law of coal measure mudstone under different pressure,stress-strain dynamic change and displacement under different N₂ injection pressure,The gas adsorption capacity,permeability change and stress-strain change of coal measure mudstone under different conditions are obtained,the differences of displacement results caused by different N₂injection pressure conditions are analyzed,and the above experimental results are comprehensively analyzed in combination with relevant theories.This paper has guiding significance for increasing permeability,eliminating outburst and development of coal measure mudstone CH₄ reservoir.The research shows that:（1）Through the permeability analysis of Carboniferous Permian coal measure mudstone in Daping mining,it is concluded that under the action of effective stress enhancement,both CH₄ and N₂ permeability show a decreasing trend.The permeability of N₂ with the same effective stress is higher than that of CH₄;Gas injection pressure is negatively correlated with permeability,and permeability is sensitive to the early response of gas injection pressure.At the same gas injection pressure,the permeability of mudstone to N₂ is always higher than that of CH₄;Under the same effective stress,the radial strain is greater than the axial strain under the same conditions.The permeability of mudstone is positively correlated with the strain increment and increases with the increase of strain increment.（2）Through the single component isothermal adsorption and strain experiment of mudstone sample,it is obtained that the mudstone sample adsorbs the same gas,and the strain of the sample increases with the increase of gas injection pressure;The radial strain increment is faster than the axial strain increment;The adsorption mainly occurred in the first and middle stage of the experiment,and was relatively stable in the later stage;The adsorption capacity of mudstone for CH₄ is stronger than that of N₂（3）During N₂ flooding experiment,both axial and radial strains showed a decreasing trend.The axial strain drop is relatively gentle in radial direction.N₂ flooding increases mudstone permeability.When the gas injection displacement pressure is 2mpa,3mpa and4MPa respectively,the permeability increases by 16.21%,44.44%and 68.06%in the same time span.The displacement efficiency increases with the increase of N₂ injection pressure,and the displacement efficiency is 69.79%,83.31%and 85.02%at 2,3 and4MPa gas injection pressure,respectively.With the development of displacement experiment,the percentage of CH₄ at the outlet of mudstone sample decreases gradually,while the percentage of N₂ increases.