Study On The Strain In Adsorption/Desorption Of CH₄+N₂ And The Permeability Enhancement By N₂ Injection And Seal In Medium And High-Order Bituminous Coal

In order to investigate the strain characteristics of medium and high-order bituminous coal in adsorption/desorption of CH₄+N₂ at different combinatorial concentrations and the improvement of porosity and permeability of coal samples by N₂ injection and seal,three experiments have been carried out on four samples of medium and high-order bituminous coal which are from Shanxi Province in Carboniferous to Permian in this paper.The first experiment is the strain test of the samples when adsorbing different combinations of CH₄+N₂.Then is the experiment of N₂ injection and seal on the samples which has different saturations of CH₄.Finally,porosity and permeability tests of CH₄-free samples during N₂ injection and seal.The results show that:1)The higher the concentration of CH₄ in CH₄+N₂ gas,the biger the adsorption strain of the sample;2)The relationship between the volume deformation and the amount of gas adsorbed by samples satisfies power function;3)The strain difference in the process of adsorption/desorption is mainly controlled by the basic physical properties of the samples,such as pore-fracture connectivity,porosity,permeability and adsorption capacity;4)At 3.5 MPa pressure,the condition that N₂injection and seal can desorb CH₄ in the sample is that the initial adsorption saturation of CH₄ is higher than a critical value,the saturation of QM sample is 85.08%,FM is70.76%,JM-1 is 74.83%,JM-2 is 52.64%;5)Porosity and permeability characteristics of samples are modified by N₂ injection and seal's positive and negative effects,positive effects include desorption and gas support,negative effects include adsorption and compression,when the positive effect is greater than the negative effect,the porosity and permeability of the samples increase,but decrease in the opposite case.6)In the process of N₂ injection and seal,the permeability experienced three stages:decrease-increase-decrease,in the stage of permeability increase,the positive effect is dominant,while the negative effect is dominant in the stage of permeability decrease.7)At 3.5 MPa pressure,when the N₂ injection and seal's time between 24 and 30 hours,the permeability of the sample reaches its maximum,and the N₂ injection and seal's effection is the best.For CBM development of medium and high-order bituminous coal reservoirs,the research results have guiding significance.