Characteristics Of Gas Adsorption-Desorption-Diffusion-Seepage And Response Mechanism Of Petrophysical Properties In Low Permeability Coal During CO₂-ECBM Process

Liuzhuang and Qidong mine are typical low permeability coal reservoirs in the Two Huai mining areas of China.The article take Liuzhuang 13 coal seam and Qidong7 coal seam as objects,conducted the experiments of adsorption-desorption-diffusion-seepage for CO₂ and CH4 based on analyzing basic physical characteristics of coal sample.Combined with low-temperature liquid nitrogen pore analysis of samples before and after experiment,explored the response mechanism of coal pore structure characteristics in CO₂-ECBM process.The above content providing theoretical basis for research the adsorption-desorption-diffusion-seepage characteristics of CO₂-ECBM process in low permeability coal seams.The results show:In the organic macerals of Liuzhuang 13 coal and Qidong 7coal which vitrinite content of 84.84%and 79.68%,inertinite content of 11.74%and15.35%,and chitin content of 3.42%and 4.95%,respectively;The minerals of Liuzhuang 13 coal and Qidong 7 coal are mainly clay minerals,accounting for 95.7%and 97.3%respectively,followed by quartz.Qidong 7 coal contains a small amount of calcite,accounting for 1.1%;The density of micro cracks in the coal sample of Liuzhuang 13 coal is about 1-2/mm,and the width of conjugate cracks is about 10-20μm.The density of micro cracks in Qidong 7 coal is about 5-10/mm,and the types of cracks are mostly static pressure cracks.Both types of coal samples have significantly developed cracks.The results of isothermal adsorption experiments indicate that:（1）in the adsorption-desorption curves of excess adsorption isotherm,both CO₂ and CH₄exhibit desorption"lag effect",and the difference between CO₂ adsorption capacity and desorption capacity is relatively small;（2）The adsorption capacity of CO₂ and CH₄decreases with the increase of temperature and increases with the increase of pressure;（3）The specific surface area of micro-pores,small pores,and large pores in coal is positively correlated with CO₂ adsorption,and significant control effect on CO₂adsorption.The moisture,ash content,and mineral content of coal inhibit CO₂adsorption,while the organic matter content of coal is generally weakly positively correlated with CO₂ adsorption capacity.The results of diffusion numerical simulation experiments showed that:（1）Fick type diffusion was the main type of gas diffusion in study area,followed by transition type diffusion,and the proportion of Knudsen type was lowest;（2）The diffusion coefficients of Fick type diffusion and transition type diffusion decrease with the increase of pressure,and tend to stabilize after the pressure reaches 20MPa,which reduced by 10 times compared with diffusion coefficient of 2MPa;（3）The Knudsen type diffusion coefficient shows a slow linear increase with increasing temperature,and the Knudsen type diffusion coefficient increases by 0.2×10^-9m²/s when temperature increases by 6℃;（4）CO₂ has the advantage over CH₄ in diffusion of micro-pores and small pores during CO₂-ECBM.Seepage experiment results found that:（1）The findings of the experiment are listed below.The N₂ permeability of the Liuzhuang sample ranges from 0.0008m D to0.0012m D,whereas the permeability of Qidong is around 0.0003m D.With an increase in gas injection duration and an expansion of the coal matrix for N₂ adsorption,the permeability steadily decreases;（2）The efficient stress compression of the coal pore fracture structure during sample preparation and testing avoids the visible fracture region,which results in poor permeability;（3）The displacement stages of CO₂ can be divided into three phases.Free nitrogen flows from the end of the position and the permeability diminishes during the phase of free nitrogen.When CO₂ is introduced into the penetration stage,the permeability tends to rise,however when there is no penetration,the permeability test values are frequently low.During the CO₂ steady displacement phase,gas permeability gradually declines;（4）Axial and radial strains are gradually reduced during the initial stage of the N₂ injection;（5）Axial and radial strains are progressively raised during the initial stage of the CO₂ injection;（6）While CO₂ is continuously supplied through the coal body stage,the axial and radial strain exhibit fluctuating changes;（7）During the stable stage of CO₂ displacement,axial and radial strains tend to stabilize;（8）The overall pore volume of the coal significantly rises following the displacement.The increase part of the pore volume is primarily focused on the pore of absorption and filling（aperture<10nm）,whereas the decreased part is mainly concentrated in the diffusion pore of the Fick type and the permeability part（aperture>50nm）;（9）The increased in pore volume ratio surface area is centered mostly in the fill pore region（aperture 10 nm）and is four times greater than it was before the displacement;（10）The CO₂ injection exerts an expansion impact on the adsorption-filled and diffusion pores during the CO₂-ECBM process,whereas the compression effect on the percolation pores results in a reduction in permeability.Figure[43]Table[21]Reference[84]...