Experimental Study On Multi-scale Pore Structure Characterization Of Coal And Its Effect On Gas Migration Characteristics

The microscopic pore structure of coal is the main factor that affects the characteristics of coal seam gas migration characteristics.The difference in adsorption and desorption performance and seepage capacity of coal with different metamorphic degrees is not only related to macroscopic factors such as temperature,pressure,coal composition,and moisture,but more importantly related to its own microscopic pore structure characteristics.Investigating the influence of the microscopic pore structure of coal on the characteristics of gas ad/desorption and seepage characteristics has important theoretical guidance for the comprehensive understanding of the structural characteristics and spatial distribution of pore-fractures in coal,and the understanding of the production and migration process of gas at the micro-scale.The main research results are as follows:Firstly,the determination of basic parameters of coal petrology was carried out as the research basis for the characterization of multi-scale pore-fracture structure.The morphology and development characteristics of the pore-fractue on the surface of different rank coals were observed and identified by FESEM.At the same time,the semi-quantitative statistics of the structural parameters of the pore-fracture of different coal ranks.The results show that the pore-fractue of different coal ranks show obvious heterogeneity in shape,size and distribution.With the increase of coal rank,the distribution range of pore diameter shows a trend of"first decrease and then increase",and area porosity shows a trend of"first increase and then decrease".Secondly,through the micro-CT scanning technology(micro-CT),based on the maximum between-class variance algorithm,the accurate threshold segmentation of the large holes and fissures in the coal is realized.On this basis,a skeletonized model and an equivalent pore network model(PNM)with pore-fracture morphological topological structure were established,and the pore-throat structure parameters,connectivity and development degree of macropores and fractures in coal with different metamorphic degrees were quantitatively analyzed.In addition,with the help of small-angle X-ray scattering technology(SAXS),the structure information of the micropores and transition pores in the coal were effectively obtained,and the pore size distribution,specific surface area and fractal dimension change law of different coal ranks were compared and analyzed.The effective porosity and pore connectivity of different coal ranks have a synergistic effect,and the two satisfy the positive correlation exponential function distribution.The degree of pore-throat development and connectivity of macropores in low-rank coals is higher than that of high-rank coals;the average pore diameter of micropores and transition pores gradually decreases with the increase of coal rank.Then,from a full-scale,non-destructive perspective,with the help of low-field nuclear magnetic resonance technology(LF-NMR),according to the relaxation characteristics of nuclear magnetic T₂ spectrum,the porosity evolution and T₂ distribution characteristics of coal samples with different metamorphism degrees in saturated water and centrifugal water are compared.The physical parameters of total porosity,residual porosity,bound porosity and permeability of coal are calculated,and the change law of porosity and permeability of coal samples with different metamorphic degrees is analyzed.It is found that low-rank coal has more seepage space composed of mesopores and macropores or fractures than high-rank coal.The permeability of low-rank coal is generally higher than that of high-rank coal,and its change trend is basically the same as the porosity.Finally,based on the NMR methane ad/desorption experiment,the NMR response characteristics of methane ad/desorption under different gas injection pressures were studied,and the content of adsorbed and free methane under different gas injection pressures was determined.The Langmuir adsorption equation was used to fit the experimental data,and the methane ad/desorption characteristics of coal samples with different metamorphic degrees were obtained.By extracting the micropore structure parameters and the ad/desorption characteristics index to establish a correlation analysis.It is found that increasing the specific surface area and reducing the fractal dimension of the structure will increase the adsorption capacity of coal for methane.Based on the pore-scale seepage simulation of the reconstructed model,the visualization seepage process of methane in the pore-fracture space was reproduced from the three angles of pressure field,velocity field and velocity streamline.By extracting the characteristic parameters of the macropore structure and the gas seepage characteristic index to establish a correlation analysis,it is obtained that the pore radius is positively correlated with the effective porosity of the coal.When the throat radius is larger,the pore-throat ratio is smaller,the tortuosity is smaller,and the coordination number is larger,the coal permeability will increase,which is conducive to gas seepage and production.There are 72 figures,25 tables and 269 references in the dissertation.