Development Characteristics Of Nanopores In Coal And Its Controlling Mechanism On Gas Migration

The accurate description of the nanopores in the coal plays an important role in the evaluation of coalbed methane?CBM?reservoir and gas production capacity.The typical coal samples with different coal ranks were selected as the research object of this paper.The method of accurate determination and quantitative characterization of nanopores in the coal was proposed by using pore structure analysis methods including low-temperature gas adsorption?LTGA?,mercury intrusion porosimetry?MIP?,high-resolution transmission electron microscopy?HRTEM?,high-resolution scanning electron microscopy?HRSEM?,and low-field nuclear magnetic resonance?LF-NMR?.Then,the scale,structure,distribution and connectivity of nanopores in different coal samples were systematically studied.Based on the experiments of permeability and isothermal gas adsorption tests of different coal samples,the mechanism of gas migration in the nanopores,the control mechanism of nanopores on migration and adsorption were discussed.The main conclusions were as follows.Based on experiments of LTGA and MIP,the method of accurate determination and characterization of nanopores in the coal was proposed by comparing the conditions of use,analysis range and fitting error of different adsorption gas and analytical models.The pore development scale and pore size distribution?PSD?of 0.3-1.5 nm,1.5-30 nm and 30-100 nm can be accurately determined by using CO₂?273 K?adsorption with NLDFT model,Ar?87 K?adsorption with QSDFT model,and high-pressure MIP test with compression correction.The morphology,spatial distribution and connectivity of micropores,mesopores and large-nanopores in the coal were deeply analyzed by using HRTEM and HRSEM.The quantitative analysis of the nanopores in the coal samples from the Chengzhuang?CZ?coal mine,Wuyang?WY?coal mine,and Yujialiang?YJL?coal mine were carried out by the methods mentioned above.Their total volume of nanopores was 0.092 cm³·g^-1,0.058 cm³·g^-1,and 0.098 cm³·g^-1 respectively.And the total specific surface area was 305.46 m²·g^-1,179.07 m²·g^-1 and 227.19 m²·g^-1respectively.It was found that the development scale of nanopores in different coals was quite different,but the pore structure and PSD were similar.The pore volume proportion of micropores,mesopores and large-nanopores was 63.69%-97.04%,1.54%-23.81%and 1.42%-12.50%respectively.The specific surface area of nanopores was mainly from micropores,the contribution proportion was 99.85%,99.8%,92.83%respectively.Micropores were the absolute space for gas adsorption,which contributed most of the surface area and volume of the coal.HRTEM images of the coal showed that the micropores were mainly from the chemical pores between the aromatic layers in the large molecular structure unit of the coal.Micropores were scattered in the coal matrix,and the pore morphology was mainly slit-pore.The results of the statistics showed that the average porosity between the aromatic layers was from 40.78%to 50.57%and the pore size was distributed in the range of 0.15nm to 0.80 nm.The difference in pore development between different coal samples depended on the length,morphology and regularity of the aromatic layer units.Mesopores and nanopores with a diameter greater than 25 nm can be observed by HRSEM.Their morphology was cylindrical pores or plate-like micro-fissures,and its distribution in the coal had remarkable inhomogeneity.The cylindrical pores were usually developed in groups or clusters,and their connectivity was poor.The elongation of micro-fissures was generally large,and it had better connectivity.Some fissures were filled with minerals,but organic/inorganic interfacial fissures often were developed between the mineral vein and the coal matrix.Nanopores and micro-fissures in the coal were necessary channels for gas migration.Their development density and connectivity in the coal had an impact on the effective gas migration.The T₂ spectra of dry,saturated and centrifugal coal samples were analyzed by LF-NMR,and the occurrence and migration characteristics of water in nanopores were studied.In the coal samples from CZ,WY and YJL,the average proportion of water in nanopores was 98.32%,84.84%and 60.93%respectively.After centrifugation,most of the water in the fissures and micro-fissures was migrated.In the nanopores,although only a little water in the connected mesopores and large-nanopores migrated,their contribution to water seepage was large,with the average proportions of 90.23%,67.90%and 48.89%respectively.Comparing the gas-measured permeability analysis results and the development scale of pores with different scales,it was found that the gas migration scale in permeability testing was not only related to the development scale of pores and fissures,but also to the internal or interconnectivity of the nanopores,micro-fissures and fissures.Gas can only migrate effectively in a few large-scale,well-connected nanopores and fissures,and the larger the scale of nanopores,the higher the contribution proportion.The development and distribution of nanopores in the coal showed that micropores were the absolute space of gas adsorption and also the space of gas desorption and initial migration.The mechanisms of gas migration in the nanopores were surface diffusion,Knudsen diffusion,transitional flow and slip flow.The change of pressure and pore structure were the main control factors of gas migration form and scale in the nanopores.The gas migration in the micropores was mainly surface diffusion,and the contribution was more than 96.93%.The gas migration scale decreased exponentially with the increased tortuosity,and increased linearly with the increased porosity.The form and scale of gas migration in mesopores and large-nanopores were changed relative to micropores.When the gas pressure was less than 1 MPa,Knudsen diffusion dominated the gas migration,and the contribution proportion was from 97.33%to99.90%.In the high pressure stage,the slip-flow was the principal gas migration,the contribution proportion was from 64.55%to 99.86%,followed by surface diffusion with the contribution ranging from 0.14%to 35.45%.There was no Knudsen diffusion in this stage.Mesopores and nanopores were the necessary channels for gas migration in micropores after releasing,and their development degree and connectivity determined the gas migration form and scale in the micropores.The isothermal adsorption experiments and pore structure analysis showed that the total specific surface area of micropores was positively correlated with the adsorption capacity of coal samples.The scale and velocity of gas migration was not only affected by the development scale,structure and distribution of nanopores,but also was closely related to the internal or interconnectivity of nanopores,micro-fissures and fissures.The velocity and scale of gas migration was large in the coal reservoirs with good connectivity and the development of nanopores.After the adsorption and desorption of the coal samples,the deformation of the nanopores structure was anisotropic.In the CZ coal samples,the total volume and specific surface area of nanopores increased.In the WY coal samples,the total pore volume decreased while the total specific surface area increased.In the YJL coal samples,the total volume and specific surface area reduced.The adsorption and desorption of gas would cause the change of the pores and fissures structure of the coal,which was controlled by the two mechanisms of coordinated integral deformation and local severe deformation.At the same time,the change of pores and fissures structure would result in the change of gas migration form,velocity and scale.