Nie Baisheng, Wang Kedi, Fan Yu, Zhao Junsheng, Zhang Letong, Ju Yiwen, Ge Ze
Beijing Key Laboratory of Precision Mining of Associated Energy, China University of Mining and Technology (Beijing), Beijing 100083, China.
Key Laboratory of Computational Geodynamics, College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China.
J Nanosci Nanotechnol. 2021 Jan 1;21(1):538-546. doi: 10.1166/jnn.2021.18725.
The complexity and multiscale structure of coal pores significantly influence the gas diffusion and seepage characteristics of coal. To apply small angle X-ray scattering (SAXS) to study the coal pore structure parameters within the scale of 1-100 nm in the methane adsorption process, the X-ray window was optimized and a gas adsorption chamber was designed to interface with the small angle X-ray scattering platform. The fractal dimension and porosity of Hami coal samples under different methane pressures were studied using the small angle X-ray scattering platform and adsorption chamber. The surface and nanopore fractal information of the nanopores in coal were distinguished. The variation trends of the pores and surface fractal dimension with time under the same methane pressure were compared. The results indicate that the surface dimension changes from 2.56 to 2.75, and the extremum point may indicate that the primary nanopore structure is crushed by the adsorbed gas after approximately 15 minutes. This work clarifies that the fractal dimension can characterize the changes in nanopores in the process of gas adsorption by using SAXS. According to the fractal characteristics, the adsorption of gas in coal nanopores is summarized as four steps: expansion from adsorbance, deformation, crushing and recombination. The minimum porosity is 0.95% and the extreme value point is 1.47%. This work also shows that decrease in surface energy affect the porosity changes in nano-size pores. This work is of some significance to coalbed methane permeability improvement and gas extraction.
煤孔隙的复杂性和多尺度结构显著影响煤的瓦斯扩散与渗流特性。为应用小角X射线散射（SAXS）研究甲烷吸附过程中1-100nm尺度范围内的煤孔隙结构参数，对X射线窗口进行了优化，并设计了一个气体吸附室与小角X射线散射平台对接。利用小角X射线散射平台和吸附室研究了不同甲烷压力下哈密煤样的分形维数和孔隙率。区分了煤中纳米孔隙的表面和纳米孔隙分形信息。比较了相同甲烷压力下孔隙和表面分形维数随时间的变化趋势。结果表明，表面维数从2.56变化到2.75，极值点可能表明在大约15分钟后，初级纳米孔隙结构被吸附气体压碎。这项工作表明，利用SAXS分形维数可以表征瓦斯吸附过程中纳米孔隙的变化。根据分形特征，将煤纳米孔隙中的瓦斯吸附归纳为四个步骤：吸附质膨胀、变形、压碎和重组。最小孔隙率为0.95%，极值点为1.47%。这项工作还表明，表面能降低会影响纳米尺度孔隙率的变化。这项工作对提高煤层气渗透率和瓦斯抽采具有一定意义。
