Quantitative analysis of coal nanopore characteristics using atomic force microscopy

Abstract

Accurate characterization of nanopore structure within coal is of great significance for understanding the mechanism of coalbed methane transportation and accumulation. In order to quantitatively analyze the nanopore characteristics and surface roughness of coal, atomic force microscopy (AFM) experiments were conducted on nine coal samples with different ranks from the Qinshui Basin in Shanxi Province, North Chia in this study. The visible surface three-dimensional topographies of coal were obtained. The nanopore parameters including pore number, mean pore size, areal porosity and form factor were acquired through AFM image processing on the scanning area of 1 μm × 1 μm. Comparison of nanopore distribution determined by AFM and low-pressure nitrogen gas adsorption (LP-N2GA) has also been made to validate the reliability of AFM. Based on the experimental data, relationships between these nanopore parameters and the degree of coalification (indicated by vitrinite reflectance, Ro = 1.19%–3.15%) were studied. The results show that the pore number and areal porosity of studied samples are 1716–6885 and 0.91%–10.47%, respectively, with the mean pore size of 8.75–24.76 nm. The linearly positive correlation between the form factor and Ro suggests the enhancement of coal maturity can make the nanopore shape within coal tend to be more regular. As a response, the values of Ra and Rq demonstrate a similar nonlinear decreasing trend with the increase of Ro. Besides, dramatic changes of nanopore distribution have been observed during coalification jump, resulting in the shift to smaller nanopores. Comparison of nanopore distribution between AFM and LP-N2GA was made, indicating that AFM result is closer to the true value than LP-N2GA. This study enables us to better understand the surface characteristic of coal at nanoscale.