The use of AFM in quantitative analysis of pore characteristics in coal and coal-bearing shale

Quantitative characterization of nanopore structure is of great importance for both accurate assessment of gas reservoir capacity and gas migration behaviors. In this work, atomic force microscopy (AFM) experiments were performed on coal and coal-bearing shale samples, attempting to gain clearer insights into the nanopore characteristics and surface roughness. Scanning electron microscopy (SEM) and low pressure N-2 gas adsorption (LP-N(2)GA) were used for qualitative and quantitative comparative analysis, respectively. SEM results are found in good accordance with AFM surface topographies. The values of R-a are 18.04, 7.24 and 14.17 nm for Coal-1, Coal-2 and shale, respectively. The obtained similar nanopore distribution curves via LP-N(2)GA and AFM indicate that the nanoporosity determined by AFM is reliable. However, the differences between these two methods can not be neglected. The percentage of nanopores with diameter < 4 nm generated from LP-N(2)GA are higher than that rendered from AFM for both coal and shale samples, due to the gas adsorption-induced swelling, but it is reverse for macropore ( > 200 nm). AFM is a high-resolution tool for nanopore characterization, and it is expected to be widely used for quantitative analysis of porous media.