A Methodology for Characterizing the Multiscale Pores and Fractures in Anthracite and Semi-Anthracite Coals and Its Application in Analysis of the Storage and Permeable Capacity of Coalbed Methane

Abstract

Summary Pores and fractures are important components in coal that provide storage space and migration channels for coalbed methane (CBM). To study the importance of pores and fractures to CBM, a combined technique of focused-ion-beam scanning electron microscope (FIB-SEM), X-ray microcomputed tomography (µ-CT), and 3D visualization technology is applied to study the quantitative characterization of multiscale pores and fractures and show their significance for the storage and permeable capacity of CBM. Two high rank coals, anthracite coal (AC) and semi-anthracite coal (SAC), were collected from the southern Qinshui Basin. The results show that two samples show primarily developed organic pores that are filled with minerals. The developmental morphology of the dissolution pores, the shrinkage-induced pores, and the microfractures have important influences on the pore connectivity. The pore connectivity of the SAC sample is poorer than that of the AC sample, and the development of pores and throats is unbalanced. The throats with smaller radii have larger contributions to the volume of the throats. Both samples are beneficial to the adsorption and storage of CBM, but the permeability of the AC sample is better than that of the SAC sample. The porosity of the AC sample is larger than that of the SAC sample, and the microscale pores and nanoscale pores have a larger contribution to the porosity of the SAC and AC samples, respectively. On the basis of the 2D morphology observation, 3D reconstruction analysis, and comparative research of the extracted data, we found that the pore and fracture network of the AC sample contributes more to the storage and permeable capacity of CBM. The introduction and application of the new methodology of the combined technology can enrich the study of multiscale pores and fractures from the nanometer scale to the micron scale and can provide new suggestions for reservoir evaluation in the coal geology sciences.