Multi-scale quantitative characterization of 3-D pore-fracture networks in bituminous and anthracite coals using FIB-SEM tomography and X-ray μ-CT

Abstract

To study the three-dimensional (3-D) pore-fracture networks of bituminous (BC) and anthracite (AC) coals, a combination of focused-ion beam-scanning electron microscopy (FIB-SEM) tomography and X-ray computed micro-tomography (X-ray μ-CT) was used to characterize 3-D pore-fracture characteristics at different scales. First, as observed in the SEM images, organic-matter (OM) pores, shrinkage-induced pores and dissolution pores are developed in BC samples with pore sizes ranging from ∼25nm to 600nm, and are independently distributed within OM and partial pores filled with minerals. In contrast, a large number of gas pores are widely developed in the AC sample with a cluster distribution in OM, and the pore widths range from ∼10nm to 2μm. Second, we reconstructed the 3-D pore-fracture networks of the BC and AC samples, and quantitatively characterized the porosity, 3-D pore-throat characteristics and its connectivity using the Pore Network Model (PNM) in Avizo. The results indicate that the pores in the BC sample are poorly connected and isolated from each other, whereas the pores in the AC sample are well connected by the throats. There is a logarithmic correlation between the cumulative throat volume and throat size, indicating that the smaller throats make the main contribution to the throat volume. Moreover, the normalized pore size distribution obtained from FIB-SEM and X-ray μ-CT analysis shows that both BC and AC samples exhibit a three-peak structure, but the nano-scale pores are more developed in the AC sample and its 3-D pore morphologies are more complex than those of the BC sample. Furthermore, the macro-pores and micro-fractures in the two coal samples show complex spatial distribution, interconnectivity and tortuosity. The total resolved porosity is 1.108% for BC samples consisting of 0.279% of FIB-SEM and 0.829% of X-ray μ-CT, and 6.082% for AC sample composed of 4.194% of FIB-SEM and 1.888% of X-ray μ-CT, including the connected and non-connected porosity. These results show that the pore-fracture networks of the AC sample are more conducive to CBM storage and flow ability than those of the BC sample. Therefore, this finding provides an accurate method of evaluating the physical properties of coal reservoirs and assists in understanding the mechanisms of CBM storage and transport.