Two-dimensional image-based nondestructive probing of micro- and nano-scale heterogeneous pore structures and their geological control

Low-rank coals have a unique multiscale microstructure, which affects their gas storage and transport characteristics. This paper proposes a nondestructive characterization method based on computed tomography (CT) scanning and small-angle X-ray scattering (SAXS) for the micro- and nano-scale characterization of the pore-fracture structures in low-rank coals. CT 3D reconstruction and positive-deviation correction of the SAXS data were performed to systematically quantify the geometry, structural parameters, and spatial arrangement of the pore-fracture. The geological control of gas transport in multilevel pore structures was discussed on the basis of the pore size distribution (PSD) and porosity characterization. The results revealed that the pore-fracture in low-rank coals have spatial abundance and orientation. The skeletal structure parameters exhibited intense directionality, which was related to microstructural anisotropy. From the perspective of connectivity, the topological pore-throat parameters decreased with increasing coal metamorphism. SAXS results revealed the existence of pore fractals in the nano-scale pores; the pore fractal dimension (PFD) increased and then decreased with increasing coal metamorphism. The evolution of the specific surface area (SSA) presented a negative correlation with the average pore diameter. In terms of the geological control of gas transport, macroporosity and microporosity were positively correlated with pore connectivity and SSA, respectively. These findings shed new light on the storage and transport behaviors of coalbed methane (CBM) reservoirs.