Multiscale Morphological and Topological Characterization of Coal Microstructure: Insights into the Intrinsic Structural Difference between Original and Tectonic Coals

Multiscale microstructure evolution of tectonic coal determines CBM storage, development and transport mechanism as well as its effect on coal and gas outburst potential. The multi-scale morphological and topological microstructure characterization of original and tectonic coals were performed via the photoelectric & radiation techniques, fluid penetration method and digital core technique, from nanometer scale to micrometer scale. This study concludes that compared to original coals, tectonism coals have enhanced sorption ability with short equilibrium time and lower structural strength basically. With the increase of metamorphism, the surface morphological features could transform from irregular to regular as the banded pore-fractures evolve into uniformly distributed pores; brittle and ductile deformations increase microstructure connectivity and promote more irregular bending coal body than original coals. Metamorphism could accelerate an evolutionary trend from larger pores to smaller pores whereas mesopore and macropore are more sensitive to tectonism than micropore; meanwhile, micropore volume and surface area in high-ranked coals are significantly influenced with the tectonism than middle-ranked coals; besides, the negligible variation in micropores volume serves as a significant contributor to a rapid rise in surface area. Tectonism restrains the increase of effective migration pathways, i.e., the constricted configuration. Physisorption isotherms indirectly verify the coal has diverse pore shapes, such as cylindrical, conical, slit and constricted pores, etc. 3D visualization of tectonic coal morphologically exhibits more microcracks and sporadically distributed pore-connected groups, completely transforming original coal matrix into smaller blocks that promote gas sorption, diffusion and seepage capacity. In this case, the equivalent topological parameters of tectonic coals are strengthened with more developed space-throat properties and higher coordination number. Through the morphological and topological analyses, tectonic coals are identified as the interconnected microstructure with lower pore-throat ratio and tortuosity than original coals, generating more diversified and accessible migration pathway. In light of the aforementioned results, a conceptual schematic presentation was summarized to illustrate the intrinsic structural difference between original and tectonic coals, which are of great guiding significance to the occurrence mechanism of outburst prediction and risk.