3D visualization of tectonic coal microstructure and quantitative characterization on topological connectivity of pore-fracture networks by Micro-CT

Research on the connectivity characteristics and topological relationships of pore-fracture networks is crucially significant for systematic understanding the microstructure evolution on original and tectonic coals. From the aspects of digitization, visualization, non-destruction, topologization and quantitation, Micro-CT is adopted to characterize the microstructure of original, tectonic and reconstructed coals. The results indicate that the roughly vertical distribution among cleat system in original coal microstructure regularly splits coal matrix into several cubic blocks. Tectonism induces the fundamental transformation on microstructure, associated with well-developed micro-fractures and sporadic pores distributed in spatial scale. With external stress loading, the internal micro-fractures of reconstructed coal sharply decline and gradually evolves into widely-distributed pore connectivity with locally disorderly and dispersed micro-fractures. Following morphological thinning algorithm, skeletonization models were created to demonstrate that the mean tortuosity of original coal has the highest value of 1.2213, followed by reconstructed and tectonic coals of 1.1741 and 1.1205, respectively. Based on skeletonization, the topological "ball-rod" model and its quantitative connectivity parameters reveal that the equivalent structure of pore space for tectonic coal significantly increases, compared to original coal; however, the reconstructed coal after stress loading decreases slightly to 37.75%. Tectonism may facilitate coal microstructure to generate a high coordination number >10, enhancing the connectivity of tectonic coal; nevertheless, the coordination number of reconstructed coal dropped dramatically due to stress loading. Based on the mentioned above, the pore-scale flow simulation of micro-topological equivalent PNMs was conducted for discovering that pressure distributions of tectonic coal in different directions are more concentrated and uniform than original coal while the latter has a non-uniform flow pattern due to its structural anisotropy and heterogeneity. The aforementioned results have guiding significance for revealing the micro-topological connectivity of pore-fracture network in coal, as well as the pore-scale fluid transporting visualization.