Pore network structure characterization based on gas occurrence and migration in coal

Microscopic pore structure of coal seam plays a key role in controlling the characteristics of gas storage and transportation, and the research on the complex pore network microstructure of coal is crucially significant for efficient gas extraction and disaster prevention. From the aspects of pore multiscale, network property, connectivity and selfsimilarity, the fluid intrusion method, photoelectric radiation and digital core techniques are adopted to characterize the morphology, topology, and geometry of pore network structure in coal. The results indicate that the morphology of multiscale pores transforms from irregular exogenous pores to round mineral pores with the pore scale decreases, while the pore size distribution shows a decreasing trend with the micropores dominating pore volume and specific surface area of 56%-85% and 92%-98%, respectively. A 3D reconstruction of multiscale pores in coal reflects the multistage feature and similarity with major micro/nano network framework and local nanopore groups, constituting the gas migration microchannels and main occurrence sites, respectively. Then, pore network connectivity is indirectly reflected via fluid intrusion method, and fractal dimension is demonstrated for the segmented feature of multiscale pores, with fitting coefficient higher than 0.9. Pore network model with topological equivalence and its coordination number are constructed by digital core technology. Based on Taylor polygon microstructure of coal, the structural configuration relationship between multiscale pores mainly in series is explained, and also the possibility of coal sample as fractal geometry is proved. Finally, it is proved that most gas molecules are adsorbed on the multiscale structure of“filling, diffusion and seepage pores”, while micropore filling is the main way of gas storage in coal with adsorption ratio roughly higher than 90%. The filling pores (<1.5 nm) act as “storage tank” of gas molecules in coal, the diffusion pores (1.5-100 nm) serve as “bridge” between the filling space and seepage channels in series, and the seepage pores (>100 nm) act as “gateway” connecting with the outside.