The Fine Characterization Of Pore Structure Of Tectonically Deformed Coals From Hancheng Mining Area On Multiscale

Tectonic coals with various deformation types were systematically collected in Hancheng mining area. In combination with present achievements on classification of tectonic coals, the tectonic coal types of each samples were determined properly.Based on the analysis of the original geological data in Hancheng mining area, combined with the observations of coal reservoir and hand specimen, and the microscopic characteristics of tectonic deformation by the scan electron microscope, qualitatively describe the characteristics of deformation of tectonic coals on multiscale. Appling Micro-CT, mercury injection and low-temperature nitrogen experiment methods, combined with box dimension, thermodynamics, FHH fractal theoretical model, quantitatively characterizing the pore structure of seepage pore and adsorption pore of different typical tectonic coals samples (cataclastic coal, granulated coal, pulverized coal, foliated coal and mylonitic coal) on multiscale. On this basis, the revolution mechanism of pore structure of tectonic coals were further discussed, which provides important theoretical supports for assessment of recoverability of coalbed methane in complicated structure area.Results show that, affected by tectonic stress, the deformation characteristics of differenttectonic coals are consistent. Overall, the coal body structure shows that, the stronger tectonic deformation, the smaller coal particle, the lower Coal hardness. Brittle deformation coal has well-developed exogenetic pore and micro-fracture due to brittle failure, however, brittle-ductile deformation coal and ductile deformation coal has corrugation texture and fluxing banding due to the ductile deformation. The deformation characteristics of different tectonic coals affected the pore structure characteristics. Weak brittle deformation make a well-developed breccia pores and microfracture, which resultes in the more macropore. Strong brittle deformation make breccia broken to particles, lead to more particle pores, reducing the percentage of macropore and increasing the the percentage of mesopore. while the ductile rheology generates mylonite, which fills the original pore space and results in the coal rock material rearrangement, the less transition pore and the more micropore. Along with the change of pore size distribution, the pore structure parameters, such as porosity, pore volume, pore specific surface area and median pore diameter, change accordingly.Fractal dimension can characterize the heterogeneity of the pore structure. The more complex pore structure, the higher heterogeneity of pore structure and the higher fractal dimension. The study found that, the granulated coal which has a strong brittle deformation shows a largest fractal dimension by the Micro-CT technology, because of its well-developed breccia pore, particle pore and the most complex pore structure. However, the foliated coal which has more strong deformation shows a smallest fractal dimension. The reason is that the ductile deformation generates mylonite, which fills the macropores and fractures and results in the less scattered macropores and the more nano pores. Micro-CT technology cannot detect the information of nano pores, which cause the appearance of foliated coal having a simple pore structure, which make it a smaller fractal dimension.By contrast, the fractal dimension increases with tectonic deformation strengthening, complex pore structure and heterogeneity in the results of injection and low-temperature nitrogen methods. the fractal dimension can quantitatively reflect the strength of tectonic stress and be used as an indicator of the deformation degree of the nano pore structure.