Quantitative Characterization Of Tectonic Coal Microstructure And Fractal Gas Transport Characteristics

The exploitation of coal reservoir resources in China has been faced with a complex geological environment of “high geostress,high gas pressure,high gas content and low permeability” with coal and gas outburst accidents frequently occurring.Thus,Studies on coal mine safety involving outbursts disaster is still an indispensable topic.The accident cases at home and abroad show that geological structure is an important controlling factor for coal and gas outburst.During the geological movement,the structural evolution of coal reservoir that strongly effected by tectonic factors may lead to tectonic coal formation with low-strength and weakly bonded properties.The macroscopic and microscopic of tectonic coal structure fundamentally determine the outburst risk in the outburst prone area.Based on the theories of surface physical chemistry,adsorption science,fluid mechanics,fractal porous media,reservoir geology,coal mechanics,this paper focused on the multi-scale multidimensional studies of pore morphology,microcrystalline morphology,topological connectivity,fractal geometry in terms of macromolecular,nanometer,micron and millimeter scales.In this case,the normalized evaluation of coal microstructure complexity and fractal transport model were comprehensively established to investigate the macroscopic and microscopic controlling effects of tectonism in outburst prone area on gas occurrence and migration.The above results may reveal the essential difference between tectonic and original coal microstructure.Further discussion focused on the outburst tendency of macro-and micro-of tectonic structure and some implications for outburst mechanism.Main conclusions are drawn as follows:(1)The pores and fracture morphology structural transformation of coal determines the existence form and transport performance of reservoir fluids.The extremely rough and uneven multiscale morphology of coal body was characterized by Photoelectric & Radiation Techniques,reflecting the possible existence of pore constricted configuration in the microstructure that may be not beneficial to fluid migration.For Fluid Penetration Method,although there are different mechanism explanations on hysteresis phenomenon of relevant curves,pore constriction exists extensively in the microstructure.Tectonic coal exhibits more irregular apparent morphology than original coal.Tectonism factors have a greater impact on mesopore and macropore development,leading to more constricted pores,and the reduction of pore throat ratio and tortuosity with structural simplification.Based on percolation theory,the mathematical expression for accessibility evaluation of simplified coal pore network was deduced,which proves that the diversified migration path and gas molecules migration efficiency of coal microstructure.(2)The degree of coalification and tectonic deformation plays a key role in the evolution of coal microcrystalline morphology,controlling the ultramicropore development at the molecular scale.The metamorphism and tectonism facilitate the microscale evolution of macromolecular morphological structure of coal,leading to the microcrystalline structure growth toward the increase of aromatization,graphitization and crystallinity degree.Meanwhile,compared with original coal,tectonic coal has a higher degree of aromatization and ordering in the length and orientation angle of lattice fringes.The densification,stacking,condensation,aromatization of microcrystalline structure unit promotes the evolution and development of ultramicropores.(3)Three dimensional(3D)reconstruction model of tectonic coal microstructure is established through the digitization,visualization and nondestructive method,to obtain topology network structure and its quantitative parameters.In the 3D reconstruction of original coal microstructure,the roughly vertically distributed cleat system divides the coal matrix into cube blocks,while the sporadically distributed microcracks and pore groups generate the decrease of coal matrix scale.Moreover,the fracture space in the reconstructed coal decreases sharply with external stress loading,inducing microfracture with disorganized pore groups.Based on the 3D reconstruction and morphological skeletonization model,the average tortuosity of original coal,tectonic coal and reconstructed coal is 1.2213,1.1205 and 1.1741,respectively.Furthermore,the topologically equivalent microstructure and its quantitative parameters were obtained based on the maximum sphere algorithm.Results show that the equivalent volume,diameter,length and connectivity of tectonic coal pore and throat are greatly increased whereas the coordination number sharply decreased with external stress loading.Ultimately,the flow simulation based on topology equivalent model was conducted.Results show that tectonic coal presents similar flow laws in different directions while the original coal presents different pressure distribution in different directions with the heterogeneity and anisotropy of microstructure.(4)Based on the fractal geometry,the complexity coefficient of coal microstructure and its mathematical fractal model of gas transport were established.In the theoretical framework of fractal geometry,the original coal reservoir is generally identified as a tree-like branching fractal structure composed of large tubular cleat system connecting bifurcated small cleat channels,while the tectonic coal reservoir is generally identified as capillary structure with tectonism.With the fractal characterization of two simplified structures,the geometric properties of original coal and tectonic coal microstructure were determined to obtain the complexity coefficient of microstructure and the mathematical fractal model of gas transport.The upper and lower limits of maximum diameter of fracture channel are 100 ?m and 20 ?m,respectively.Eventually,the reliability of the fractal seepage transport model was verified by the measured permeability data and the fractal permeability fitting curves.(5)The geological tectonism evolution leads to the multiscale essential microstructure difference between the original coal and tectonic coal,which indirectly controls the macroscopic and microscopic gas occurrence and migration characteristics of the reservoir.Due to the influence of multi-stage tectonic geological movement,the macroscopic structure of original reservoir evolved into brittle,soft and dull tectonic particles.According to the topological structural parameters,fractal transport model and multi-scale multidimensional variation,the complexity degree of tectonic coal is lower than original coal,causing the decrease of pore length,the improvement of connectivity,and the reduction of gas migration efficiency.However,the fractal complexity degree of reconstructed coal rebounds,leading to the constricted pore development,the decrease of low coordination number,the increase of tortuosity and pore-throat ratio.The geological structure is the dominant control on gas occurrence in tectonic coal reservoir,verified by the controlling effects of regional geological structure on gas migration in Qinan mine.With the increase of tectonism factors,the fractal complexity coefficient decreased from 1.701 to 1.565,the Protodyakonov's coefficient decreased from 0.7 to 0.2,the pore volume of seepage channel decrease from 0.016 mL/g to 0.004 mL/g.In this case,the difficulty in gas migration decreased while gas transport efficiency increased in the microstructure.The above results may contribute to demonstrate the outburst tendency of tectonic coal reservoir structure.The paper contains 109 Figures,12 Tables,and 227 references.