| Coal mine gas disaster is very serious in China,especially in the southwest region.More than 80 % of the coal mines in Guizhou are coal and gas outburst mines or high gas mines,which seriously restricts the sustainable and safe development of the coal industry.From a large number of outburst accident cases in China in recent years,the occurrence of outburst accidents is mostly tectonic coal.Compared with raw coal,the physical and chemical structure of this kind of tectonic coal has essential changes compared with raw coal,which significantly improves the gas adsorption and desorption capacity.As one of the important clean energy,coalbed methane has become an unconventional natural gas resource in China as an independent mineral,and it is also an important associated energy in coal development.In this paper,the primary coal and tectonic coal in three different mining areas are selected as the research object.Based on the theoretical guidance of coal mechanics,adsorption and molecular dynamics,the microscopic model of pore structure of tectonic coal is constructed by combining theoretical analysis,physical experiment and numerical simulation.Through sample testing and other methods,from the aspects of adsorption,desorption,sealing,connectivity,pore fracture shape and size,the differences between primary coal and tectonic coal in structure are compared and analyzed.A theoretical model was established to analyze the occurrence characteristics of pore-fracture structure.Through the reservoir reconstruction experiment,the evolution characteristics of the pore fracture structure during the reconstruction process are analyzed,so as to study the evolution process of the pore fracture structure from a dynamic perspective.Based on fractal theory,multifractal theory and molecular simulation,the complexity of tectonic coal is analyzed,and the transformation mechanism of pore structure of tectonic coal is proposed.The main results are as follows :(1)Tectonic action can change the macromolecular structure of coal and affect the chemical structure of coal.Through FT-IR and XRD experimental analysis,the results show that the fat structure,-OH and-NH content of tectonic coal will decrease accordingly,and-O-will also increase.The structure significantly improves the aromatization index of coal and promotes the transformation of non-aromatic compounds into aromatic compounds;Through the analysis of low temperature liquid nitrogen and high pressure mercury injection experiments,the results show that the tectonic coal is mainly composed of micropores and small pores,and the pore volume and pore specific surface area are larger than those of the primary coal.It shows that tectonic action will promote the development of pore structure,and micropore and transition pore structure make coal samples have better gas adsorption and flow capacity.The fractal dimension of tectonic coal is larger than that of raw coal,indicating that the pore structure of tectonic coal is more complex than that of primary coal.(2)The pore distribution characteristics,chemical structure properties,fractal dimension and porosity of coal before and after acidification are revealed.The overall transformation mechanism of acidification transformation on the pore structure of coal is pore-increasing effect.It can be seen from the XRD spectrum of the acidified coal sample that the acid solution neutralizes the carbonate in the coal sample.The pore volume and pore specific surface of coal samples after acidizing transformation are larger than those before transformation.The fractal dimension is closer to 3,indicating that the acidification transformation makes the pore structure and surface pore fissure more complex.(3)Through the injection pressure modification experiment based on nuclear magnetic resonance technology,the pore structure of coal samples was modified by injection pressure.The results showed that the porosity of coal samples increased from 18.91 to 23.03 after injection pressure of 5 Mpa.The modification effect was mainly aimed at the micropore structure in coal samples.The number of micropores in coal samples was significantly increased,and the proportion of micropores increased from 23.52 % to 40.69 %.(4)Through multifractal analysis,it is found that the Hausdorff dimension of tectonic coal is slightly lower than that of primary coal,and the Hausdorff dimension of tectonic coal after transformation is also lower than that of tectonic coal before transformation,indicating that tectonic action and reservoir transformation will increase the local density of coal pore size distribution and promote the uniform distribution of pore size.After the transformation,the Δf value of Wenjiaba tectonic coal is the largest,indicating that the pore size distribution in the coal sample is more uniform,which further proves that the transformation experiment can affect the adsorption and desorption capacity of coal samples by changing the distribution of pore size structure.(5)Using MS molecular simulation software,a coal reservoir reconstruction model was constructed.By changing the size of slit pores to simulate the amount of methane adsorption and desorption,it was found that the amount of free space gas increased with the increase of slit pores.The pores are getting larger and larger,but the adsorption capacity of coal reservoirs is getting weaker and weaker,so that more methane gas can be desorbed to achieve the reconstruction effect.In this paper,the differences between primary coal and tectonic coal are systematically compared from the pore structure,adsorption capacity,pore volume and pore specific surface area of coal.The pore structure of tectonic coal is effectively transformed by acidification transformation and pressure injection transformation,and the changes of microscopic pore structure of tectonic coal before and after transformation are comprehensively analyzed,which provides a theoretical basis for coalbed methane mining,coal and gas outburst prevention and control. |
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