Study On The Mechanism Of Methane Desorption Controlled By Pore Fracture Structure In Tectonic Coal Reservoir Reconstructio

China’s tectonic coal and coalbed methane are rich in storage,especially in southwest China,the tectonic coal reservoir has high gas content,poor permeability of coal seam,uneven coal quality,and various forms of coal seam occurrence,which increases the difficulty of coalbed methane mining and utilization.In view of the low porosity,low permeability and high heterogeneity of tectonic coal,physical and chemical transformation methods can be used for permeability enhancement to effectively improve coalbed methane production and reduce the occurrence of dynamic gas catastrophes.The control of methane adsorption/desorption by complex evolution of pores and fracture structures in the coal reservoir reconstruction process needs to be investigated.In this paper,the original structural coal and tectonic coal of Fa’er Coal Mine are selected as the research object.Through the macroscopic and microscopic description of the coal body,the external morphology,internal structure and physical properties of the coal are understood.Based on the principle of high-pressure mercury injection and adsorption of liquid nitrogen at low temperatures,a comparison and analysis of the pore and fracture structure characteristics of the primary structural coal and tectonic coal was performed.Based on the material composition of tectonic coal mine,the best acid solution was selected by dissolution experiment,and the changes of pore fracture structure and surface functional groups before and after acidification of tectonic coal were analyzed.Through methane adsorption/desorption experiments,the influence of pore fracture structure on methane adsorption/desorption was analyzed by using pore fracture structure parameters,and the mechanism of pore fracture structure controlling methane desorption in coal reservoir reconstruction was comprehensively expounded.The main research results are as follows:（1）Compared with primary structural coal,tectonic coal has a more developed pore-fracture system,loose internal structure,high content of inorganic minerals,and contains a certain amount of ink bottle-like pores and cylindrical pores.（2）The mineral composition of tectonic coal is mainly silicate minerals,a nd the content of kaolinite is high,which seriously affects the gas conductivity of coal.The best acid ratio of tectonic coal is selected by single component and multi component dissolution experiments:6%HCL+6%HF+3%CH₃COOH+2%KCL or 3%HCL+6%HF+6%CH₃COOH+2%KCL.（3）Acidification will change the physical and chemical structure of coal and affect the ability of coal to adsorb/desorb methane.High-pressure mercury injection experiments are used to analyze the pore fracture structure characteristics of coal.This demonstrates that some micropores are produced following the acidification of the tectonic coal,and transition pores in the coal transfer to medium and large pore sizes and porosity increases.The results show that the total pore volume and total specific surface area of tectonic coal after acidification are larger than those before acidification,The proportion of specific surface area of micropores increased by 5.61%,the proportion of micropore volume decreased by 8.06%,the proportion of pore volume and specific surface area of transition pores decreased significantly,the proportion of mesopore and macroporous pore volume increased,and the acidification effect was obvious.Through the analysis of low-temperature liquid nitrogen adsorption experiments,it was found that the number of pores in the 10-100 nm pore size section of the acidified tectonic coal increased,and some closed pores were converted into open pores.The specific performance is that the liquid nitrogen adsorption capacity increases from 3.79m L/g to 10.58m L/g,The total pore volume of tectonic coal increases and the specific surface area decreases.Based on infrared spectroscopy,the aromatic structure,aliphatic hydrocarbon structure,hydroxyl structure and oxygen-containing functional groups of coal were analyzed.The results showed that the content of oxygen-containing functional groups increased by 51.6%after acidification of tectonic coal,the hydroxyl content decreased overall,the side chains of-CH₂ and-CH₃ fell off,the length of aliphatic chain decreased,the degree of branching increased,and the aromaticity increased slightly,indicating that acidification would dissolve some minerals,which enhanced the hydrophilicity of coal and reduced the methane adsorption site.（4）The methane adsorption and desorption properties of tectonic coal before and after acidification were compared and analyzed by methane isothermal adsorption/desorption experiments.The results showed that the adsorption performance of methane decreased after acidification of tectonic coal.From the perspective of diffusion coefficient,adsorption potential,adsorption volume,average pore throat ratio and average tortuosity,the control mechanism of pore fracture structure on methane desorption in acidification transformation was revealed.After acidification,the adsorption volume of methane molecules in tectonic coal decreased,the average pore throat ratio decreased,and the average tortuosity decreased,which promoted the desorption of methane in tectonic coal.