Evolution Of Coal Microstructure And Adsorption Characteristics Under The Action Of ScCO₂-H₂O

The deep coal bed methane reserves are abundant in China,but the low permeability problem of coal seam in China leads to the low efficiency of coal bed methane exploitation.For this reason,many scholars have put forward technical measures to strengthen coal bed methane exploitation,in which carbon dioxide in deep coal reservoir can not only safely store carbon dioxide for a long time,reducing carbon dioxide emissions,but also effectively improve the effect of coal seam gas extraction.However,CO₂ exists in a supercritical state in the deep high-temperature and pressure environment.After injected into the coal seam,a series of physical and chemical reactions will occur with the coal matrix,which will have a certain influence on the microscopic organic structure and pore structure of the coal seam.At the same time,the change of coal structure will affect the adsorption capacity of coal to methane and affect the later gas recovery.So this thesis is based on independent research and development of deep coal and rock environment ScCO₂-H₂O-reaction experiment platform,and the N₂-H₂O and the reaction of coal sample as the control experiment.The effects of CO₂ injection on the microstructure and adsorption capacity of water-bearing coal under deep temperature and pressure were studied with three coal samples of different metamorphic degrees as experimental subjects.By means of physical experiment and theoretical analysis,the physicochemical modification of coal matrix by supercritical CO₂ and the effect of supercritical CO₂ on methane adsorption capacity were discussed.The main results are as follows:Firstly,infrared spectroscopy was used to study the evolution characteristics of the microscopic organic structure of the coal sample after the mixed reaction of ScCO₂-H₂O,and the variation rules of the content of functional groups and structural parameters in the coal were analyzed.The results show that with the increase of reaction time,the total amount of functional groups of the three kinds of coal samples decreases first and then increases.The aromatic condensation degree of middle rank bituminous coal increases gradually,but the length of aliphatic hydrocarbon chain or side chain branch becomes shorter.The variation trend of structural parameters of anthracite decreases first and then increases,and the aromatic condensation degree increases continuously.Secondly,the evolution characteristics of pore structure of coal before and after reaction are revealed.With the increase of the reaction time with ScCO₂-H₂O,the pore volumes of the three kinds of coal samples with different metamorphism degrees all decrease first and then increase,while the pore volumes of the coal samples with N₂-H₂O show a decreasing trend.The effect of ScCO₂-H₂O environment on the pores of anthracite and high-rank bituminous coal is mainly reflected in the micro-pore segment,while the effect of ScCO₂-H₂O environment on the medium-rank bituminous coal is mainly reflected in the micro-pore and small pore.The large pore volume of the three kinds of coal samples increased to varying degrees after the reaction with ScCO₂-H₂O or N₂-H₂O,and the increase of the latter was greater overall.After the reaction of ScCO₂-H₂O with anthracite,the number of medium,large and superlarge pores increased significantly,while the number of superlarge pores only increased in bituminous coal.Furthermore,the evolution law of specific surface area of coal before and after reaction was found.The results show that with the increase of ScCO₂-H₂O reaction time,the specific surface area of coal micropores decreases first and then increases.In the reaction process with N₂-H₂O,the specific surface area of the three kinds of coal samples decreased continuously.Under the two reaction conditions,the specific surface area of the large pores of the three coal samples increased continuously,and the increase of the reaction with ScCO₂-H₂O was greater than that with N₂-H₂O.Finally,the effects of the two reactions on methane adsorption capacity of coal were studied,and the evolution mechanism of methane adsorption capacity of coal was revealed.After the reaction of coal with ScCO₂-H₂O or N₂-H₂O,the methane adsorption capacity increased,but the former enhanced the effect more obviously.During the reaction,the decrease of oxygen-containing functional groups,the increase of aromatic condensation and the increase of large pore volume and specific surface area are the key factors to improve methane adsorption capacity of coal.The research results are helpful to improve the evolution characteristics of coal microstructure and adsorption capacity during CO₂ injection in deep water-bearing coal seams,and provide theoretical support for the engineering application of"CO₂-ECBM".During the master's degree,1 academic paper was published.There are 42 pictures,20 tables and 110 references in this paper.