Structural Response Of Bituminous Coal Under Supercritical Co₂ And Its Influence Mechanism On Mechanical And Permeability Properties

China has abundant reserves of deep coalbed methane resources,but its complex geological structure and low permeability have become one of the main factors limiting the efficient development of coalbed methane.The use of CO₂ injection to enhance deep coalbed methane recovery technology can not only effectively increase the recovery rate of coalbed methane,but also geologically store the greenhouse gas CO₂.This technology is considered to be one of the effective measures to achieve"Carbon Neutrality".At present,CO₂ injected into deep coal seams under high pressure usually occurs in a supercritical state.Compared with the subcritical state,many physical and chemical properties of supercritical CO₂ have changed significantly,and its interaction with coal will be more complicated.The research background of this thesis is to inject CO₂ to enhance deep coalbed methane recovery,and the research object is bituminous coal in the Ordos Basin and Fuxin Basin.This thesis comprehensively uses the interdisciplinary research methods of surface physical chemistry,coal petrology,adsorption science,fractal geometry,fluid mechanics,coal mechanics,etc.,from the macromolecular structure,micro pore structure,mechanical damage characteristics and permeability of coal.Then,a systematic study of the interaction between supercritical CO₂ and bituminous coal has been carried out in the aspects of evolution.The research results reveal the influence mechanism of coal macromolecular structure evolution on pore structure,and clarify the effect mechanism of pore structure change on coal mechanics and seepage characteristics.The main research results and understandings obtained in this thesis are as follows:(1)Supercritical CO₂ saturation destroys the crystal structure of coal,resulting in a decrease in carbon order and crystal structure integrity.After supercritical CO₂saturation,the characteristic peaks of carbonate minerals in the X-ray diffraction pattern of coal samples were significantly weakened,indicating that some carbonate minerals in the coal were eroded during the supercritical CO₂ saturation process.The saturation of supercritical CO₂ promotes the reduction of the diffraction angle and shifts the 002 peak to the left,which indirectly leads to the further increase of the coal-like aromatic layer spacing d₀₀₂.On the other hand,the microcrystalline structure packing degree L_c,the number of aromatic layers N_ave and the graphitization degree g all show a decreasing trend after supercritical CO₂ saturation.In addition,the weakening of the absorption peak of the infrared spectrum of the coal sample after saturation indicates that supercritical CO₂ can extract part of the organic matter in the coal,such as aliphatic hydrocarbons,hydroxyl and other oxygen-containing compounds and some hydrocarbons.(2)Supercritical CO₂ has an obvious effect on the pore distribution and morphological transformation of coal.After supercritical CO₂ saturation,the total pore volume of coal samples shows an increasing trend,indicating that supercritical CO₂ has a significant pore expansion effect on the pore structure of coal.The pore size distribution of coal samples before and after the supercritical CO₂ saturation obtained by mercury intrusion method and low-field nuclear magnetic resonance method is consistent with the change trend obtained by low-temperature nitrogen adsorption experiment.In addition,after supercritical CO₂ saturated,the fractal dimensions of seepage pores and adsorption pores of coal samples are reduced to varying degrees,indicating that supercritical CO₂ has modified the pore surface morphology of coal to make it smoother and more regular.(3)The strength of coal samples deteriorates significantly after CO₂ saturation,and the degree of deterioration depends on the CO₂ phase and saturation time.After supercritical CO₂ saturation,the reduction in peak strength of coal samples is significantly higher than that of subcritical CO₂ saturated coal samples,which is related to the greater adsorption expansion of coal samples caused by supercritical CO₂ and its unique extraction capacity.The destruction of coal samples after subcritical CO₂saturation presents a simple combination of shear failure and tensile failure,while after supercritical CO₂,the destruction of coal samples presents complex multi-faceted failures.In addition,the mechanical strength of coal samples decreased with the increase of the supercritical CO₂ saturation time,but the decrease range decreased with the increase of the saturation time.(4)After supercritical CO₂ injection,the permeability of coal samples decreased significantly,and with the increase of injection pressure,the permeability rebounded.Supercritical CO₂ injection resulted in a signficant decrease in the permeability of the coal sample compared to before injection,which is consistent with the phenomenon observed in the CO₂ injection to strengthen the deep coalbed methane pilot project.When the confining pressure is 12 MPa and 16 MPa,with the increase of CO₂ injection pressure,the permeability has rebounded,and the corresponding rebound points are about 7 MPa and 10 MPa,respectively,which indicates the effective stress and the expansion caused by the adsorption of CO₂ by the coal matrix the effect is competitive.Through numerical simulation,the permeability rebound and recovery phenomenon in the process of gas migration in coal reservoirs is analyzed,and it is found that the rapid decrease of fracture gas pressure is the root cause of permeability rebound,and for deep coal seams,the permeability rebound phenomenon is more obvious.(5)A multi-scale structure evolution model of coal reservoirs under the action of supercritical CO₂ was proposed,and the influence of coal body microcrystalline structure parameters on the evolution of pore structure was analyzed,and the influence mechanism of coal body microstructure response on macro characteristics was revealed.Under the action of supercritical CO₂,the microcrystalline structure ductility L_a,the microcrystalline structure stacking degree L_c and the number of aromatic layers N_ave of the coal body are reduced,resulting in the reduction of the size of the coal sample microcrystalline structure carbon.On the other hand,Fourier infrared spectroscopy experiments show that some functional groups are extracted after the coal sample is saturated,which may lead to the fracture of the amorphous carbon in the coal,thereby destroying the integrity of the crystal structure of the coal.After supercritical CO₂saturation,the voids between the microcrystalline structural units of the coal sample and the molecular barrier of the internal carbon atom network will increase,which leads to an increase in the number and volume of pores in the coal sample to a certain extent.The analysis of the mechanical process of coal crack propagation found that the pores and cracks formed after saturation may become mechanical weak surfaces,accelerating the expansion and communication of the cracks,and ultimately leading to the deterioration of the mechanical properties of the coal sample.The research in this paper has deepened the understanding of the interaction mechanism between supercritical CO₂ and coal,and provided theoretical support for the exploration of CO₂ injection-enhanced coalbed methane recovery projects in deep coal seams in China.During the thesis research period,a total of 7 academic papers were published,of which 5 academic papers were published as the first author(5 were indexed by SCI),and 2 invention patents were authorized.This thesis has 106 pictures,38 tables,and 329 references.