Study On Changes Of Fracture Characteristics In Deep Coal Seams By Experimental Simulation

The carbon emission reduction of the coal industry directly affects the achievement of China’s carbon peaking and carbon neutrality goals.At present,carbon capture,storage and utilization（CCUS）technology and underground coal gasification（UCG）technology are considered to be effective means to reduce carbon emissions in the coal industry and achieve clean development and utilization of coal seams.Through the research on CO₂ storage and UCG technology,it is found that the size and structure of coal seams have a direct impact on CO₂ injection and migration during CO₂ storage,as well as gasification agent injection and gasification product production during UCG.The larger the volume and number of fractures in the coal seam,the more favorable it is for the fluid movement in the coal seam,which is reflected in the fracture structure of the coal seam.However,there are few studies on coal seam CO₂ storage and UCG fracture structure,and there is a lack of field data,which fails to reveal the change law of coal seam fracture structure in the actual construction process.Therefore,this paper takes the fracture structure change of coal seam under different conditions in the process of CO₂ storage and UCG as the research goal.Firstly,the mineral composition of coal and rock was obtained by X-ray diffraction,and then the mineral distribution and fracture structure characteristics of coal samples were obtained by nanomechanical probe experiment and scanning electron microscope,and the relationship between fracture characteristics and mineral components was analyzed.Finally,the coal heating experiment,CO₂ storage simulation experiment and scanning electron microscope experiment were used to obtain the change of the fracture structure of coal after heating and exposure to CO₂ environment.Through the simulation experiment of saturated CO₂ storage,we observed the obvious expansion,formation and closure of the sample fractures,independent narrow fractures connected with each other to form large fractures after the reaction,new fractures were generated in the non-fractured area in the coal sample,and some fractures shrink or even disappear;the observation of coal heating experiments showed that the fracture structure of coal samples shows a decreasing trend at 0⁸0 ℃,an increasing trend at 80¹50 ℃,and finally a weak downward trend at 150²00 ℃;The stiffness of coal samples is concentrated in the range of 0.2⁰.4GPa,and the Young’s modulus is between 1⁵GPa.Combined with the results of scanning electron microscopy and X-ray diffraction experiments,it is found that the coal fracture structure,morphology and distribution of different mineral components are between a direct relationship exists.In this study,it was found that the mineral composition of coal is closely related to the structural characteristics of fractures.The presence of minerals with high Young’s modulus and stiffness such as quartz/siderite is the main reason for the development of large fractures in coal samples;coal rock will change the fracture structure in the CO₂ environment,causing the fracture porosity and permeability of coal rock to increase,which has a positive impact on coal seam CO₂ storage;the fracture structure of coal and rock is the best at 150 ℃,which is the most suitable temperature for fluid to move in the coal seam.