| China is rich in coalbed methane(methane)reserves,but the low permeability of coal seams restricts the clean utilization of coal and the large-scale development of coalbed methane,thus affecting the development process of the country to achieve the goal of carbon peaking and carbon neutrality.Changing coal seam structure through artificial disturbance can promote coal seam permeability increase and thus enhance gas extraction.Considering that the thermal effect of microwave irradiation on coal body can cause damage to coal body,which is beneficial to gas transportation,this thesis investigates the graded thermal response and permeability enhancement characteristics of coal body under microwave irradiation with the help of physical experiments and theoretical analysis based on microwave cycle experiments,the following main results were obtained:First,the thermal weight loss characteristics of coal under microwave irradiation were investigated,and it was found that the coal samples showed three temperature change phases in the microwave field: rapid warming,decelerated warming and accelerated warming phases.It also showed two stages of mass loss: rapid weight loss and decelerated weight loss.Temperature rise and weight loss exist in the phase of mutual correspondence;the amount of change in the rate of reduction of bound water inside the coal sample tends to level off from fast during microwave heating,reflecting that the thermal response of moisture in coal under microwave irradiation is first fast and then slow,while the loss of minerals in coal is first slow and then fast,while moisture removal and mineral loss correspond to each other with the process of temperature rise and weight loss rate change,and it is found that the graded thermal response of coal under microwave irradiation is divided into moisture thermal response main control phase and mineral thermal response main control phase.Secondly,the law of graded thermal response of coal body under microwave irradiation is elaborated.In the main control stage of moisture thermal response(temperature range below 200℃),a large amount of moisture is evaporated by heat,which generates gas pressure and causes pore rupture of coal sample,while the air pressure difference pushes moisture migration,causing pore expansion and fissure extension;in the main control stage of mineral thermal response(temperature above200℃),minerals in coal absorb microwave energy and become hot spots.Resulting in the disintegration of mineral removal pores,fracture expansion and penetration of pores under the action of thermal stress;the migration of moisture and minerals leads to the generation of a large number of pores and fractures,while the development of pores and fractures provides a channel for the removal of moisture and loss of minerals.Then,the evolution of the physical structure of coal samples in the process of graded thermal response of coal body under microwave irradiation was discussed.During the graded thermal response stage of coal body under microwave heating,the oxygen-containing functional groups of coal samples tended to decrease,the fat structure was gradually decomposed by heat,the fat side chains were shortened,and the degree of cyclization and aromatization of coal samples increased;the shedding of fat side chains and small molecular structures might be an important reason for the reduction of microporosity,the opening of closed pores and the enhancement of pore connectivity of coal body.At the same time,the thermal effect of coal body under microwave radiation leads to the deepening of the dense degree of carbon molecular structure of coal body and the maturation of coal sample crystal structure toward graphitization,which reduces the gas adsorption capacity.The water in the coal migrates under the effect of microwave radiation,and the minerals are shed and lost,which makes more pores and small cracks form inside the coal body and enhances the inter-pore connectivity inside the coal body;meanwhile,it is found that the increase of total volume and porosity,and the decrease of specific surface area of the coal body are less in the main control stage of mineral thermal response than in the main control stage of hydrothermal response.Microwave irradiation improves the fractal geometric parameters as well as the fractal dimension,which makes the fractal network enriched;under the continuous effect of microwave,the degree of structural damage of the coal body increases continuously,and the accumulated damage of the coal samples is roughly in three stages of development: rapid,gentle and accelerated;reconstructing the pore-fracture structure inside the coal body during the graded thermal response of the coal body under microwave radiation,the isolated pore fractures of SM and YC coal samples are reduced to the original coal after microwave action After the microwave action,the isolated pore fissures of SM and YC coal samples were reduced to about 0.5times of the original coal,and the whole pore fissures were increased to about 2 times of the original coal,and the fissure connectivity of coal body was improved.Finally,the gas adsorption,desorption and percolation characteristics of coal samples in the graded thermal response phase of coal under microwave irradiation were studied.Through gas adsorption experiments,it was found that the oxygen-containing functional groups of coal samples decreased after microwave action,leading to the reduction of gas adsorption capacity of coal;through gas desorption experiments,it was found that the gas binding resistance decreased,gas desorption increased and desorption rate accelerated after microwave irradiation,and the cumulative gas desorption of coal samples all increased in the microwave;Through the permeability experiment and the three-dimensional reconstruction of the pore and fissure structure of coal samples after microwave irradiation,we found that the permeability of coal samples increased and the fluid transport channels were improved after microwave action.The research results obtained in this thesis further improve the theory of thermal response and fracture-permeability characteristics of coal bodies in microwave fields,and lay the theoretical foundation for the engineering application of microwave heat injection to enhance the efficient extraction of gas from low-permeability coal seams.One piece of Core paper was published during the master’s degree,5 invention patents were applied for,and 3 were authorized.There are 55 figures,4 tables and 118 references in this thesis. |
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