Study On Fracturing And Permeability Enhancement Of Water Bearing Coals Under Microwave Thermal Cycling

The low permeability problems of coal seams in China,such as developed pore structure,high gas content,and poor permeability,are the main factors restricting the efficient and safe mining of coal and the utilization of gas resources,and also seriously affect the national development process of low carbon emission reduction.Among them,Intensified gas drainage technology is an indispensable means to increase permeability of coal seams.Taking into account the high efficiency and superiority of microwave heating,this paper proposed a method of microwave cyclic heating to fracture and increase the permeability of coal.Combining physical experiments and theoretical analysis,the thermal gradient effect of the coal during microwave dehydration period was studied.The influence mechanism of microwave on the structure evolution and fracturing and permeability enhancement of water-bearing coal was expalnied,the main results were as follows:(1)The thermal response characteristics of coal under microwave radiation were deeply analyzed.It was found that the temperature rise process of different rank coal samples in the microwave dehydration period could be divided into preheating period,continuous action period and accelerated heating period according to the heating rate,while the rate of water content of coal samples showed a "slow-fast-slow" downward trend.Each stages of the two processes corresponded to each other and the time was consistent,which confirmed the moisture is the main factor affecting the temperature rise of coal under microwave radiation.Meanwhile,the regional distribution of moisture leads to the characteristics of asynchronous and uneven heating in the process of microwave radiation,while the heat transfer modes such as conduction and convection tend to homogenize the temperature field.(2)The thermal driving characteristics of multiphase coal in microwave field was studied.It was found that coal is mainly affected by air pressure and thermal stress in the microwave field.The driving force of air pressure mainly comes from the pressure difference formed by uneven heating and evaporation of water.On the micro level,the impact of water bearing pore wall leads to the opening and expansion of pores,and on the macro level,it drives part of moisture to move outward in the form of liquid water.There are obvious water droplets and water stains on the surface.The temperature gradient formed by uneven heating greatly promotes the development and expansion of fractures,and also provides channel conditions for the migration of water and other substances,forming the phenomenon of interdependence and interaction between fractures and water migration.(3)The evolution law of coal structure under microwave radiation was explored,and it was found that microwave heating could effectively frauture coal,and the increase of water content significantly improved the fracturing effect.From the perspective of the microscopic pore structure,the pore size of the water-bearing coal body increased after microwave radiation and the pore connectivity was enhanced.With the detachment of some minerals and the migration of water,the small pores can be expanded.From the perspective of molecular structure,the microwave thermal effect can lead to the disintegration and removal of molecular groups in coal,in which the changes of hydroxyl and oxygen-containing functional groups lead to the decrease of hydrophilicity of coal and the enhancement of gas-liquid fluidity,and the decrease of fatness and the increase of aromaticity reflect the improvement of coal structure.From the perspective of macroscopic fracture structure,microwave radiation increased the density and fractal dimension of fractures,promoted the development and connection of fracture networks,and the degree of internal damage in coal was aggravated with the increase of radiation time and moisture content.(4)The characteristics of gas storage and transportation in coal under microwave radiation were explored.The effects of microwave radiation time and moisture content on coal desorption capacity,desorption rate and initial effective diffusion coefficient were analyzed through gas desorption experiments.It was found the increase of radiation time and moisture content can contribute to the value of the above indexes,but there was an optimal value for moisture content under a certain radiation time.Based on the principles and advantages of microwave heating technology,an application proposal of microwave synergistic hydraulic measureas was discussed to strengthen extraction,and it is committed to further deepen the practice of improving the permeability of coal by microwave.Based on the above research content,one piece of SCI paper was published during the master’s degree,with an impact factor of 3.42.4 invention patents were applied for,and 1 was authorized.There are 45 figures,5 tables and 122 references in this paper.