Electromagnetic-thermal-hydraulic-mechanica L Coupling And Numerical Simulation Of Coal Reservoir Under Microwave Irradiation

Because of the characteristics of "high storage,low pressure and low permeability" in gas reservoirs in China,Some scholars put forward the technical measures to strengthen gas drainage through the modification of physical structure such as hydraulic fracturing,hydraulic slit,pre-splitting blasting,hydraulic punching,high pressure pulse flushing jet and gas injection displacement,but these measures have some limitations in practical application.Microwave irradiation can increase the temperature and structure of coal,showing the characteristics of favorable gas transport output,so it is a new attempt to use microwave irradiation to strengthen gas drainage.In this paper,the electromagnetic-thermal-hydraulic-mechanical coupling mechanism of coal reservoir under microwave irradiation is discussed by theoretical analysis,parameter testing,numerical simulation.Mainly includes:(1)The microwave heating technology is introduced,and the thermal effect,damage mechanism of coal and the influence mechanism on gas in coal under microwave irradiation are analyzed by means of physical experiments and theoretical analysis.The increase of coal temperature under microwave irradiation is realized in the form of dipole turning polarization loss.Coal contains a certain proportion of pyrite,kaolinite,feldspar mineral components and moisture can enhance its own dielectric response ability.The thermal effect of microwave leads to the increase of gas molecular energy,the shallow depth of gas molecular adsorption potential well,and the damage effect expands the gas flow channel,which is beneficial to the escape and seepage of gas.(2)The electromagnetic-thermal coupling effect and thermodynamic response mechanism of coal body are studied by using the microwave-radiated coal simulation model.The results show that the electromagnetic field re-distribution will be caused by the wave absorbing effect of the dielectric coal,and the electromagnetic field in the cavity and coal body will be distributed in a continuous state.The electromagnetic power loss density is affected by the electromagnetic field distribution in coal.The greater the electric field intensity,the greater the electromagnetic power loss density.The evolution trend of coal body temperature rise is determined by dielectric loss,surface convection heat dissipation,liquid-water phase change heat dissipation,fluid heat transfer and internal heat conduction,in which the overall heat dissipation mainly depends on surface convection heat dissipation and liquid-water phase transformation heat dissipation.Because of the characteristics of coal body selection heating,the high and low temperature domain zoning is obvious,which leads to the inconsistency of liquid water evaporation rate and water vapor migration rate in different positions inside,and the difference of fluid distribution.The uneven distribution of temperature and pressure will cause the relative flow of fluid and promote the deformation and rupture of coal body.(3)By means of numerical simulation of coal irradiated by microwave,the heating condition of coal body under different variables such as microwave frequency,input energy and position of coal sample in resonant cavity was investigated.The results show that the average electric field intensity of coal is the largest and the heat injection effect is the best for coal at 2.45 GHz frequency.Energy input determines the average temperature rise of the coal body.The larger the input power,the more obvious the high and low temperature zone is,and the power controls the uniformity of coal sample temperature field.The electric field distribution of the cavity is more uniform in the mode of turning on the double microwave source than in the one-sided microwave source.The different positions of coal samples will also affect the electromagnetic field distribution,so the coal body should be placed in the center of the bottom of the cavity.(4)The electromagnetic-thermal-hydraulic-mechanical multi-physical fields coupling simulation model of microwave heating coal reservoir is constructed,and the variation law of gas migration and coal permeability during microwave heating is analyzed.It is found that the strain caused by the desorption process of adsorbed gas is the leading factor in the evolution of coal permeability.when using low power microwave to assist the extraction,it is helpful to increase the amount of gas extraction and ensure the durability of gas extraction.The increase of coal seam permeability is the largest at 2.45 GHz,and the effect of microwave radiation is the best.the effect of microwave heat injection on coal seam reconstruction is most prominent under the condition of high gas pressure.