Research On Temperature Variation Characteristics Of Loaded Coal And Migration Mechanism Of Coalbed Methane Under Microwave Radiation

China's coalbed methane reserves are abundant,but most reservoirs have the characteristics of "three lows" such as low gas saturation,low permeability and low pressure.Existing conventional methods for extracting coalbed methane are economically low.Using microwave to radiate coal rock is a new type of enhanced coalbed methane extraction method.The rapidity,integrity and environmental protection of its heating have attracted extensive attention from academia and industry.It is of great significance to discuss the temperature variation law of coal under microwave radiation in natural environment and the migration mechanism of coalbed methane in loaded coal under microwave radiation.Based on multidisciplinary theories such as Electromagnetism,Thermodynamics,Rock Mechanics,Elasticity Mechanics,Seepage Mechanics and Control Science,through mathematical modeling,laboratory experiments,numerical simulations and other methods,the temperature variation characteristics of loaded coal under microwave radiation,the migration mechanism of coalbed methane and engineering application were studied systematically.The main research results are as follows:(1)The triaxial loading experiment system for microwave radiation was developed independently.Simultaneous loading of coal samples by microwave field,stress field and seepage field was realized,and the axial pressure,confining pressure,pore pressure,temperature and gas flow data of coal samples during the experiment can be obtained in real time.The experimental system has the characteristics of simple structure,high safety,good sealing,large pressure range and stable value.(2)The microwave heating experiment under the combined action of stress field and seepage field was carried out,and the temperature variation law of loaded coal under microwave radiation was obtained.The average variation of the coal temperature is directly proportional to the microwave power,and the variation rate of coal temperature decreases with the pore pressure.In the cooling process of coal samples,the temperature variation rate can be maintained for a period of time when the pore pressure is large,and the larger the microwave power,the longer the retention time.(3)Desorption-diffusion experiments and seepage experiments of coalbed methane under microwave radiation were carried out,and the desorption kinetics and permeability variation law of coal radiated by different power microwaves were obtained.During microwave radiation,the average desorption speed of coalbed methane increases with the microwave power,and the effect of temperature on the desorption speed is greater than that of the pore pressure.Affected by pore pressure,the desorption speed changes at the same power are different.During microwave radiation,the maximum permeability of coal samples decreases with pore pressure.When the pore pressure is low,the permeability is greatly affected by microwave.The variation rate of permeability decreases rapidly with increasing pore pressure.During the microwave radiation,the variation trend of permeability is basically consistent with the pore pressure,but lags behind the change of pore pressure.In different pore pressure ranges,temperature has different effects on permeability and there are temperature-sensitive regions of permeability.The permeability increases with the pore pressure,and the increase speed is determined by the microwave power.(4)According to the dynamic law of coal rock desorption under microwave and considering the effect of desorption time,the existence of lag time was confirmed.Based on the control theory,a desorption-diffusion model of coalbed methane under microwave radiation was established by considering the lag time,and the mechanism of desorption-diffusion of coalbed methane under microwave radiation was revealed.Due to the stress field,the basic physical parameters of coal rock change.Stress causes changes in the dielectric parameters of coal rock,which weakens the microwave response capability of coal rock.Thermal parameters change with stress,which improves the energy storage performance of coal rock.Under the influence of microwave energy,coal permeability can rapidly increase following the exponential curve.Because temperature,pore pressure and matrix adsorption performance have different effects on permeability,permeability fluctuates up.Porepressure affects the energy storage characteristics of coal rock,and microwave power determines the energy input speed.(5)The electromagnetic-thermal-fluid-solid coupling model of porous media was established,and the effectiveness of pulsating microwave heat injection to promote coalbed methane production was verified by comparing the variation law of coal permeability under microwave radiation and natural cooling.Using multi-field coupling software for engineering application simulation research,the results show that:in the process of microwave radiation loaded coal,pulsating microwave heat injection method has an important impact on coalbed temperature,gas pressure,seepage velocity,permeability,coalbed methane content and cumulative gas production.As the pulsed microwave cycle heating progresses,the average temperature of the coalbed gradually increases,and the temperature difference decreases continuously.Then,a large amount of gas is desorbed,and the pulsed microwave heating significantly increases the permeability of the coalbed in the heat affected area.The research work enriches the theory and application of microwave heating,and the research conclusions can provide theoretical support and application guidance for the engineering application of microwave radiation to promote coalbed methane production.