Simulation on the temperature evolution law of coal containing gas in the freezing coring process

Freezing coring technology is a kind of coalsampling method,which can significantly improve the measurement accuracy of underground coalbed gas content.Notably,the temperature of coal core directly affects the inhibition effect of gas loss.To investigate the temperature evolution law of coal core during the freezing coring process,a series of physical simulation experiments of coal cooling under different freezing environments were carried out on the self-developed platform of freezing response characteristics of coal containing gas; and then a gas-solid coupling model of heat transfer was established with COMSOL software to predict the temperature field distribution of coal core.The results showed that in the conventional coring,the axial temperature of coal rises with the increase in axial height and time,which can be fitted by Poly2D function,and so does the radial temperature.When the temperature of coring tube wall is 90-150 ℃,the midpoint temperature reaches 46.3-62 ℃ respectively at 30 min,and then the midpoint temperature is close to the tube wall temperature at 60 min.While in the freezing coring,the cryogen in the tube can effectively prevent the heat of the tube wall and ensure the coal core cool down rapidly.The temperature of coal core rapidly cools down in the first 60 min,and subsequently it enters the slow cooling stage.The axial temperature of coal hardly changes,but there is a significant gradient in the radial temperature,which presents a negative exponential decline with the radial distance and time.When the freezing intensity remains constant,the cooling efficiency of coal can be improved with the decrease in the tube wall temperature.When the tube-wall temperature is 90,110,130 and 150 ℃,the temperature of coal midpoint decreases to -27.3,-13.2,2.05 and 16.8 ℃ respectively at 30 min,and the temperature of each point in the coal core almost decreases to the same after 60 min.Moreover,the sample thermal conductivity linearly decreases with the ambient temperature.