Dynamic Response Characteristics Of Coal Core Temperature Change And Gas Desorption During Drilling And Coring

In the process of conventional coring with coring tube,due to the dual role of heat generated by friction between the coring bit and the hole wall,and heat generated by cutting the bit and the coal body at the hole bottom,the heat is transmitted from the bit and the coring tube to the coal core,and the temperature of the coal core increases,which leads to promote gas desorption from the coal core.However,the underground gas desorption amount is used to calculate the gas loss amount in the coring process at present.Due to the large difference between the underground desorption temperature and the coal core temperature in the coring process,the gas desorption speed also varies greatly,resulting in a large error between the calculated gas loss amount and the actual loss amount.Therefore,it is very important to obtain(theoretical research)the evolution characteristics of coal core temperature and gas desorption characteristics during coring.This paper independently developed and designed a set of simulation testing system for thermal effect and gas desorption in coring process,which realized real-time quantitative testing of coal core temperature and gas desorption during coring.A thermo mechanical multi field coupling model of bit coring tube coal body gas in coring process was established,and the temperature change of coal core and the gas desorption law of coal core during coring process under different influencing factors(coring depth,coal core particle size and gas pressure)were numerically simulated,revealing the evolution mechanism of coal core temperature and the time-varying characteristics of coal core gas desorption during coring process.Based on the results of gas desorption rate of coal core in the process of coring affected by many factors,the applicable conditions of conventional coring for determining coal seam gas content are proposed.The main conclusions are as follows:(1)By using the thermal effect and gas desorption simulation test system in the coring process,the temperature variation law of the coring tube during the process of entering the hole bottom is obtained: the temperature of the coring tube and the friction time(coring depth)conform to the power exponential function,which can reach about 110 ℃.In the process of coring,the temperature of the coal core gradually increases,and the central temperature of the coal core can reach more than 60 when coring for ℃100 m.The gas desorption under the influence of coal core temperature during coring is higher than that under constant temperature,and the former is 1.5～2.5 times of the latter in the same desorption time.(2)Based on the theory of heat transfer and material damage model,the thermo mechanical model of core pipe coal body friction and the thermo mechanical model of drill bit coal body cutting were established respectively,and the dynamic variation law of core pipe friction bit cutting thermo mechanical was analyzed numerically.The results show that the temperature of coring pipe changes periodically with the friction of drill pipe rotation;during the rotation of the cutting teeth,the temperature of the cutting teeth gradually increases and heat transfer is conducted,which provides a basic model for studying the thermal effects(heat generation and heat transfer)in the coring process(3)The multi field coupling model of bit coring pipe coal body gas was established.The evolution mechanism of coal core temperature during coring was analyzed numerically,and the time effect characteristics of coal core temperature change and gas desorption during coring were revealed.The error between the measured temperature value and the numerical simulation value of the temperature measuring point of the coring tube is less than 10%.The relative error between the simulation results of gas desorption and the measured results is basically within 10%,which indicates that the mathematical model of gas desorption during coring is practical and reliable.The greater the coring depth is,the higher the coal core temperature rises(2.5～3.5 ℃/10m);The larger the particle size of coal core,the smaller the temperature rise of coal core.The higher the gas pressure is,the higher the coal core temperature rises.At the end of coring,the gas desorption rate of coal core increases linearly with the coring depth.The gas desorption rate of coal core decreases exponentially with the equivalent particle size of coal core.At the end of coring,the gas desorption rate and gas adsorption equilibrium pressure of coal core increase exponentially.(4)The power function method is used to fit and regress the gas desorption speed of coal core in the process of coring,especially in the initial stage of coal core gas desorption,the regression desorption speed is several times of the measured result.Based on the simulation data of gas desorption rate of coal core during coring under the influence of multiple factors,the SVM-PSO model was used to analyze the correlation between the main influencing factors and the gas desorption rate.The gas pressure is positively correlated with the gas desorption rate,but the correlation is the smallest.The equivalent particle size is negatively correlated with the gas desorption rate,but the correlation is the highest.According to the characteristics of the influence of equivalent particle size,coring depth and gas pressure on the gas desorption rate of coal core during coring,the applicable conditions of conventional coring in coal seams are proposed: meeting the relationship between coal core particle size and coring depth Hd->59.62025.0.The dissertation includes 126 figures,22 tables and 166 references.