Temperature of the Core Tube Wall during Coring in Coal Seam: Experiment and Modeling

Abstract

Temperature is the primary factor affecting the law of coal gas desorption. When the core method is used to measure the coal seam gas content (CSGC), the temperature of the coal core sample (CCS) will increase because the heat generated by the core bit cutting and rubbing the coal is transferred to the CCS through the core tube. To solve the above problems, the temperature of the core tube wall during coring at core depths of 10, 20, and 30 m was measured by a self-designed temperature measuring device. The thermodynamic models of the core bit and the core tube during coring were established. The thermal flux of the system at different stages was inverted numerically by the dichotomy method. The reliability of the model was verified by comparing the numerical simulation results with the field measurement results. The main influencing factors during coring were studied by numerical simulations. The results show that the temperature change of the core tube wall goes through four stages: slowly rising, fast rising, slowly rising, and slowly falling, which correspond to the process of pushing the core tube, drilling the CCS, and the early stage and later stage of withdrawing the core tube, respectively. The maximum temperature of the core tube wall appears in the first 5 min of withdrawing the core tube and increases with the increase of core depth. When the core depth is 30 m, the maximum temperature of the core tube wall reaches 105.17 °C. The temperature of the measuring point at the end of drilling the CCS and the maximum temperature during coring linearly increase with the core depth, friction heat generated while pushing the core tube, and coal strength. This study can provide a basis for further research on the dynamic distribution characteristics of temperature in the CCS during coring, which is of profound significance to calculate the gas loss amount and CSGC.

Generation of Cutting Heat and Simulation of Coring Tube Wall Temperature During Coring in Primary Structure Coal Seam

“…At the same time, the average rising temperature of the coring bit was proportional to the square root of the propulsion speed when other conditions were the same. Wang Zhaofeng et al [41][42][43][44][45] found the temperature curve of the tube wall during the whole sampling process was divided into three phases: a relatively stable period in the drilling process, a sharp rise period in the cutting process, a slow increase period and slow cooling period in the withdrawal process by real-time monitored of the temperature of the coring tube wall during coring.…”

Generation of cutting heat and simulation of core tube wall temperature during coring in primary structure coal seam

“…At the same time, the average rising temperature of the core bit was proportional to the square root of the propulsion speed when other conditions were the same. Zhaofeng Wang et al [ [41] , [42] , [43] , [44] , [45] ] found the temperature curve of the tube wall during the whole sampling process was divided into three phases: a relatively stable period in the drilling process, a sharp rise period in the cutting process, a slow increase period and slow cooling period in the withdrawal process by real-time monitored of the temperature of the core tube wall during coring.…”

Low-Temperature Response Characteristics of Coalbed Methane Desorption under High and Low Destruction Degrees

“…The national standard “Method for direct downhole determination of coal seam gas content” (GB/T23250-2009) prefers the coring tube sampling method. The frictional heat generated between the tube wall and the coal wall during coring and the cutting heat generated between the coring tube coring bit and the cutting coal body will cause the temperature of the coal core to rise, resulting in excessive gas loss during the sampling process, and the deduced gas loss will have a large deviation from the actual gas loss, thus causing the inaccurate measured value of coal seam gas content. – Temperature is one of the important factors affecting gas desorption. – Wang and Yue et al – proposed a freezing coring technique based on the temperature drop response characteristics of gas-bearing coal cooling and desorption inhibition, which aims to block the heating of the coal body by the frictional and cutting heat of the coring tube and reduce the temperature of the coal body at the same time, thus reducing the amount of gas loss during the sampling process and improving the accuracy of the coal seam gas content measurements.…”