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 (CSGS), the temperature of the coal core sample (CCS) will increase. The heat generated by the core bit cutting and rubbing the coal during coring is transferred to the CCS through the core tube, resulting in the temperature rising of the CCS. Because the CCS entering the core tube during coring is a dynamic process, the temperature of CCS is difficult to measure. To solve this problem, the temperatures of the core tube wall during coring in the Jiulishan coal mine (JLS), Guhanshan coal mine (GHS) and Zhaogu coal mine (ZG) at the core depth of 20 m were measured by the self-designed temperature measuring device. The thermodynamic models of the core bit and the core tube during coring were established. The reliability of the model was verified by comparing the numerical simulation results with the field measurement results. The verified model was used to predict the temperature changes of the core tube wall during coring in different strength coal seams and different core depths. The results show that the temperature change of the core tube wall was divided into a slowly temperature rising stage Ⅰ, a fast temperature rising stage Ⅱ and a slowly temperature rising and slowly temperature falling stage Ⅲ, which correspond to the process of pushing the core tube, drilling the CCS and early stage and later stage of withdrawing the core tube, respectively. The maximum temperature of the core tube wall appears in the first 3 min of withdrawing the core tube, and increases with the core depth increasing. 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. The temperature heating rate of the core tube is negatively linear, with the coal seam strength during pushing the core tube wall process. However, the temperature heating rate of the core tube wall is positively linear with the coal seam strength during drilling the CCS process. 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.
Highlights
The accurate determination of the coal seam gas content (CSGC) is significant for the safety of coal mines and CBM exploitation [1,2]
(different strengths of coal) during coring at the core depth of m are as shown temperature change of the core tube wall goes through three stages during coring:inI
By observing the boreholes with an endoscope, we found that the holes after drilling in Jiulishan coal mine (JLS) are shown as Figure 9a, while the holes in Guhanshan coal mine (GHS) and Zhaogu coal mine (ZG) are shown as Figure 9b,c, respectively
Summary
The accurate determination of the coal seam gas content (CSGC) is significant for the safety of coal mines and CBM (coalbed methane) exploitation [1,2]. The indirect method has many parameters, resulting in cumulative error and a long measurement period; the direct method is the primary method to measure the CSGC, which is widely applied both in China and the United States [6,7,8] The error of this method comes from the calculation of the gas loss amount [9,10,11,12]. Babak et al [27] systematically studied the influence of bit design parameters on cutting temperature, and concluded that the bit screw angle, drilling location and lithology had a significant impact on the rising of temperature caused by the clog of the drain hole. The study results provide a basis for the study of CCS temperature change law and gas desorption law during coring, so as to further improve the CSGS determination of coring technology
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