Study on the attenuation coefficient of gas flow rate and the variation law of coal seam permeability coefficient in Zhaotong Tunnel boreholes

The permeability coefficient of coal seams and the attenuation coefficient of gas flow rate in boreholes are important basis for the safety assessment of gas tunnels. Their measurement and calculation work usually consume a lot of time due to the complexity of on-site conditions. Therefore, conducting research on the temporal variation of the above two coefficients is of great significance for reducing the duration of on-site measurement. This article takes the Zhaotong Tunnel project of the Yungui section of the Chongqing Kunming high-speed railway as the background to evaluate the difficulty of gas drainage. At the same time, function fitting is performed on the calculated borehole gas flow attenuation coefficient and coal seam permeability coefficient at different times. Research has shown that coal seam permeability is poor in high stress and deep environments, making gas drainage difficult. Therefore, it is necessary to adopt reasonable pressure relief and permeability enhancement techniques to eliminate gas hazards. At the same time, the fitting curve of the attenuation coefficient of drilling gas flow shows that it conforms to the negative exponential attenuation law. The corresponding relationship between the attenuation coefficient of drilling gas flow and time can be fitted by the BoxLucas 1 model. After a certain increase, the attenuation coefficient of drilling gas flow will approach a fixed value. Similarly, the coal seam permeability coefficient is highly sensitive to the value of drilling gas flow rate, and the calculated value of coal seam permeability coefficient also shows a negative exponential decay trend over time, reflecting the possibility of using the BoxLucas 1 model to predict the coal seam permeability coefficient after gas flow rate stabilization to reduce the duration of gas flow measurement.