Gas Migration Law and Precision Extraction in Close Distance Coal Seam Goaf: A Case Study

Gas disasters are more prominent in complex coal conditions, especially for the close distance coal seam mining that the gas control in the upper corner of the return airflow roadway in lower coal seam has seriously restricted the safe production of coal mine. In this study, taking Anshun coal mine as engineering background, the accumulation law of large amounts of gas in the goaf in the low-medium positions of overburden fracturation zone is revealed after considering the desorption of the residual coal in the goaf and the gas emission from the upper adjacent layer through comprehensively research methods including geophysical survey, porous medium seepage theory and numerical simulation. The directional drilling is proposed to control gas, and the influence of drilling layer, extraction negative pressure and horizontal distance of directional drilling on gas migration in goaf is analyzed to optimize the precision extraction scheme. Finally, the optimization gas extraction scheme is adopted on filed applications. The results show that the gas concentration in the upper corner of the return airflow roadway can reduce to less than 0.4% meeting the requirements of safe mining when the extraction negative pressure is 12.5 kPa, the borehole is arranged in the carbonaceous clay rock with the horizontal distance of 10 m from the return airflow roadway and the height of 12 m from the coal seam. The gas extraction scheme is of great theoretical and practical significant for the similar conditions of close distance coal seam safety mining.