The stage evolution characteristics of gas transport during mine gas extraction: Its application in borehole layout for improving gas production

Abstract Diffusion and seepage play a significant role in the mine gas extraction, while their influence degree is dynamic with the change of time or location, showing a notable dynamic stage characteristic. Therefore, it is significant to master the conversion node of gas transport for improving the gas production. However, it is difficult to determine the conversion node and master controlling roles during mine gas extraction due to the lack of judgment index. In this work, a dual-porosity model was constructed to describe the gas transport in coal seam. Then, a transfer coefficient ratio between gas diffusion and gas seepage was used to define as the conversion node. Furthermore, our model was validated by comparing with the previous model, showing that our model can better describe the evolution of gas pressure under different stress conditions. The influence of stress, initial permeability and initial diffusion coefficient on the conversion node were investigated. Results showed that the initial permeability shows the most notable influence on the conversion node, followed by the stress. The initial diffusion coefficient has the relatively complex effect on the conversion node depending on the specific reservoir conditions. Finally, the transfer coefficient ratio was used to determine the best distance of boreholes for improving the gas production. The research results are important for CBM and mine gas extraction.