1. School of Mechanics and Civil Engineering,China University of Mining and Technology(Beijing),Beijing　 100083,China; 2. School of Energy and Min- ing Engineering,China University of Mining and Technology(Beijing),Beijing　 100083,China; 3. State Key Laboratory of Coal Resources and Safety Mining, China University of Mining and Technology(Beijing),Beijing　 100083,China; 4. Helmholtz Centre for Environmental Research-UFZ,Leipzig　 04318,Ger- many

In order to study the permeability of deep coal seam with time-dependent effect during the long-term extrac- tion process,the improved porosity and permeability model coupling with gas desorption effect and time-dependent effect was developed by incorporating mean effective stress into the gas migration equation. By using COMSOL Mul- tiphysics,the process of gas migration around the borehole was calculated quantitatively. In combination with the field data,the variation law of gas seepage field with or without time-dependent effect was compared and analyzed,and the law of gas migration in deep coal seam during long-term extraction was simulated and analyzed. The results show that coal seam permeability increases exponentially with gas pressure decreasing. The porosity and permeability simulation results considering time-dependent effect are significantly smaller than those without considering the time-dependent effect model,and as the extraction time increases,the viscoelastic elements make the coal body denser in the creep constitutive. With the increase of extraction time,the time-dependent effect of deep coal seam becomes more obvious, and the difference increases gradually between the simulation results of porosity and permeability considering time-de- pendent effect or not. The time-dependent simulation results are in good agreement with the field data and are more in line with the actual evolutionary characteristics. At the same time,as the distance from the center of the borehole de- creases,the permeability shows an increasing trend,while the pressure shows a decreasing trend. When the extraction time is 1 d,the permeability is large and the gas pressure is small near the center of the borehole. At different extrac- tion times,when the extraction time gradually increases,the permeability gradually increases at the same position,and the gas pressure gradually decreases. When the extraction time increases from 1 d to 30 d,the permeability increases by nearly 1. 4 times near the center of the borehole,the gas pressure decreases by nearly 3. 8 times,and the evolution of permeability and gas pressure tends to balance in the model.

国家重点研发计划资助项目(2016YFC0600704);国家自然科学基金资助项目(51674266);中国矿业大学(北京)越崎杰出学者奖励计划资助项目

ZHANG Lei,ZHOU Hongwei,WANG Xiangyu,et al. Characteristics of deep coalbed gas migration based on the time- dependent effect[J]. Journal of China Coal Society,2019,44(6):1771-1779.