On the evolution mechanism of permeability during gas drainage: Insights from deformation field, gas pressure field and temperature field

Permeability is an important factor affecting efficient gas drainage, ensuring coal mine process safety, ecological environment protection and clean energy capture. We develop a gas drainage physical simulation device with multi-physical coupling to study the dynamic response mechanism of permeability during gas drainage and explore the basic relationship between the interaction between boreholes and permeability during parallel borehole drainage. By installing 48 pressure sensors, 14 temperature sensors, and 9 displacement sensors in a test box, gas drainage tests of parallel boreholes with spacings of 250, 504, and 784 mm were carried out. The results show that during gas drainage in parallel boreholes, the dynamic evolution law of permeability in different spatial positions of the coal reservoir has apparent differences. The permeability near the borehole area exhibits a fast decline rate and a large recovery amplitude, and vice versa. The gas pressure, coal deformation, and coal temperature influence the periodical permeability evolution. In the early stage of drainage, gas pressure and coal temperature play a leading role in reducing permeability. In the middle and late stages, coal deformation slowly increases permeability. When the borehole spacing was 250 mm, the permeability decay rate and rebound rate were high with interaction between the boreholes; a drainage superposition area makes the permeability spatial variation difference between the boreholes significant. A correlation is observed between borehole spacing, gas migration rate, and permeability. The study is expected to have a substantial guiding significance for efficient gas drainage, decreasing greenhouse gas emissions, and improving coal mine process safety.(c) 2022 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights reserved.