The correlation between crushed coal porosity and permeability under various methane pressure gradients: a case study using Jincheng anthracite

As a greenhouse gas (GHG), methane (CH4) emitted from underground mines contributes 17% to total anthropogenic greenhouse gas emissions. Gas drainage is the main measure to resolve this problem, and knowledge of methane flow in surrounding coal and rocks greatly influences our thinking and handling strategies for reducing CH4 emissions. However, due to the lack of matched testing systems, current studies are generally confined to considering methane flow properties in porous and fractured coal and rocks, whereas methane flow behaviors in crushed coal have not been extensively studied and understood yet. In this study, the relationships between crushed coal porosity and permeability under various methane pressures are investigated using a self‐designed gas permeation testing system, and continuous measurements of methane pressure and flow rate have been conducted. The results show that the permeability properties of crushed coal are closely related to the coal porosity, methane pressure, and axial stress. Darcy flow transfers to slip flow in crushed coal in the latter stage of coal‐mine methane drainage in crushed coal, and the permeability of crushed coal has a magnitude of 10−12–10−11 m2, which is rather larger than that of porous and fractured coals. The pseudo‐threshold pressure gradient (TPG) decreases with the growth of porosity, and it is rather smaller than that of lower permeability reservoirs (LPR). With an increase in methane pressure, the permeability of crushed coal presents a logarithmic growth trend; by contrast, it displays an exponential growth trend with an increase in porosity. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.