A Mercury Intrusion Porosimetry Method for Methane Diffusivity and Permeability Evaluation in Coals: A Comparative Analysis

Mercury intrusion porosimetry (MIP) has been utilized for decades to obtain the pore size, pore volume and pore structure of variable porous media including inorganic rocks and organic rock (e.g., shales and coals). Diffusivity and permeability are the two crucial parameters that control gas transport in coals. The main purpose of this work is to derive the CH4 effective gas diffusivity and permeability in different rank coals with vitrinite reflectance of 0.46–2.79% Ro,m by MIP. Furthermore, regular CH4 diffusivity and permeability measurements are conducted to compare with the results of the derived CH4 diffusivity and permeability with MIP data. In this work, CH4 diffusivity and permeability of different rank coals are acquired with established equations, which are basically in accordance with the experimental values. However, the coal rank (maximum vitrinitere flectance, Ro,m) exhibits no significant relation to the effective diffusion coefficient (De) and gas diffusivity (D′). The cementation factor (m values) varies from 2.03 to 2.46, which tends to exhibit a semi-consolidated structure for coals compared with other rocks (e.g., dolomite, limestone, sandstone and red brick). The results show that the cementation factor could be an important factor for gas flow in coals. The correlation of CH4 diffusivity to porosity and permeability of 12 coal samples were explored, and it appears that CH4 diffusivity exhibits an increasing trend with an increase of permeability, and two different exponential relationships respectively exist in diffusivity versus porosity and permeability versus porosity. Therefore, this study could be conducive to gas sequestration or gas production during enhanced coalbed methane (CBM) recovery.