Effect of maceral composition and coal rank on gas diffusion in Australian coals

Gas diffusion within the coal matrix plays a key role in determining the rate of natural gas depletion and enhanced coal bed methane production via CO2 sequestration (CO2-ECBM) in coal seam gas reservoirs. In this work, we investigated the influence of maceral composition and coal rank on CO2 and CH4 diffusion rates of 18 bituminous and sub-bituminous Australian coals. We obtained measures of the gas diffusion rate and the spread of diffusion times. Gas diffusion rate through coal pores was found to vary over 6 orders of magnitude depending on the coal rank and maceral composition. This diffusion rate was independent of pressure in the range 1-5 bar. It increased substantially with inertinite content of the coal in the lower rank and medium rank coals examined. In the high rank coals, the diffusion rate was less sensitive to maceral composition, but alternatively, this may reflect regional variations in the dependence of diffusion rate with maceral composition. The CO2 diffusion rate was faster than the CH4 diffusion rate. The factor describing the spread of diffusion times generally increased with increasing vitrinite content but for a given coal was similar for both CH4 and CO2. This suggests the gases penetrate the same parts of the coal structure. Based on the experimental data, different synthetic coalbed simulation models were constructed to analyse the impact of CH4 and CO2 diffusion coefficients on ECBM and CO2 sequestration performance. The numerical simulation results showed that CH4 production rate is inversely proportional to the sorption time, if the bulk flow in the cleats does not create any restriction. The results also indicated that CO2 breakthrough time is a function of the CO2 sorption time - if the CO2 adsorption is not fast enough, the injected CO2 will be spread into the seam, resulting in an early breakthrough. (C) 2017 Elsevier B.V. All rights reserved.