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

Author:
Alireza Keshavarz  Richard Sakurovs  Mihaela Grigore  Mohammad Sayyafzadeh  


Journal:
International Journal of Coal Geology


Issue Date:
2017


Abstract(summary):

Abstract 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 CO 2 sequestration (CO 2 -ECBM) in coal seam gas reservoirs. In this work, we investigated the influence of maceral composition and coal rank on CO 2 and CH 4 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 CO 2 diffusion rate was faster than the CH 4 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 CH 4 and CO 2 . 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 CH 4 and CO 2 diffusion coefficients on ECBM and CO 2 sequestration performance. The numerical simulation results showed that CH 4 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 CO 2 breakthrough time is a function of the CO 2 sorption time - if the CO 2 adsorption is not fast enough, the injected CO 2 will be spread into the seam, resulting in an early breakthrough. Highlights • Influence of coal type and rank on gas diffusion rate in a wide range of Australian coals • Six orders of magnitude variation in gas diffusion rate in the tested coal samples • The higher the inertinite content, the higher the diffusion rate • The same dispersion parameter for both CO 2 and CH 4 gases in the same coal


Page:
65-65


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