Understanding the role of the co- and counter-diffusive properties of CO_2 and CH_4 in coal has important implications for enhanced coalbed methane recovery (ECBM) and CO_2 sequestration in coals and for related issues of gas outbursts during mining. This study addresses how coal-gas interactions affect CO_2 injectivity, in particular the roles of coal deformation, gas flow, CH_4-CO_2 counter-diffusion and gas absorption/desorption on the evolution of transport and mechanical properties of fractured coals, and therefore on ECBM recovery. A fully coupled coal deformation, gas flow, CH_4-CO_2 counter-diffusion and gas absorption/desorption finite element (FE) model was developed to investigate the combined net effects on evolutions of CO_2 injection related parameters. The FE model was successfully applied to match the experimental data; and a field scale model was constructed to quantify CO_2 injection rate and other transport parameters for ECBM under in-situ conditions. Model results indicated that (1) Coal rank has a converse influence on the CO_2 injection performance, lower coal rank reservoir could be more suitable to carry out CO_2-ECBM technology; (2) Initial permeability has positive impact on the performance of CO_2 replacing methane. This finding also has an important indication for the implementation of this technology in shale and other low permeability media; (3) CO_2 injection is very sensitive to changes in the injection gas Langmuir strain constants, so trying to reduce the resultant strain constant, such as using mixed gases like N_2 and CO_2 or flue gas, is an efficient solution to improve the methane recovery efficiency.
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