Super-critical carbon dioxide flow behaviour in low rank coal: A meso-scale experimental study

The carbon dioxide (CO2) adsorbed in coal seams during CO2-enhanced coal bed methane recovery (CO2-ECBM) causes substantial coal matrix alterations, resulting in significantly reduced flow performance. Many studies have been conducted to date on the effect of CO2 phase on coal mass permeability. However, the effect of coal rank on these permeability changes with CO2 phase has not yet been studied. Therefore, the main aim of this study is to investigate how the influence of CO2 phase condition on coal flow performance varies with rank. A series of tri-axial permeability tests was conducted using Australian brown coal samples for both CO2 and N2 under various confinements and injections at 35 °C. The results were then compared with those for high-rank coal reported in the literature. According to the test results, greater coal macro-pore-structure rearrangement occurs with super-critical CO2 adsorption, resulting in lower permeability in coal, regardless of rank. However, this CO2 phase influence is much greater for high-rank coal. Although coal permeability reduces with depth for any rank of coal, this depth effect reduces with increasing rank. Furthermore, although N2 has the ability to recover CO2 adsorption-induced swelled areas in coal regardless of rank, that capability is much greater for high-rank coal.