Carbon dioxide flow behaviour in macro-scale bituminous coal: an experimental determination of the influence of effective stress

Experimental studies on CO₂ flow behaviour in coal are generally performed on meso-scale specimens (less than 100 mm in length), the results are not applicable at a larger scale. In this study, CO₂ flow behaviours were experimentally tested on macro-scale reconstituted high rank coal samples (203 mm in diameter and 1 m in length). The permeability values, pressure development profiles, volumetric strains of the sample and the CO₂ storage characteristics were recorded and compared to interpret the results. It was observed that CO₂ permeability reduces with injection pressure, especially for supercritical CO₂ injections but lower reduction rates were observed at higher CO₂ injection pressures. With the increase of depth, CO₂ permeability reduces and this reduction is greater for higher injection pressures. Maintaining a relatively high CO₂ injection pressure creates greater pressure development in coal, and the effective zone of influence (areas where the CO₂ pressure may remain at least 90% of the injection pressure) decreases with depth but increases with injection pressure to some extent. CO₂ storage capacity reduces with depth, and the CO₂ injection rate is greater for lower injection pressures during early stages of CO₂ injection. However, the ultimate CO₂ storage capacity increases with CO₂ injection pressure for each injection.