Sub- and super-critical carbon dioxide flow variations in large high-rank coal specimen: An experimental study

Many experimental studies to date have investigated CO2 sequestration in coal using small-scale samples (usually less than 100 mm in length), and the results may not be applicable to large-scale samples which better represent in-situ conditions for the estimation of CO2 flow behaviours in coal. This study was therefore initiated to determine the sub- and super-critical CO2 flow characteristics in a large reconstituted bituminous coal sample (203 mm in diameter and 1 m in length) by performing three sets of N2 and CO2 injections with injection pressures from 6 MPa to 10 MPa under 11 MPa axial stress and at 37 °C. It was observed that, unlike N2 permeability which increases with injection pressure, CO2 permeability exhibits reductions with injection pressure due to greater swelling effects at elevated CO2 pressures. The second N2 injections into the coal sample previously flooded with CO2 showed reductions in permeability compared to the first N2 injection into the original coal sample, because the coal structure had been altered considerably by the CO2 flows. The pressure build-up at downstream for CO2 injection is always lower than that for the first N2 injection due to reduced CO2 flows as a result of decreased permeability, which also contributes to the lower pressure development at downstream for the second N2 injections. The pressure profiles along the sample are similar for the first N2 and CO2 injections with relatively greater pressure decays for CO2, while considerable pressure reductions were observed for the second N2 injections, especially in the regions near the injection point where greater CO2 pressures previously existed, causing greater structural rearrangement in those regions. The second N2 injection causes less volumetric strain of the sample than the first due to the altered coal structure induced by the prior CO2 flows which impede gas flow in the sample.