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

Many experimental studies to date have investigated CO 2 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 CO 2 flow behaviours in coal. This study was therefore initiated to determine the sub- and super-critical CO 2 flow characteristics in a large reconstituted bituminous coal sample (203 mm in diameter and 1 m in length) by performing three sets of N 2 and CO 2 injections with injection pressures from 6 MPa to 10 MPa under 11 MPa axial stress and at 37 °C. It was observed that, unlike N 2 permeability which increases with injection pressure, CO 2 permeability exhibits reductions with injection pressure due to greater swelling effects at elevated CO 2 pressures. The second N 2 injections into the coal sample previously flooded with CO 2 showed reductions in permeability compared to the first N 2 injection into the original coal sample, because the coal structure had been altered considerably by the CO 2 flows. The pressure build-up at downstream for CO 2 injection is always lower than that for the first N 2 injection due to reduced CO 2 flows as a result of decreased permeability, which also contributes to the lower pressure development at downstream for the second N 2 injections. The pressure profiles along the sample are similar for the first N 2 and CO 2 injections with relatively greater pressure decays for CO 2 , while considerable pressure reductions were observed for the second N 2 injections, especially in the regions near the injection point where greater CO 2 pressures previously existed, causing greater structural rearrangement in those regions. The second N 2 injection causes less volumetric strain of the sample than the first due to the altered coal structure induced by the prior CO 2 flows which impede gas flow in the sample. • CO 2 permeability reduces with increasing CO 2 injection pressure. • Super-critical CO 2 injection causes greater permeability reductions and reduced pressure development in the sample. • Similar pressure profiles along the sample are found for the first N 2 and CO 2 injections. • The second N 2 injections exhibit lower permeability values and greater pressure decays along the sample. • CO 2 injections produce greater volumetric strain of the sample. [ABSTRACT FROM AUTHOR]