Heterogeneous sorption and swelling in a confined and stressed coal during CO2 injection

A confined Pittsburgh seam (DECS-12) coal sample was pressurized with carbon dioxide (CO2) and the changes in the sorbed gas concentration and matrix properties were studied using dual-energy X-ray computed tomography (X-ray CT). The use of dual-energy technique enabled the quantification of spatial and temporal variations in bulk density and effective atomic number. These two quantities were used to calculate separately the amount of sorbed gas and the changes in the coal matrix and to reveal the kinetics of the complex heterogeneous processes occurring with CO2 injection in a consolidated bituminous coal sample kept under a constant effective stress. The swelling and CO2 Sorption in coal are heterogeneous processes and different parts of the coal behaved differently. Vitrite, liptite, and clarite microlithotypes swelled due to dissolution of CO2, while the clay + inertite region was compressed, even though it was the region that had the highest CO2 concentration. The vitrite swelled and de-swelled on the time scale of the experiment (5000-7000 min at each pressure) demonstrating that CO2 dissolution enabled rapid coal structure changes. Sorption kinetics were also heterogeneous: CO2 uptake was fastest for the clay + inertite region. Vitrite swelling reached a maximum then decreased with the expulsion of CO2, a behavior that has been observed in polymers.