Synchrotron X-ray tomographic characterization of microstructural evolution in coal due to supercritical CO2 injection at in-situ conditions

Microstructural evolution in coal due to CO2 injection directly influences the gas transport and storage properties of coal during CO2 sequestration in deep coal seams. Using synchrotron X-ray tomography, we performed CT imaging under simulated reservoir conditions with a novel X-ray transparent core holder and investigated the time-dependent evolution of 3-D microstructures of an anthracite coal with 1.37% moisture as-received interacted with supercritical CO2 (ScCO2). Three sets of CT scans were taken: (1) coal was subjected to 10 MPa confining pressure before exposed to ScCO2, (2) coal was subjected to 10 MPa confining pressure and after 7 h of exposure to ScCO2, and (3) coal was subjected to 10 MPa confining pressure and after 53 h of exposure to ScCO2. When ScCO2 interacts with coal, complex physico-chemical reactions occur, changing coal microstructures in multiple ways and resulting in significant permeability changes. After 7 h ScCO2 injection, we directly observed the pre-existing microfracture closure, which was attributed to coal swelling. A significant permeability reduction was measured from 3.0 to 0.24 mu D with up to 26 h CO2 injection. With increasing injection duration, interestingly, we observed the wormhole growth in coal due to hydrocarbon mobilization and mineral dissolution, which accordingly caused a permeability rebound to 1.2 mu D after 53 h CO2 injection. Coal swelling occurred readily upon ScCO2 injection but hydrocarbon mobilization and mineral dissolution were delayed to affect the permeability probably due to the slower reaction kinetics. This study suggests that ScCO2 injection has the potential to improve the permeability and enhance the coalbed methane recovery due to the effects of hydrocarbon mobilization and mineral dissolution.