Pore-fracture alteration of different rank coals: implications for CO₂ sequestration in coal

A comprehensive characterization of pore-fracture alteration of coal after exposure to CO₂ is critical to CO₂ sequestration in deep coal seams. In order to investigate the effect of supercritical CO₂ (ScCO₂) on pore-fracture structure of coal, three coal samples with different ranks (lignite, bituminous and anthracite) were saturated with ScCO₂ for 14 days at 50 ℃ temperature and 10 MPa pressure. Multiscale characterization techniques, i.e. low-temperature N₂ adsorption, mercury intrusion porosimetry (MIP) and X-ray CT scanning, were adopted to capture the changes of pore-fracture characteristics from nanometre to millimetre. Results show mesopores and macropores were well-developed in lignite due to its low maturity, whereas fractures were well developed in bituminous and anthracite. Lignite was relatively non-reactive with slight changes in mesopores and macropores as determined by MIP and N₂ adsorption. An increase of macropore volume in lignite was observed from CT scanning, which possibly resulted from the shrinkage of coal matrix and collapse of pores caused by the drying effect of ScCO₂. However, the induced macroporosity was not well-connected. The mesopores, macropores and fractures increased after ScCO₂ treatment in bituminous and anthracite. Swelling-induced cracking, mineral and maceral dissolutions were the main causes for porosity increase in bituminous and anthracite and the induced fracture networks were well-connected. The fractal dimension of pore in three coals reduced after ScCO₂ treatment, which indicates the surface of pore surface became smooth and homogeneous. Overall, ScCO₂ has great potential of increasing coal porosity and enhancing coal permeability under unconstrained conditions, although the effect is rank dependent, which provides additional pathways for carbon storage and methane recovery.