Pore structure changing of coal during the CO2 geo-sequestration is one of the key issues that affect the sequestration process significantly. To address this problem, the CO2 sequestration process in an anthracite coal was replicated using a supercritical CO2 (ScCO2) reactor. Different coal grain sizes were exposed to ScCO2 and water at around 40 degrees C and 9.8 MPa for 72 h. Helium pycnometer and mercury porosimetry provide the density, pore size distribution and porosity of the coal before and after the ScCO2 treatment. The results show that after exposure to the ScCO2-H2O reaction, part of the carbonate minerals were dissolved and flushed away by water which made the true density increased as well as total pore volume and porosity most importantly in the micro-pore range. Hysteresis between mercury intrusion and extrusion was observed. Ink bottle shaped pores can be either damaged or created compared with the ScCO2 treated coal samples. This suggests that the ScCO2 treatment most likely increase the volumes of pores in anthracite coal, which also contributed to the increase in porosity of the treated samples. Therefore the CO2 sequestration into coal appears to have the potential to increase significantly the anthracite micro-porosity which is very advantageous for CO2 storage. (C) 2010 Elsevier Ltd. All rights reserved.