Emissions of greenhouse gases have been a major problem for the world for many decades, and the geo-sequestration of carbon dioxide (CO2) has been proposed as one of the best solutions for the reduction of anthropogenic greenhouse gas emissions. The injection well and the well cement play vital roles in the sustainability of a sequestration project, and CO2 leakage through the wellbore and well cement has been found to be the major threat to the success of sequestration projects. Therefore, the permeability of well cement should be low to avoid leakage in the wellbore. To date, ordinary Portland cement (OPC) based well cement has been used, and it has been found to be unstable in the CO2 environment as it provides pathways for CO2 leakage. One of the best replacements for OPC would be geopolymer cement as it possesses excellent strength, acid resistance and durability compared to OPC. Therefore, this paper addresses geopolymer as well cement in place of OPC and discusses its permeability. In the first phase of this work, previous studies relevant to the permeability of existing oil well cement and geopolymer cement were analysed. In the second phase, an experimental study was conducted to study the CO2 permeability of geopolymer under tri-axial conditions, using the high pressure tri-axial set-up available in the Monash University laboratory. Based on the experimental results, the apparent CO2 permeability of geopolymer is in the range of 2 x 10(-21)-6 x 10(-20) m(2) and it is less than the CO2 permeability of typical oil well cement (10(-20)-10(-11) m(2)) This indicates that geopolymer is a good replacement for existing OPC-based cement as it has lower CO2 permeability. At a given confining pressure, the permeability of geopolymer reduces with the increase in injection pressure and this is attributed to Klinkenberg's effect. When the confining pressure is increased from 10 to 25 MPa, the permeability reduction is 60% at lower injection pressures (5 MPa). The corresponding reduction is 35% at higher injection pressures (>10 MPa) as the permeability value reaches its intrinsic permeability at higher injection pressures. In addition, CO2 permeability reduces with confining stress for all the injection pressures studied. (c) 2012 Elsevier Ltd. All rights reserved.