The wellbore integrity is a key factor for a successful oil, gas and carbon capture and storage (CCS) projects and the durability of the cement used in the wellbore plays a vital role in the long term safety of those projects. To date, Ordinary Portland cement (OPC) has been used in the wells used in oil and gas industry, and it is found to be unstable under down-hole pressures and temperatures conditions. Therefore, this research work intends investigating an alkali activated inorganic binder, geopolymer, as well cement and study the apparent CO2 permeability of different types of geopolymer. Three different types of geopolymer was prepared by adding 0%, 8% and 15% of alkali activated slag (by mass) with fly ash, and existing class G oil cement was tested for the comparison of results. Sub and supercritical CO2 permeability was experimented at different injection and confining pressures expected under deep down-hole environment. The experimental results reveal that the apparent CO2 permeability of geopolymers (0.0005-0.002 mu D) is two to three orders lower than class G cement (0.12-2.6 mu D) depending on the mix compositions of geopolymer. The addition of 15% slag reduces the permeability by approx. 10 times compared to fly ash based geopolymer and 1000 times compared to class G cement. Alkali activated geopolymer materials can be a good replacement for existing OPC based cement as they have lower CO2 permeability, and can be employed in shallow and deeper depths of injection wells by changing the mix composition. (C) 2013 Elsevier Ltd. All rights reserved.