"Carbon Dioxide (CO2) sequestration into porous and permeable brine-filled aquifers is seen as one of the most feasible solutions for reducing the amount of greenhouse gases released into the atmosphere from coal-fired power plants. To safely store the CO2, it must be trapped under an impermeable rock acting as a seal. One of the concerns with CO2 sequestration is the generation of new fractures or reopening of existing fractures caused by CO2 injection in the sealing formation. This project evaluates the potential of sealing these fractures by injecting sealing materials into them. These sealing materials need also to stay in place over long term. Therefore the long term thermo-stability of the sealing materials exposed to CO2 has to be addressed. Four sealing materials have been investigated, at subsurface conditions, to study their ability to effectively seal CO2 migration through fractures ranging in size from 250 [mu] up to 1 mm. The four sealant materials were: paraffin wax, silica-based gel, polymer-based gel, and calcium aluminate-based cement. All four materials significantly reduced the fracture permeability. However, the calcium aluminate-based cement was the most effective sealant agent and was the only sealant that was able to withstand the large differential pressure caused by CO2 or brine injection pressure. Based on the experiments conducted, gels cannot be expected to withstand large pressure differentials in a parallel fracture and therefore the calcium aluminate-based cement is recommended for sealing of fracture widths above half a millimeter. Since cement exposed to CO2 is subjected to the reaction of carbonation, a potential injection scenario is to inject cement first to create a barrier to differential pressures and then follow with a gel as a secondary seal to create a chemically stable sealing agent exposed to CO2"--Abstract, page iii.