Transport of perfluorocarbon tracers and carbon dioxide in sediment columns – Evaluating the application of PFC tracers for CO2 leakage detection

Perfluorocarbon compounds (PFCs) have high chemical and thermal stability, low background levels in natural systems, and easy detectability. They are proposed as tracers for monitoring potential CO2 leakage associated with geological carbon sequestration (GCS). The fate of the PFCs in porous media, and in particular, the transport of these compounds relative to CO2 gas in geological formations, has not been thoroughly studied. We conducted column tests to study the transport of perfluoro-methylcyclo-pentane (PMCP), perfluoro-methylcyclo-hexane (PMCH), ortho-perfluoro-dimethylcyclo-hexane (ortho-PDCH), and perfluoro-trimethylcyclo-hexane (PTCH) gas tracers in a variety of porous media. The influence of water content and sediment minerals on the retardation of the tracers was tested. The transport of PFC tracers relative to 13CO2 and the conservative tracer sulfur hexafluoride (SF6) was also investigated. Results show that at high water content, the PFCs and SF6 transported together. In dry and low-water-content sediments, however, the PFCs were retarded relative to SF6 with the degree of retardation increasing with the molecular weight of the PFC. When water was present in the medium, the transport of CO2 was greatly retarded compared to SF6 and the PFC tracers. However, in dry laboratory sediments, the migration of CO2 was slightly faster than all the tracers. The type of minerals in the sediments also had a significant impact on the fate of the tracers. In order to use the PFC tracer data obtained from the ground surface or shallow subsurface in a GCS site to precisely interpret the extent and magnitude of CO2 leakage, the retardation of the tracers and the interaction of CO2 with the reservoir overlying formation water should be carefully quantified.