PNNL

Abstract

Perfluorocarbon compounds (PFCs) have high chemical and thermal stability, low background levels, 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 particular the transport of these compounds relative to CO2 gas, has not been thoroughly studied. We conducted column tests to study the transport of perfluoro-methylcyclo-pentane (PMCP), perfluoro-methylcyclo-hexane (PMCH), othro-perfluoro-dimethylcyclo-hexane (ortho-PDCH), and perfluoro-trimethylcyclo-hexane (PTCH) gas tracers in a variety of porous media. The influence of water content and sediments minerals on the retardation of the tracers was tested. The transport of tracers relative to 13CO2 and 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 as the molecular weight of the PFC increases. When water was present in the media, the transport of CO2 was greatly retarded compared to SF6 and PFC tracers. However, in dry laboratory sediments, the migration of CO2 was slightly faster than all the tracers. The minerals in the sediments also had significant impact on the fate of the tracers. In order to use the PFC tracer concentration data obtained from the ground surface or shallow subsurface in a GCS site to precisely interoperate the extent and magnitude of CO2 leakage, the retardation of the tracers and the interaction of CO2 with reservoir overlying formation water should be carefully quantified.