Tagging CO 2 to Enable Quantitative Inventories of Geological Carbon Storage
In the wake of concerns about the long term integrity and containment of sub-surface CO 2 sequestration reservoirs, many efforts have been made to improve the monitoring, verification, and accounting methods for geo-sequestered CO 2 . Our project aimed to demonstrate the feasibility of a system designed to tag CO 2 with carbon isotope 14 C immediately prior to sequestration to a level that is normal on the surface (one part per trillion). Because carbon found at depth is naturally free of 14 C, this tag would easily differentiate pre-existing carbon from anthropogenic injected carbon and provide an excellent handle for monitoring its whereabouts in the subsurface. It also creates an excellent handle for adding up anthropogenic carbon inventories. Future inventories in effect count 14 C atoms. Accordingly, we have developed a 14 C tagging system suitable for use at the part-per-trillion level. This system consists of a gas-exchange apparatus to make disposable cartridges ready for controlled injection into a fast flowing stream of pressurized CO 2 . We built a high-pressure injection and tagging system, and a 14 C detection system. The disposable cartridge and injection system have been successfully demonstrated in the lab with a high-pressure flow reactor, as well as in the field at the CarbFix CO 2 sequestration site in Iceland. The laser-based 14 C detection system originally conceived has been shown to possess inadequate sensitivity for ambient levels. Alternative methods for detecting 14 C, such as saturated cavity absorption ringdown spectroscopy and scintillation counting, may still be suitable.