Developing Radioactive Carbon Isotope Tagging for Monitoring, Verification and Accounting in Geological Carbon Storage

In the wake of concerns about the long-term integrity and containment of sub-surface CO2 sequestration reservoirs, many efforts have been made to improve the monitoring, verification, and accounting methods for geo-sequestered CO2. This Ph.D. project has been part of a larger U.S. Department of Energy (DOE) sponsored research project to demonstrate the feasibility of a system designed to tag CO2 with radiocarbon at a concentration of one part per trillion, which is the ambient concentration of 14C in the modern atmosphere. Because carbon found at depth is naturally free of 14C, this tag would easily differentiate pre-existing carbon in the underground 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 14C atoms. Accordingly, we developed a 14C tagging system suitable for use at the part-per-trillion level. This tagging system uses small containers of tracer fluid of 14C enriched CO2. The content of these containers is transferred into a CO2 stream readied for underground injection in a controlled manner so as to tag it at the part-per-trillion level. These containers because of their shape are referred to in this document as tracer loops. The demonstration of the tracer injection involved three steps. First, a tracer loop filling station was designed and constructed featuring a novel membrane based gas exchanger, which degassed the fluid in the first step and then equilibrated the fluid with CO2 at fixed pressure and fixed temperature. It was demonstrated that this approach could achieve uniform solutions and prevent the formation of bubbles and degassing downstream. The difference between measured and expected results of the CO2 content in the tracer loop was below 1%. Second, a high-pressure flow loop was built for injecting, mixing, and sampling of the fast flowing stream of pressurized CO2 tagged with our tracer. The laboratory scale evaluation demonstrated the accuracy and effectiveness of our tracer loops and injection system. The 14C/12C ratio we achieved in the high pressure flow loop was at the part per trillion level, and deviation between the experimental result and theoretical expectation was 6.1%. Third, a field test in Iceland successfully demonstrated a similar performance whereby 14CO2 tracer could be injected in a controlled manner into a CO2 stream at the part per trillion level over extended periods of time. The deviation between the experimental result and theoretical expectation was 7.1%. In addition the project considered a laser-based 14C detection system. However, the laser-based 14C detection system was shown to possess inadequate sensitivity for detecting ambient levels of 14CO2. Alternative methods for detecting 14C, such as saturated cavity absorption ring down spectroscopy and scintillation counting may still be suitable. In summary, the project has defined the foundation of carbon-14 tagging for the monitoring, verification, and accounting of geological carbon sequestration.