Isotopic and Chemical Tracers

Natural and introduced tracers in the atmosphere can be used for monitoring of a possible CO₂ release from geologic storage reservoirs. Natural tracers are chemical compounds that are associated with CO₂ in the subsurface, near-surface, or atmosphere. These include CH₄, radon, noble gases, and isotopes of CO₂. Introduced tracers, such as sulfur hexafluoride (SF6) and perfluorocarbon tracer (PFT), are chemical compounds that may be injected into a geologic reservoir along with the CO₂ in order to give the injected CO2 a unique fingerprint that can be recognized in aboveground emissions. Tracer monitoring can take place in the near-surface and subsurface regions as well.¹

Isotopic and Chemical Tracers Tool Summary

Description: Natural and injected chemical compounds that are monitored in air to help detect CO₂ released to the atmosphere.
Benefits: Used as a proxy for CO₂, when direct observation of a CO₂ release is not adequate. Also used to track potential CO₂ plumes.
Challenges: In some cases, analytical equipment is not available onsite, and samples need to be analyzed offsite. Background/baseline levels must be established. Tracers may not behave the same as CO₂ along the migration pathway.²

A research team injects a fluorinated tracer into the subsurface at a test site in Tennessee. Monitoring will then involve collecting data to look for evidence of the tracer.

Application

Atmospheric perfluorocarbon tracer (perfluoroethylcyclohexane) sampling was part of a shallow, controlled underground CO₂ leak experiment held at Montana State University in Bozeman, Montana in 2008. Tracer dispersion was measured using a tethered balloon system with multi-tube remote samplers at elevations of 10 m, 20 m, and 40 m above ground level, as well as a ground-based portable tower with monitors containing sorbent material to collect the tracer at 1 m, 2 m, 3 m, and 4 m above ground level. As anticipated with a controlled release, tracer was consistently detected at elevated concentrations downwind of the release point. This field study demonstrated the feasibility of remote sampling with unmanned aerial systems at future carbon storage sites.³

Sampling balloon towed to deployment in the field (left) at the field experiment station in Montana. At right, the balloon is shown in flight with the multi-tube remote samplers (MTRS) in tow.⁴

Image Credits

fluorinated tracer: National Energy Technology Laboratory
sampling balloon 1: National Energy Technology Laboratory
sampling balloon 2: National Energy Technology Laboratory