Tracers can be added to the injected CO₂ stream in geosequestration projects to demonstrate safety and containment to stakeholders by verifying the presence of injected CO₂ (other CO₂ may exist), monitoring the possible leakage of CO₂ (fingerprinting tool), and measuring the behavior of CO₂ under injection (e.g., saturation, fluid mixing, calibrating models, etc.).

Monitoring may be required in a variety of settings (e.g., reservoir fluids, shallow groundwater, deep soil, surface emissions, and formation gases). Thus, an appropriate tracer or suite of tracers is required for clear identification if the CO₂ migrates out of the injection zone. Although the injected CO₂ in some study areas has been isotopically distinct from the existing CO₂ present in the aquifer or reservoir (e.g., the Encana Enhanced Oil Recovery Program at Weyburn, Saskatchewan, Canada), this is not always the case.

Tracers are required to be chemically inert, environmentally safe, nontoxic, persistent, and stable for purposes of the desired monitoring time scale. Low-volume usage (for cost, availability, and ease of handling) and sensitivity to detection by analytical methods are also of key importance. Application methods (e.g., pulsed or continuous addition) may also affect costs. More commonly than not, the analytical costs are seen to be the most prohibitive cost overall (specialist tracers, very precise measurements by specialized methods or equipment, etc.).

Based on the general requirements for carbon storage, several possible tracer compounds have been identified by various authors and summarized here. Their behavior, availability, ease of analysis, and indicative costs are evaluated. This list is by no means exhaustive for all likely tracers.

The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is conducting a CO₂ storage pilot program in Victoria, Australia. The Otway Project site is a depleted natural gas reservoir with roughly 20% residual methane saturation and a remaining methane-rich gas cap. In addition, the natural source of CO₂ for injection is mixed with about 20% methane content. Thus, tracers are required that may give information on the fate of the injected CO₂-methane combination. Some preliminary work on CD₄ (fully deuterated methane) is presented, and further work on the evaluation of this potential tracer is discussed.