Monitoring well
CO injection well
Accumulation in surface depression
Surface CO sensor
Surface CO sensor
Wind
Accumulation in surface water
Injection & pressure pump
CO
Water table
CO
Lake
Plume
Plume
Di usion
Plume
CO dissolved
Separate phase CO
in groundwater
Annular ow
CO in low permeability environment
Di usion
CO
CO
Plume
Cap rock
Fault
Figure 1 CCS relies on transparent MMV systems that can demonstrate containment beyond doubt
carefully selected compound, stable under the temperature and pressure conditions of storage, and chemically inert so it does not react with formation fluids or alter the CO₂ itself. Detection begins through a network of observation or monitoring wells placed at strategic points around the injection zone. Tiny samples of formation water or gas are collected at intervals, then analysed using ultra-sensitive spectrometric methods capable of identifying parts per trillion. The presence, absence, or concentration of a tracer provides direct evidence of where injected CO₂ has migrated, and whether it has remained securely trapped. Different tracers can be designed for different purposes. Some are tailored to mimic the behaviour of CO₂ precisely, ensuring they move through the formation in the same way the injected gas does. Others are developed to respond uniquely to certain reservoir conditions, allowing operators to discriminate between zones, layers, or time periods of injection. By deploying several tracers simultaneously, complex multi-well systems can be monitored with precision, providing clear evidence of where the CO2 is located or where it may have migrated within the formation.
Importantly, tracer monitoring does not interfere with normal operations. Samples are taken without interrupting injection or production, meaning data collection can be continuous and non-intrusive. This makes chemical tracers particularly suited to long- term carbon storage, where verification must continue for years after injection has ceased. The result is a robust, quantitative dataset that complements and strengthens more interpretive measurements such as seismic imaging or geochemical analysis. Verification and assurance Within carbon storage projects, measurement and verification are more than technical requirements – they are instruments of trust. Regulators, investors, and local communities all need clear proof that CO₂ remains where intended. Tracer data provides a measurable, long-term objective line of evidence that directly links to regulatory frameworks and carbon accounting standards. For operators seeking to qualify for incentives such as the US 45Q tax credit, tracer results can form part of the documentation that demonstrates secure geologic storage. Since
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