Tracers are marker chemicals used for many processes. In CCS they are injected along with the CO2, to help distinguish it from naturally occurring CO2.
They are useful to locate the plume and recognise its migration character. If well understood, they can also be used to determine trapping processes. When a tracer is injected with the CO2, some of the tracer dissolves (partitions) into the formation water whilst the rest of it remains in the CO2 cloud.
Knowing the partition information for a range of tracer chemicals will result in:
- Improved estimates and correlations for calculating reservoir capacity, and
- Improved and accurate simulation of the tracers within a range of subsurface temperatures.
This project established general methods for determining the coefficients for other chemical tracers. The study has experimentally determined the partition coefficients for a number of chemical tracers relevant to CCS. These included both reactive ester tracers, useful for determining residual CO2 saturation using the single well chemical tracer test, and inert gas tracers, useful for inter-well tests and reservoir to surface tests. This data was then incorporated into computational simulations of CCS scenarios to understand the impact of partition coefficients on the interpretation of tracer field data.
- The temperature dependent behaviour of the partition coefficients for a number of reactive ester tracers was determined. This information will lead to more accurate predictions of residual CO2 saturation in instances where these chemical tracers are used for the single well chemical tracer test.
- In computational simulations of inert gas tracers, it was determined that, for bounded reservoirs, the behaviour of a chemical tracer is affected to a very minor extent by changes in the partition coefficient (air/ water vs. supercritical CO2/water); while for unbounded reservoirs, the differences are somewhat significant and could possibly be differentiated in a field trial.
- Future work could include slim tube experiments packed with various sediments to experimentally determine the breakthrough curves for chemical tracers. This information could be used with future field studies to determine the possible influence that sorption onto sediment surfaces actually has on the behaviour of chemical tracers.