MOVING BEYOND TRACERS: 13C INSIGHTS INTO CARBON SEQUESTRATION

Science and society are faced with the inextricably linked challenges of fossil fuel energy dependence and rising atmospheric CO₂ levels. There is growing recognition that management of remaining hydrocarbon resources, exploration of less CO₂-emitting "cleaner" CH₄ and natural gas fuels, and investigation of subsurface capture and storage of anthropogenic CO₂ require a better understanding of the deep terrestrial carbon cycle [1]. Noble gases provide a tracer set that preserves a signal of fluid origin, transport, and age. Coupling this tracer set with carbon isotope geochemistry enables resolution of the inorganic and organic reactions controlling carbon mobility.

Prediction of the subsurface behaviour of CO₂ on a decadal time scale can be achieved through laboratory and field experiments in engineered systems and modeling. Understanding the fate of injected CO₂ on the millennia time scale needed to assure reservoir competence and safety remains a challenge. A key unknown is the CO₂ residence time and mobility once injected. Addressing this requires understanding how much CO₂ ultimately reacts with the rock to form immobile carbonate rocks; how much remains in a near pure, but buoyant, CO₂ gas phase; and how much is dissolved into the groundwater.

Traditional use of carbon isotope signatures in investigations of CO₂ sequestration in saline aquifers and oil field brines focuses on injected tracer study approaches, are dependent on having a sufficiently different carbon isotopic signature between injected CO₂ and background CO₂ and dissolved inorganic carbon species (DIC) in the subsurface, and yield information on a timescale of years to decades. Recently the use of in situ carbon isotope studies in naturally occurring CO₂-rich gas fields has shown the insights available on CO₂ trapping mechanisms and water-rock interreactions over millenia from geologic analog sites worldwide.

[1] Shewrood Lollar, B. and Ballentine, C.J.. (2009) Nature Geoscience 2, 543-547.