Near-surface monitoring strategies for geologic carbon dioxide storage verification

Geologic carbon sequestration is the capture of anthropogenic carbon dioxide (CO{sub 2}) and its storage in deep geologic formations. Geologic CO{sub 2} storage verification will be needed to ensure that CO{sub 2} is not leaking from the intended storage formation and seeping out of the ground. Because the ultimate failure of geologic CO{sub 2} storage occurs when CO{sub 2} seeps out of the ground into the atmospheric surface layer, and because elevated concentrations of CO{sub 2} near the ground surface can cause health, safety, and environmental risks, monitoring will need to be carried out in the near-surface environment. The detection of a CO{sub 2} leakage or seepage signal (LOSS) in the near-surface environment is challenging because there are large natural variations in CO{sub 2} concentrations and fluxes arising from soil, plant, and subsurface processes. The term leakage refers to CO{sub 2} migration away from the intended storage site, while seepage is defined as CO{sub 2} passing from one medium to another, for example across the ground surface. The flow and transport of CO{sub 2} at high concentrations in the near-surface environment will be controlled by its high density, low viscosity, and high solubility in water relative to air. Numerical simulations of leakage and seepage show that CO{sub 2} concentrations can reach very high levels in the shallow subsurface even for relatively modest CO{sub 2} leakage fluxes. However, once CO{sub 2} seeps out of the ground into the atmospheric surface layer, surface winds are effective at dispersing CO{sub 2} seepage. In natural ecological systems with no CO{sub 2} LOSS, near-surface CO{sub 2} fluxes and concentrations are controlled by CO{sub 2} uptake by photosynthesis, and production by root respiration, organic carbon biodegradation in soil, deep outgassing of CO{sub 2}, and by exchange of CO{sub 2} with the atmosphere. Existing technologies available for monitoring CO{sub 2} in the near-surface environment include (1) the infrared gas analyzer (IRGA) for measuring point concentrations using IR absorption by the CO{sub 2} molecule, (2) the accumulation chamber (AC) method for measuring soil CO{sub 2} fluxes at discrete points, (3) the eddy correlation (EC) tower that measures net flux over a given area, and (4) light distancing and ranging (LIDAR) that can measure CO{sub 2} concentrations over an integrated path. Novel technologies that could potentially be useful for CO{sub 2} concentration and flux measurement include hyperspectral remote sensing of vegetative stress as an indication of elevated CO{sub 2} concentrations, tunable lasers for long distance integrated concentration measurements, microelectronic mechanical systems (MEMS) that can be dispersed to make widespread point measurements, and trained animals such as dogs as used for landmine detection.

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