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type     June, 2005

Vol 2 Chapter 21: Detecting Leaks from Belowground CO2 Reservoirs Using Eddy Covariance

Natasha L. Miles, Kenneth J. Davis and John C. Wyngaard

Abstract: We describe the eddy covariance method of measuring earth–atmosphere CO2 exchange, including past applications to measurements of volcanic venting of CO2. The technique involves continuous atmospheric measurements of both CO2 mixing ratio and atmospheric winds from a tower platform. Equipment is robust and commercially available, and the methodology is well established. The surface area covered by the measurement is described. The upwind coverage is typically ð10–100Þzm; where zm is the measurement height, and the cross-wind extent of this area is of the order of the upwind distance. Thus, a 10-m high tower detects fluxes from an upwind distance of 100–1000 m, and an area of order 104–106 m2. The eddy covariance method yields continuous measurements of earth–atmosphere exchange over such areas, typically expressed as averages over hourly or half-hourly time periods. The area measured depends on wind speed, wind direction, surface roughness, and stability of the atmospheric surface layer. The measurement works best under well-mixed atmospheric conditions which frequently occur on a daily basis, often for a majority of the day. We assess the ability to detect leaks from geologic CO2 reservoirs by comparing expected leakage rates to typical ecological flux rates. While the character and magnitude of ecological fluxes are well established, reservoir leakage rates and areas are uncertain. Fairly conservative estimates based on ensuring the economic viability of CO2 storage are constructed. Our estimates of leakage rate and area yield leakage fluxes that range from 1 to 104 times the magnitude of typical ecological fluxes. The flux measurement areas readily encompass the assumed leakage areas (10–105 m2). We conclude that this approach shows promise for the monitoring of belowground CO2 storage. Leak detection is shown to be a simpler problem than leak quantification, but both can in principle be accomplished using eddy covariance under conditions favorable for the measurement.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO2 Capture Project Geologic Storage of Carbon Dioxide with Monitoring and Verification - Volume 2
Edited by:
Sally M. Benson, Lawrence Berkeley Laboratory, Berkeley, CA, USA

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