Measurement of atmospheric near-surface carbon dioxide CO2 by open-path laser and portable infrared techniques is showing promise as an effective tool for the monitoring of gas emissions from CO2 storage sites. BGS has been developing the techniques to be used in CO2 monitoring strategies.
Measurement of near-surface CO2 at geological storage sites is likely to be increasingly required as part of a monitoring strategy for CO2 containment. Commercial storage sites will probably be large (many tens of square kilometres) and potential leakage sites small (order of a few metres across). Measurement techniques therefore need to be capable of covering large areas within a reasonable timeframe, yet with sufficient precision and sensitivity.
Conventional techniques for near-surface measurement of gas include both field- and laboratory-based methods. Field measurements are usually rapid and can provide instant results but tend to be of low precision. Laboratory-based techniques can produce results of higher precision but involve delays in reporting and can be more laborious to carry out. Both approaches provide data for a single point.
Open-path laser and portable infrared techniques offer the potential for rapid surveying of near-surface gases over large distances. The techniques have been trialed at natural-analogue research sites in Italy and Germany and tested at the CO2 storage sites at In Salah (Algeria) and Weyburn (Canada).
Commercial open-path laser systems are available for CO2, methane (CH4) and a number of other industrial gases. Modifications to a tripod-mounted GasFinder 2.0 CO2 unit (Boreal Laser Inc.) were developed and trialed for the investigations. Rapid surveying (slow walking pace) was also carried out using a portable pumped infrared analyser. Field trials were undertaken in areas of known CO2 emissions: the volcanic centres of Latera (Italy) and the Laacher See (Germany). Trials were undertaken during 2005—2008. In each case, the survey results were tested against established soil-gas methods. These included CO2 flux measurements, and measurement of gases in the field using infrared and electrochemical sensors and in the lab by gas chromatography. Location information was collected by handheld GPS.
Open-path laser surveying of an area (100 000 m2) on the western shore of the Laacher See (Figure 1) showed clearly the location of two gas anomalies, and a repeat survey of the area, on the same visit and in a different season, were able to reproduce results. Soil analysis revealed that these vent centres had concentrations of CO2 approaching 100% at 60—80 cm depth and flux rates of 0.5—1.2 kg/m2/day. The survey also identified other areas of diffuse gas escape. The concentrations of CO2 showed a large variability in and around the vents.
Infrared measurements at Latera caldera in Italy (Figure 2) by colleagues at Sapienza University of Rome showed that observed concentrations diminished greatly with height of measurement and so after initial testing at 0, 10, 20 and 50 cm above ground surface, all subsequent measurements were made at ground level. The stability of the infrared detector proved critical in being able to detect weak CO2 signals.
Work is ongoing to improve survey speed and sensitivity, to monitor conditions closer to background, and to test the instruments under variable weather and vegetation conditions. For both techniques, good GPS location information is critical to data accuracy and reproducibility; sub-metre positional accuracy has been used in the most recent survey investigations.
Jones, D G, Barlow, T, Beaubien, S E, Ciotoli, G, Lister, T R, Lombardi, S, May, F, Möller, I, Pearce, J M and Shaw, R A. 2009. New and established techniques for surface gas monitoring at onshore CO2 storage sites. Energy Procedia, 1, 2127—2134.
Please contact Dr David Jones for further information