Eddy Covariance (EC) has been proposed as a surface monitoring solution for long-term deployment at CCS sites. However, its suitability when applied to a highly inhomogeneous source area- as would be the case for a small-scale CO2 surface leak- has been poorly established. For this reason, EC has been implemented for two controlled CO2 releases conducted at the Ginninderra controlled release facility, with the aim of determining the technique's suitability for the location, detection and quantification of a small magnitude CO2 leak (144 kg/d). By comparing results from the two release experiments, this poster highlights the variable success of using EC for detection, and how this may depend on changing experimental and climatic variables such as leak location, tower height and depth to groundwater. The detection significance of grouped EC measurements will be established through statistical analysis using Cramer-Von Mises tests. In addition, the application of two EC towers concurrently for leak detection and location will be explored, with a second tower deployed for the latter portion of the 2013 release experiment. Quantification of the leak using EC was attempted, but due to the problems in the fundamental assumptions of the technique, no substantive progress could be made. This will be explained with respect to the 'lost' CO2 from the system in part due to advection and diffusion. Presented at the 2014 CO2CRC Research Symposium

Following the drilling of a shallow natural CO₂ reservoir at the Qinghai research site, west of Haidong, China, it was discovered that CO₂ was continuously leaking from the wellbore due to well-failure. The site has become a useful research facility in China for studying CO₂ leakage and monitoring technologies for application to geological storage sites of CO₂. During an eight day period in 2014, soil gas and soil flux surveys were conducted to characterise the distribution, magnitude and likely source of the leaking CO₂ . Two different sampling patterns were utilised during soil flux surveys. A regular sampling grid was used to spatially map out the two high-flux zones which were located 20–50 m away from the wellhead. An irregular sampling grid, with higher sampling density in the high-flux zones, allowed for more accurate mapping of the leak distribution and estimation of total field emission rate using cubic interpolation. The total CO₂ emission rate for the site was estimated at 649-1015 kgCO₂/d and there appeared to be some degree of spatial correlation between observed CO₂ fluxes and elevated surface H₂O fluxes. Sixteen soil gas wells were installed across the field to test the real-time application of Romanak et al.’s (2012) process-based approach for soil gas measurements (using ratios of major soil gas components to identify the CO₂ source) using a portable multi-gas analyser. Results clearly identified CO₂ as being derived from one exogenous source, and are consistent with gas samples collected for laboratory analysis. Carbon-13 isotopes in the centre of each leak zone (−0.21‰ and −0.22‰) indicate the underlying CO₂ is likely sourced from the thermal decomposition of marine carbonates. Surface soil mineralisation (predominantly calcite) can be used to infer prior distribution of the CO₂ hotspots and as a consequence highlighted plume migration of 20m in 11 years. The broadening of the affected area beyond the wellbore at the Qinghai research site markedly increases the area that needs surveying at sufficient density to detect a leak. This challenges the role of soil gas and soil flux in a CCS monitoring and verification program for leak detection, suggesting that these techniques may be better applied for characterising the source and emission rate of a CO₂ leak, respectively. <b>Citation:</b> I.F. Schroder, H. Zhang, C. Zhang, A.J. Feitz, The role of soil flux and soil gas monitoring in the characterisation of a CO2 surface leak: A case study in Qinghai, China, International Journal of Greenhouse Gas Control, Volume 54, Part 1, 2016, Pages 84-95, ISSN 1750-5836, https://doi.org/10.1016/j.ijggc.2016.07.030.