Understanding the near surface migration patterns and rates of efflux of CO₂ is important for developing effective monitoring and verification programs for the geological storage of CO₂. Soil flux surveys are a well-established technique for characterising surface CO₂ emission sources from controlled release sites, CO₂storage sites or natural CO₂seeps. The performance of four interpolation methods; arithmetic mean (AM), two minimum variance unbiased estimators (MVUE), and a newly developed geostatistical cubic surface were evaluated using 21 soil flux surveys conducted over two controlled release experiments in 2012 and 2013, at the Ginninderra controlled release facility, Australia. Data was binned to approximate a regular sampling grid for improved performance of the whole-of-field AM and MVUE averaging techniques. The AM and MVUE methods were highly sensitive to deviations in the statistical distribution of the data, and performed inconsistently across the two experiments. These two methods proved ill-suited for application to CO₂ leak quantification due to their inflexible sampling and distribution requirements. The cubic technique provided the best net emission estimates across both experiments, and when applied at different bin sizes, estimating the true release rate to within 20% for the 2012 experiment and 45% below the release rate for the 2013 experiment. The cubic method is well-suited for CO₂ leak quantification because it is not limited by assumptions of the data’s spatial or statistical distribution. Net H₂O emissions of 29 kg/d were observed coincident with the high CO₂ flux zones in the field. The interpolation methods were applied with similar results on soil flux surveys taken from a natural seepage site in Qinghai, China. Gravity currents appear to describe the observed soil flux and soil gas behavior at Ginninderra, i.e. the observed lateral migration of CO₂in the subsurface. Subsurface migration was also strongly influenced by the relative depth of the groundwater. Thus the low water table and greater vadose zone in the 2013 experiment is suspected to facilitate greater lateral CO₂ migration and explain the poor closure of the CO₂ balance. <b>Citation:</b> I.F. Schroder, P. Wilson, A.F. Feitz, J. Ennis-King, <i>Evaluating the Performance of Soil Flux Surveys and Inversion Methods for Quantification of CO2 Leakage</i>, Energy Procedia, Volume 114, 2017, Pages 3679-3694, ISSN 1876-6102, https://doi.org/10.1016/j.egypro.2017.03.1499.

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.