Evaluating the performance of soil flux surveys and inversion methods for quantification of CO2 leakage

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, CO2 storage sites or natural CO2 seeps. 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 CO2 in 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. (C) 2017 The Authors. Published by Elsevier Ltd.