Quantitative assessment of soil CO2 concentration and stable carbon isotope for leakage detection at geological carbon sequestration sites

This study presents a quantitative method to evaluate CO2 concentration ([CO2]) and stable carbon isotope ratio (delta C-13) as indicators for leakage detection at a geological carbon sequestration site by combining use of field release tests and a numerical modeling approach. A numerical model was developed to simulate CO2 dynamics by considering diffusion, dissolution in soil water, and soil respiration. The numerical model fits the background dynamics of [CO2] (360 to 550 ppm) and delta C-13 (-16% to -6%) well and reproduces fairly the overall trend observed during the CO2 release test. The model was further applied to assess detection probability (DP) of [CO2] and delta C-13 for leakage detection in terms of various factors, such as CO2 leakage rate, background variations, delta C-13 of the CO2 leaked, and the threshold value of signal-to-noise ratio. Modeling results suggest that delta C-13 may have a higher DP than [CO2]. This study also shows that DP of delta C-13 for the IEA Weyburn project is close to 0, implying delta C-13 is inappropriate to be an indicator for CO2 leakage at the site. The quantitative method developed can also be used to design a monitoring plan or strategy in a near-surface environment for geological carbon sequestration. (C) 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.