Surface CO 2 leakage during two shallow subsurface CO 2 releases

A new field facility was used to study CO 2 migration processes and test techniques to detect and quantify potential CO 2 leakage from geologic storage sites. For 10 days starting 9 July 2007, and for seven days starting 3 August 2007, 0.1 and 0.3 t CO 2 d −1 , respectively, were released from a ∼100‐m long, sub‐water table (∼2.5‐m depth) horizontal well. The spatio‐temporal evolution of leakage was mapped through repeated grid measurements of soil CO 2 flux (F CO2 ). The surface leakage onset, approach to steady state, and post‐release decline matched model predictions closely. Modeling suggested that minimal CO 2 was taken up by groundwater through dissolution, and CO 2 spread out on top of the water table. F CO2 spatial patterns were related to well design and soil physical properties. Estimates of total CO 2 discharge along with soil respiration and leakage discharge highlight the influence of background CO 2 flux variations on detection of CO 2 leakage signals.