Characterization of natural surface CO2 flux: Implication for leakage detection monitoring at CO2 geological sites

Carbon Capture and Storage (CCS) technology are believed to be the effective way to mitigate the greenhouse gases emission, especially CO2, to the atmosphere, given their huge capacity for geological storages[1]. The main principle of this technology is to capture the CO2, especially from fossil fuel burning and industry before emitted to the atmosphere and compressing it to be injected further to geological formations and sequestrated for a long time. However, CO2 leakage from CCS geological sites is likely to be unavoidable in the future. Injected or transported CO2 may migrate out of the geological formations through the subsurface, fractures, and faults depending on the well and caprock integrity and the trapping mechanisms[2]. Therefore, monitoring of the gas leakages after injection will be an obligatory part of any CO2 sequestration projects to ensure the functioning of the CCS technology without significant contaminant to the environment.
A monitoring in the surface of the CCS sites is a crucial part that have to be conducted during and after CO2 injection. The purposes are to ensure the safety of the CCS technology to the surrounding environment and to guarantee that injected CO2 safely sequestrated in the geological formations and no leakage occurred even after the injection. However, natural activity in the soil layers such as root respiration, microorganisms’ respiration and decomposition of organic matter can generate abundant CO2 that can emit to the atmosphere, shares about one-fourth of the global CO2 emissions in the atmosphere[3]. It causes the CO2 concentration in the soil layer is much higher than in the atmosphere, even can be more than 10 times higher and vary through the depth[4,5]. This natural gas fluctuations in soil layers complicate the detection and/or identification of gas leakage.