Estimating a baseline of soil CO2 flux at CO2 geological storage sites

This study aims to determine a baseline for natural soil carbon dioxide (CO2) flux at the surface based on long-term field measurements, with the ultimate purpose to detect the gas leakage atCO2 geological storage sites. CO2 surface monitoring is a tool that measures the safety and effectiveness of CO2 capture and storage (CCS), a technology which is believed to be a reliable approach to mitigate the CO2 emission. However, the fluctuations of naturally occurring CO2 in soil layers complicate the leakage detection as the soil connects both the underground layers and the atmosphere. In this regard, this study not only investigates the natural surface CO2 flux behavior but also develops an equation to estimate the surface CO2 flux with respect to the soil moisture content and temperature. To meet this end, two values within the CO2 flux equation were defined and calculated based on the field measurements; a,representing a water saturation–dependent value, and b,representing the temperature sensitivity (independent of the water saturation). The results show a good agreement between estimated and measured data. Upon which, the maximum baseline for surface CO2 flux was derived and used as a threshold to detect the potential CO2 leakage in the candidate field (INAS, Japan).