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 at CO₂ 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).