Capillary trapping in CO2 storage Sites: A traffic light system for preliminary leakage risk assessment

Geological storage of CO₂ offers a promising solution to mitigate global warming. In this context, the structural trapping capacity of subsurface formations is crucial. This study investigates the CO₂ storage capacity of shale caprocks under static conditions by estimating the maximum safe column height of the CO₂ plume beneath them. A modified formulation was developed to incorporate capillary pressure contributions from the reservoir rock. Depth-dependent variations in CO₂–water interfacial tension, wettability, and fluid density were calculated using experimental data and established correlations. Particular attention was given to the effect of total organic content (TOC) on the wettability of caprocks, as some organic-rich source rocks may serve as potential seals for CO₂ storage. Simple and practical equations were proposed to relate wettability to TOC and depth. The results indicate that the capillary pressure of the reservoir rock can often be neglected when estimating CO₂ column height, particularly when the ratio of the reservoir to caprock pore radius exceeds 50. However, neglecting these contributions can lead to overestimations in storage sites when reservoir rock is dense or the caprock is not strongly water-wet. For shale with low TOC (e.g., 0.1 wt%), the optimum storage depth at which the maximum mass of CO₂ can be stored is approximately 1500 m. However, as the TOC of the shale increases, the optimal storage depth and the corresponding mass of CO₂ that can be safely stored decrease significantly. A practical guideline and a traffic light system for preliminary assessment were then proposed to assess the risk of CO₂ capillary leakage. These risks are particularly significant when i) the lowest CO₂ injection point exceeds the safe column height, ii) the gravitational number is less than 0.5, iii) rocks with poor properties hinder horizontal plume propagation beneath the caprock, or iv) CO₂ leakage occurs from underlying formations.