Reply to Discussion: “Haiyang Zhang and Muhammad Arif. Residual trapping capacity of subsurface systems for geological storage of CO₂: Measurement techniques, meta-analysis of influencing factors, and future outlook. Earth-Science Reviews (2024): 104764.”

Geological storage of CO₂ is a promising technique to mitigate anthropogenic CO₂ emissions. The effectiveness of CO₂ storage in the subsurface formations relies on various trapping mechanisms that immobilize the injected CO₂. Among these mechanisms, residual trapping has been identified as a critical factor, closely associated with residual CO₂ saturation. The extent of residual CO₂ saturation is strongly influenced by the petrophysical physicochemical and hydrodynamic properties of CO₂/fluid/rock systems and operational conditions, thereby governing the overall residual trapping efficiency. This article reviews the published experimental datasets on the initial and residual CO₂ saturation and analyzes the corresponding trapping efficiency for a range of in-situ CO₂/fluid/rock systems. We explore the factors that influence trapping efficiency, including wettability, rock type, rock properties, and flow rate. The gas saturations and trapping efficiencies of different gas types (i.e., CO₂, N₂, and H₂) are also discussed. Finally, we present the knowledge gaps and outline prospects for future research. This review establishes a state-of-art data repository of gas saturations in different conditions, enhancing our understanding of residual trapping in subsurface gas storage.