Cross-scale studies in geomechanical and hydrological systems employ a variety of approaches, either experimental, simulation or theoretical, each characterized by corresponding scale-specific methodologies. This perspective identifies and discusses challenges encountered at various scales, ranging from molecular to field scale, and examines issues related to integrating these scales. It highlights discrepancies in resolution and data compatibility, emphasizing the necessity for improved scale transition techniques. Insights and recommendations are proposed for future research to enhance multiscale modeling frameworks. These suggestions are crucial for bridging knowledge gaps on geological systems and improving the analyses accuracy for better engineering applications or earth system modelling.

The underground storage of gases, such as CO₂ and H₂, in the porous media is a critical component for achieving carbon neutrality and economical energy storage. While previous research has predominantly focused on gas injection in one piece of uniform porous media, and gravity is often neglected, the reality is that natural storage formations are typically multi-layered porous systems. An in-situ gas injection apparatus based on high-resolution micro-CT was utilized to investigate gas injection behaviors and failure patterns in layered porous media systems. The system includes a reservoir layer and a cap layer, where both capillarity and permeability are meticulously controlled. Our findings reveal that all cases experience cycles of a pressure built-up period and a sudden pressure release when a barrier, either capillarity or effective stress, is overcome. Drainage conditions within the layered system significantly impact both the volume of gas trapped and the failure patterns observed. Effective stress analyses show that the key determinants of failure patterns are capillarity, effective stress, and excess pore fluid pressure, affected by pore size, cap layer thickness, gas injection rate and permeability. Five distinct failure patterns are categorized: capillary invasion, fracture opening, integral uplifting, local heaving, and violent liquefaction-based on two dimensionless parameters. This work provides new insights into understanding the gas injection dynamics in layered porous media.