The heterogeneities of complex reservoirs are expressed in terms of multi-scale pore structure, different pore type and multiple occurrence mode. Fluid transport mechanisms notably differ from that in conventional sandstone reservoir. Conventional core scale experimental methods are not applicable to complex reservoirs because of nanoscale pore size and strong heterogeneity. Investigating pore scale fluid flow is the key to reveal flow mechanisms while the current pore scale modelling framework fails to consider the multi-scale structure, multiphase fluid-rock interaction and confined phase change. This work leverages the recent advances in pore scale modeling methods of fluid transport in complex reservoirs. The developing trend of multi-scale digital rock construction, flow experiments and simulation methods are elaborated in detail. The mentioned pore scale modeling methods in this work form the future research paradigm for understanding fluid transport mechanisms in complex reservoirs.

Capillary trapping is an important strategy to prevent CO₂ from escaping. Meanwhile, under immiscible conditions, CO₂ may travel upwards by gravity. Studying the long-term effects of gravity and layered heterogeneity on CO₂ transport is crucial for ensuring CO₂ storage security in aquifers. In this work, fluid flow experiments driven by inertial force and gravity are conducted in a specially constructed layered sandstone. Whether driven by inertial force or gravity, the variation in CO₂ distribution in the high-permeability layer is consistently the most significant factor. In the low-permeability layer, the saturation and capillary pressure distribution of CO₂ clusters vary less and the geometric shapes are also more complex, thus the CO₂ capillary trapping in this layer is more stable. This work demonstrates that the low-permeability layer can effectively prevent CO₂ from escaping upwards when the permeability ratio between layers approaches two.