Effective permeability of two-phase flow: The role of pore-scale displacement patterns

The effective permeability is a core macroscopic parameter describing two-phase flow in porous media, and it plays a crucial role in fields such as oil and gas exploitation and CO2 geological sequestration. However, the mechanism by which it is affected by the capillary number (Ca) remains unclear. In this study, with the pore-scale two-phase displacement pattern as a bridge and combined with the numerical model constructed based on real pore structures, immiscible two-phase flow simulations were conducted over a wide Ca range of 10−6 to 10−3. The results demonstrate that significant variations in pore-scale interfacial dynamics under different Ca conditions give rise to a series of distinct displacement patterns, which are the key factors responsible for the differences in effective permeability. Specifically, the invading phase fluid consistently flows within its “unique” spatial domain at different displacement stages, and the effective permeability is largely governed by the spatial distribution characteristics of this phase. Further investigations reveal that both inactive zones and active zones coexist during the seepage process in porous media. The evolutionary characteristics of effective permeability strongly depend on the saturation properties of the active zones; in other words, the effective permeability under different displacement patterns corresponds to the intrinsic permeability associated with the active flow paths of the invading phase under that specific pattern. Finally, the aforementioned conclusions were verified by employing an effective permeability calculation method based on fluid distribution images.