Understanding the microscopic characteristics and evolutionary patterns of pore structures during high-PV waterflooding is critical for improving the accuracy and efficiency of oil field development. While previous studies have primarily emphasized the geometric and morphological features of overall pore structures, they often overlook local pore-scale properties and their relationship with fluid transport capacity. This study proposes a novel classification method for microscopic pore structures that integrates both pore size and local flow conductivity, enabling a more physically grounded and quantitatively robust evaluation of pore systems across rocks with varying permeabilities. The classification scheme divides microscopic pores into six distinct types based on key parameters such as pore diameter and flow flux area. To validate this approach, high-PV waterflooding experiments were performed on six sandstone samples with different permeabilities. High-resolution micro-computed tomography (micro-CT) imaging was employed to capture the internal pore structures before and after flooding. The results reveal that while low-connectivity small pores dominate numerically across all samples, high-connectivity small pores account for the largest volumetric share in medium-permeability rocks. Although overall pore size distributions remain relatively stable during high-PV waterflooding, transitions between pore types occur, driven by localized structural changes. Notably, in medium-permeability rocks, the number of low-connectivity small pores increases, whereas high-connectivity small pores decline. These findings deepen our understanding of microscopic heterogeneity and provide a theoretical foundation for evaluating the occurrence of residual oil. Moreover, the proposed classification framework offers valuable guidance for optimizing enhanced oil recovery strategies in the late stages of ultra-high water cut development.

The immiscible displacement behavior in porous media is crucial for oil recovery and subsurface remediation, yet how wettability influences this process across different pore structures remains unclear. Using a color-gradient lattice Boltzmann model, this study investigates immiscible displacement dynamics in porous media. Heterogeneous porous structures with various degrees of permeability were reconstructed using the quarter structure generation set algorithm, and wettability effects were analyzed with the contact angle set in the range of 30◦to 150◦. The numerical results showed that porous heterogeneity greatly affects the displacement efficiency via permeability-dependent flow pathway optimization. Enhanced efficiency was observed in high-permeability media through low-tortuosity channels, whereas low-permeability systems exhibited reduced efficiency due to capillary trapping in pores with lower flow capacity. Wettability alters displacement patterns via capillary forces – under hydrophilic condition, the displacing fluid preferentially enters smaller pores. Fractal dimension and Euler number were used to quantify flow heterogeneity, revealing that increased permeability reduces flow complexity and improves connectivity. Moreover, permeability heterogeneity and wettability interact to disrupt classical linear flow responses, leading to non-monotonic efficiency trends in lowpermeability systems. These findings highlight the importance of pore-scale multiphase flow in heterogeneous media and offer new insights for predicting wettability effects in subsurface flows. Document Type: Original article Cited as: Gong, W., Liu, Y., Dai, C., Zheng, J., Chen, Z., Zhao, W., Li, J. Non-monotonic effect of permeability and wettability on immiscible displacement dynamics in porous media. Capillarity, 2025, 17(2): 54-67. https://doi.org/10.46690/capi.2025.11.02