Immiscible Fluid Displacement in Porous Media ( <i>M</i> &lt; 1): Effects of Fractal and Heterogeneous Pore Structures on the Displacement Process

To investigate the flow mechanisms and efficiency response of immiscible two-phase displacement in complex porous media under unfavorable viscosity ratios (<i>M</i> < 1), we performed visualization experiments using microfluidic platforms. Four homogeneous pore-network micromodels with distinct fractal dimensions, together with a layered heterogeneous model, were fabricated to systematically examine the effects of three viscosity systems (low/medium/high) and two injection rates on displacement behavior. The results show that pore-structure complexity governs displacement patterns. In the low- and medium-viscosity systems, increasing the injection rate stabilizes the displacement front and partially offsets the front perturbations induced by increasing fractal dimension, thereby significantly improving oil recovery. In contrast, in the high-viscosity system, viscous fingering and channeling dominate; thus, a higher injection rate intensifies fingering and reduces displacement efficiency, resulting in a markedly weakened correlation between fractal dimension and recovery. Layered heterogeneity further amplifies spatial disparities by altering local pressure gradients and the distribution of flow resistance. Specifically, low-viscosity conditions promote branching and bypassing, medium-viscosity conditions exhibit a competitive interplay between capillary and viscous forces, and high-viscosity conditions favor early breakthrough in high-porosity zones while leaving low-porosity zones bypassed. Pore-scale residual-oil analysis further reveals that Clustered oil predominates across most conditions (>60%), and its morphological distribution is modulated by both fractal dimension and heterogeneity. Overall, this study clarifies how fractal complexity and macroscopic heterogeneity regulate immiscible displacement, providing an experimental basis for optimizing waterflooding strategies in structurally complex reservoirs.