Non‐Monotonic Dependence of Mixing on Phase Saturation in 3D Porous Media

Abstract Two‐dimensional (2D) experiments and simulations are commonly used as a representations of three‐dimensional (3D) flow and transport in porous media. However, these 2D approaches may yield misleading results due to fundamental differences in flow dynamics between 2D and 3D systems. This discrepancy becomes especially pronounced under unsaturated conditions, where phase connectivity and flow pathways are highly sensitive to saturation in 2D. In this study, we perform high‐resolution 3D pore‐scale simulations to quantify mixing‐limited reactions across a wide range of water saturations. Our results reveal two key findings: (a) a non‐monotonic relationship between reaction product mass and saturation emerges in 3D, with peak reactivity occurring at intermediate saturations; and (b) the pronounced mixing enhancement often reported at low saturations in 2D simulations is substantially weaker in 3D. Together, these findings challenge the validity of 2D experiments and simulations and highlight the critical role of dimensionality in controlling pore‐scale mixing and reactivity.