Macro-Meso-Micro Cross-Scale Correlation of Rheological Evolution in Waxy Crude Oil Emulsions under Dynamic Cooling Conditions

Abstract The rheological deterioration of waxy crude oil emulsions poses a significant challenge to the flow assurance of subsea and long-distance pipelines. By integrating simultaneous rheology in situ microscopy with molecular dynamics (MD) simulations, the evolution of apparent viscosity was investigated under varying temperatures and shear rates. A cross-scale investigation was conducted to establish the correlations among macroscopic rheological response, mesoscopic structural evolution, and microscopic molecular interfacial behavior. The results indicate that the stepwise elevation in apparent viscosity during cooling is attributed to the formation of a composite network consisting of precipitated wax crystals and entrained water droplets. At a constant temperature (16 °C), the apparent viscosity exhibits a pronounced sensitivity to the water cut, increasing approximately 7.5-fold as the water cut rises from 10% to 40%. The mesostructure of high-water-cut systems is characterized by greater complexity, where dense droplets act as discrete resistance units that significantly impede fluid flow. Under shearing, the deconstruction of the three-dimensional wax-water network induces a distinct shear-thinning behavior. The heightened shear-thinning effect in high-water-cut emulsions is derived from the structural instability of Structure C composite units, which are dominated by heterogeneous interfaces and prone to disintegration. MD simulations further reveal a hierarchical mechanical resistance at the micro-interface, explaining the stability variations in mesoscopic aggregates. Specifically, the bonding strength of homo-aggregates significantly surpasses that of heterogeneous flocs interfaces; the latter exhibits interaction energy per unit area, peak stress, and fracture work of only 25.32%, 31.98%, and 10.96% of the former, respectively, identifying them as the structural weak points within the bulk mechanical network. Through the development of a quantitative model linking mesoscopic units with microscopic interfacial attributes, a meso-micro cross-scale correlation was established. The underlying mechanism is thus elucidated: macroscopic shear stress preferentially ruptures the heterogeneous bridging interfaces with lower micro-bonding energy, triggering a bottom-up cascading fragmentation of the mesoscopic network (Structure C → Structure B → Structure A), which macroscopically manifests as a sharp decline in apparent viscosity. Targeted weakening of the heterogeneous bridging interfaces provides an effective regulatory strategy for further enhancing the flowability of the system.