Structural failure process of gelled waxy crude oil emulsion based on rheological-in-situ microscopic synchronous measurement

LAOS tests were conducted on waxy crude oil emulsions with varying water content accompanied by in situ microscopic observation, which provides insight into the dynamic response of flocculation network structure and its effect on macroscopic viscoelasticity. The emulsion gelation has the composite structure character, whose failure is the result of the interactions at different levels and scales. The failure evolution can be divided into five stages. Stage I: Alteration of surface molecular interaction; Stage II: Structure and morphology changing of force chain net; Stage III: Edge fracture of the bulk flocculation structure; Stage IV: Regional fracture of the bulk flocculation structure; Stage V: Complete cracking of the bulk flocculating structure. Structural changes in Stages I and II occur at the nanometer scale, and the transition from the linear viscoelastic region to the nonlinear viscoelastic region and the yield process cannot be observed under microscope. Only when the strain exceeds 3 times the yield strain value can significant changes be observed. Two newly defined strain values including "initial visual structural change strain gamma i" and "visual structure cracking strain gamma d" have been presented to describe the signature changes in the microstructure and their specific values have been obtained. The interaction between wax crystals and the resulting network structure primarily governs the system's structural strength, brittleness, and stability. After the failure of the network structure of wax crystals, the determinants are transformed into interactions between emulsified water droplets and the resulting flocculation structure. Therefore, the relationship between water content and structural stability demonstrates an inverse trend across different strain ranges, with the transition point occurring at a strain value of 21.8%.