Mechanistic investigation of ultrasonic demulsification of water-in-oil emulsions.

Oilfield-produced fluids commonly contain high water fractions and form stable water-in-oil (W/O) emulsions, complicating downstream treatment and resource recovery. In this paper, experimental characterization and numerical simulation were combined to elucidate the power-dependent mechanism of ultrasound-assisted chemical demulsification. We first optimized ultrasonic (US) power, irradiation time, and demulsifier concentration through an orthogonal experimental design and then conducted single-factor experiments to examine changes in dehydration efficiency, viscosity, and droplet characteristics. On this basis, confocal laser scanning microscopy (CLSM) was used to relate the redistribution of light and heavy oil-phase components to droplet-morphology evolution, and a Pressure Acoustics, Frequency Domain model was finally developed in COMSOL Multiphysics to interpret the experimental observations in terms of acoustic-pressure distribution and cavitation-regime evolution. The results show that a maximum dehydration efficiency of 70.13 % was achieved at 80 %P, 9 min, and 75 ppm. With increasing US power, dehydration efficiency and mean droplet diameter first increased and then decreased, whereas droplet number exhibited the opposite trend and emulsion viscosity decreased continuously. CLSM revealed oil-phase component redistribution during water-droplet coalescence. Consistent with these experimental observations, the calibrated acoustic-field model indicated a larger region classified as effective cavitation at 80 %P and the emergence of regions classified as excessive cavitation at 100 %P under the adopted pressure thresholds. These findings suggest that effective cavitation promotes interfacial-film disruption and droplet coalescence, whereas excessive cavitation induces droplet breakup and re-emulsification, explaining the non-monotonic dehydration response to US power.