In oil fields, the
formation of water-in-waxy crude oil emulsion
is inevitable. The dissolved/crystallized state wax can interact with
asphaltenes and then greatly affect the emulsion stability. However,
studies on this aspect are still insufficient. In this work, the effects
of the test temperature (30 °C well above the wax appearance
temperature (WAT) and 15 °C well below the WAT) and asphaltene
concentration (0∼1.5 wt %) on the stability of the water-in-model
waxy crude oil emulsions containing 10 wt % wax were systematically
investigated. When the model crude oils contain no wax, the flowability
of the oils is good and the asphaltene concentration has little influence
on the oil rheology. Increasing the asphaltene concentration facilitates
the adsorption of asphaltenes to the oil–water interface, thus
reducing the interfacial tension and water droplet size while enhancing
the interfacial dilatational modulus. The stability of the emulsions
improves with the increase in the asphaltene concentration, but the
emulsions are still unstable. When the model crude oils contain 10
wt % wax, the WAT slightly decreases from the initial 25 to 24 °C
after the addition of asphaltenes. The oil rheology is greatly improved
by the addition of 0.05 wt % asphaltenes. With the further increase
of the asphaltene concentration, the improved rheological ability
of the asphaltenes deteriorates rapidly. At the asphaltene concentration
of 1.5 wt %, the oil rheology is dramatically aggravated. The stability
of the emulsion containing 10 wt % wax is mainly controlled by two
aspects: on the one hand, the dissolved-state wax (30 °C) could
facilitate the adsorption of asphaltenes to the interface, further
reduce the interfacial tension and the water droplet size, and enhance
the interfacial dilatational modulus; on the other hand, the wax crystals
precipitated in the oil phase (15 °C) can form a stronger network
structure at relatively high asphaltene concentrations (0.5∼1.5
wt %) and then immobilize the water droplets. The above two aspects
greatly improve the sedimentation and coalescence stabilities of the
emulsions at 15 °C. In addition, we did not find persuasive evidence
showing that the wax could crystallize around the water droplets and
strengthen the oil–water interfacial films.