Study On Freeze-Thaw Stability Of Crude Oil Emulsions

The crude oil containing water is harmful to the transportation, refinery, and quality of the products so that demulsification is one of the key steps in various sectors of the oil industry, including crude oil transportation, petroleum refinery, as well as oil production. The influencing factors on the stability of emulsion and the demulsifying mechanism were studied in this paper from a new point of view---- freeze and thaw.The important components in the crude oil emulsions of the enhanced oil recovery system's components (ASP: alkali-surfactant-polymer), nature surfactants (asphaltene and resin), inorganic salts and solid particles were chosen to study the freeze-thaw stability of the emulsions. Their roles in stabilizing water-in-crude oil emulsions were investigated by measuring the boundary tension, Zeta potential of the dispersive drops, freezing temperature, dehydrating ratio of the model emulsions which were demulsified via the freeze/thaw method and the light transmittance of separated water. Three thawing types were used in the demulsifying experiments: air (room temperature), 40°C of water bath, and microwave irradiation. At last, the relations were analyzed between the freezing velocity and the stability of the emulsions.The results indicate that the influences of ASP on the surface tension of emulsions are limited. The surface tensions decline with the increase of the asphaltenes or resins concentration, while it increases with the content of graphite or clay particles added. The cations with high valence number are more effective in raising the surface tensions of the emulsions than that with low valence number. Among the ASP's components, the action on the absolute value of Zeta potential is largest with surfactant added, while it is smallest with the polymer. The absolute value of Zeta potential rises with the increase of asphaltenes and resins concentration, while this changing trend is reverse with clay particle added.Surfactant (petroleum sulfonate) and alkali (Na2CO3) can influence the dehydrating ratio obviously in the freeze-thaw demulsifying experiments. The dehydrating ratios decrease with the rise of the surfactant, alkali, asphaltenes or resins concentration. Addition of inorganic salts is helpful for the enhancement of the dehydrating ratio, especially with Al3+ ion added. A transition location, or call it a minimum point, appears on the curve of the dehydrating ratio. The dehydrating ratios of the O/W emulsions decline with the adding amount increasing of clay. The petroleum sulfonate and polymer (HPAM) can affect the light transmittance of separated water (LTSW) seriously among the components of ASP. The increase of concentration of the petroleum sulfonate, HPAM, asphaltenes, resins and solid particles can reduce the LTSW. The LTSW with bivalent or trivalent ions added is higher than that with monovalent ion. Thawing with microwave radiation yields the highest dehydrating ratio in the demulsifying experiments on the ASP emulsions, asphaltenes or resins stabilized emulsions and W/O emulsions with inorganic salts or graphite added.The forces acted on the dispersive particle during the unidirectional freezing process were also studied in this paper. The critical value of freezing velocity was obtained. It goes against the stability of the emulsion if the moving velocity of the freezing interface is lower this value. Thermal conductivity's ratio of oil and water can influence the figure of freezinginterface, and dispersive phase having larger thermal conductivity than continue phase is helpful for the freeze-thaw stability of emulsion.This paper provides a theory direction and abundant experiment data for the improvement of crude oil dehydration process in oil field. It can also be referenced for the studies of freeze-thaw stability on other types of emulsions.