Predicting water-in-oil emulsion coalescence from surface pressure isotherms
Stable water-in-crude oil emulsions are spontaneously formed during oil production when oil and water are stirred together and naturally occurring surfactants such as asphaltenes, resins and clays are also present. The surfactants stabilize these emulsions by forming highly viscous or rigid films at...
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| Format: | Dissertation |
| Language: | English |
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| Online Access: | Get full text |
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| Summary: | Stable water-in-crude oil emulsions are spontaneously formed during oil production when oil and water are stirred together and naturally occurring surfactants such as asphaltenes, resins and clays are also present. The surfactants stabilize these emulsions by forming highly viscous or rigid films at the oil-water interface. For economical and operational reasons, water-in-crude oil emulsions need to be destroyed in order to recover both oil and water phases. To develop more effective emulsion treatments it is necessary to have a better understanding of the factors that affect emulsion stability.
Asphaltenes play an important role in the stability of water-in-oil emulsions because they irreversibly adsorb at surface of the water droplets and form a rigid film (skin). One hypothesis is that the coalescence of these emulsions depends on the compressibility of this asphaltene film. In this work, a new experimental technique was developed to determine the compressibility of asphaltene monolayers from surface pressure isotherms measured with an IT Concept axisymetric drop shape analyzer. Surface isotherms show the relationship between interfacial tension and interfacial area and reflect the compressibility and "phase behavior" of interfacial film.
A droplet of a solution of asphaltenes, n-heptane and toluene was formed and aged at the tip of a capillary in an aqueous medium. Then fluid was withdrawn to decrease the surface area of the drop and compress the interfacial film. The compression was done in steps at intervals of approximately 20 seconds and at each step, time, surface pressure, area and volume data was collected. Surface pressure was plotted versus film ratio, where the film ratio is the fraction of the droplet surface area at a given compression to that of the original drop.
The effects of asphaltene concentration, solvent, and aging time on the film properties were determined. Irreversibly adsorbed films were observed to form rapidly at all asphaltene concentrations and rigid films form least rapidly at intermediate asphaltene concentration (10 kg/m3). A "phase change" from a compressible film to an almost incompressible film occurred upon compression in most cases. At sufficient compression, the film became completely incompressible and crumpled. The film ratio at which the "phase change" occurs, increases in poorer solvent and as the interface is aged.
The coalescence rates of model emulsions, consisting of asphaltenes, toluene, n-heptane, and water, were determined from the change in the mean drop diameter over time. A correlation between the initial coalescence rate and the initial compressibility of the asphaltene film was found. The measured mean droplet diameter of the coalescing emulsion was then predicted over time from the film compressibilities, accounting for aging time and the change film ratio as the emulsion coalesced. |
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| Bibliography: | Source: Masters Abstracts International, Volume: 45-05, page: 2561. |
| ISBN: | 0494256753 9780494256756 |