Inversion of heavy crude oil-in-brine emulsions
A large portion of Canada's reserves of crude oil consists of extra heavy crude and natural bitumens. As the reserves of conventional crude oil continue to decline, heavy oil and bitumen are becoming increasingly important sources of hydrocarbon liquids. Oil-in-water emulsions provide an alternative to heating and dilution methods for transportation of these viscous heavy oils through conventional pipelines. One of the most important preconditions for implementing this technology is prediction of the flow behavior and stability of the heavy oil-in-water emulsions. Inversion, a process by which one type of emulsion (such as oil-in-water) is converted to another (such as water-in-oil), is critical because it could cause a region of very high viscosity to form in the pipeline. This research attempts to investigate the causes of inversion, and to develop a simple and effective laboratory method for selecting suitable surfactants and their concentrations at the emulsion pipeline operation conditions. The inversion of heavy crude oil-in-brine emulsions stabilized with non-ionic surfactants has been studied experimentally. Tests were carried out in beakers and in a cone and plate viscometer. The effects of shear rate, surfactant species and concentration, temperature, and oil fraction on emulsion inversion were studied. Toroid tests were carried out to compare the data generated from the cone and plate viscometer to that for a pipeline. It was found that inversion of oil-in-brine emulsions was always associated with the disappearance of the non-ionic surfactant from the aqueous phase. The surfactant affinity (i.e., hydrophilic property) for the aqueous phase thus appeared to be critical in the stabilization of these emulsions. Increasing the number of ethylene oxide units increased the hydrophilic property of the non-ionic surfactant and the stability of the emulsion. The emulsion stability also depends on the strength of the interfacial adsorption film. High shear rates not only destroyed the film, but also displaced fragments of the film back to the aqueous phase. The emulsions remained stable only when the surfactant concentration in the aqueous phase was above the C.M.C. level, meaning there was enough surfactant stock to maintain the stability of the interfacial adsorption film. High temperatures reduced the physical strength of the interfacial adsorption film and the repulsive force between oil droplets, encouraging the flocculation and inversion. The toroid tests indicated that the cone and plate viscometer is probably a good simulation of the emulsion flow behavior in pipelines. A laboratory method to select a suitable surfactant type and concentration has been developed using the cone and plate viscometer.