Effect of Diluents on Interfacial Properties and SAGD Emulsion Stability: I. Interfacial Rheology

Abstract

Abstract In SAGD (Steam Assisted Gravity Drainage) operations the produced fluids are a complex water-in-oil-in-water (W/O/W) emulsion. A diluent, which is a major operational cost item, is often added to the produced fluid to reduce the density and viscosity of the heavy crude, thereby facilitating oil and water separation. The variable quality of the diluent and various types of diluent can increase the emulsion stability and cause the dehydration of the oil to be more difficult as there are more surface-active agents added to the oil from the diluent stream. In this work, the effect of various diluents on interfacial films was studied by measuring interfacial elasticity and "dynamic" surface pressure isotherms and then correlated with emulsion stability and oil dehydration. Compressibility, crumpling film ratio (ratio of the compressed surface area to the original area at which the interface starts to collapse), and surface pressure were determined from the surface pressure isotherms. When interfacial rheological measurements were conducted on fluids treated with the diluent, the system behaved as if a bidimensional gel near its gelation point had organized at the interface. In other words, adsorbed amphiphilic materials such as asphaltenes and resins had self-aggregated at the interface and formed a network, which presents all the rheological characteristics of a gel, acting as a stabilizing mechanism for produced emulsions. When the interface was aged for 20 hrs, its gel strength was nearly 70% higher than the fresh interface, suggesting an explanation why aged emulsions are more difficult to treat. As the surface area of an oil droplet submersed in water is reduced, asphaltene molecules adsorbed onto the interface are drawn closer together and resist compression. This gives information about how rigid the drop surface becomes. The software of interfacial rheological measurements can be set up to capture drop images during compression measurements, allowing us to observe the formation of rigid interfacial skins at the oil-water interface. The amount of skin formed at a droplet surface increased with increasing asphaltene concentration and varied in different diluents. Unlike elasticity and interfacial tension measurements, this method can predict emulsion stability in different diluents and at various asphaltene concentrations. A correlation exists between the interfacial compressibility, crumpling film ratio, and emulsion stability: the higher the crumpling film ratio and the lower the compressibility, the more stable the emulsion. This new test method gives more insights into mechanisms of emulsion stabilization caused by the diluent and asphaltene and potentially can be employed to study the structure and demulsifying performance relationships of EBs (emulsion breakers) and REBs (reverse emulsion breakers).