Temporal and Spatial Evolution of Oil and Water Emulsion at Static Conditions

Abstract

Abstract Understanding emulsion evolution at static conditions is crucial for pipeline operations during shut-in and startup and separator optimal design. This study experimentally investigated the temporal and spatial evolution of oil-water emulsion characteristics at static conditions, which include rheological behaviors, droplet size and distribution, water-cut/density, and stability. Emulsion samples were created in a mixer at ambient conditions, which were transferred into graduated glass cylinders where the volumes of separated phases were measured at different times. Phase density, rheological behavior, and droplet sizes were also measured at different times and positions in the cylinders. Isopar V and tap water were used as the testing fluids and Span80 as the surfactant. Four water cuts, two mixing speeds, and two surfactant concentrations were investigated. Experimental results showed that the apparent viscosity of the emulsion layer increased significantly with time and the shear-thinning behavior became more obvious over time. The emulsion apparent viscosity also variated vertically along with the glass cylinder. These phenomena were believed to be related to water volumetric fraction which was estimated from density measurements. The results showed that the water cut (or density) in the emulsion layer increased with time and eventually plateaued out near the inversion point, consistent with the rheological behaviors. Besides, it was observed that the droplet sizes increased with the depth, which could contribute to the gradually stronger shear-thinning behaviors. The water-in-oil emulsion became less stable at lower water cuts, lower surfactant concentrations below CMC, and slower mixing speeds. It was observed that lower surfactant concentration or slower mixing speed resulted in faster separation, leading to a faster increase of the apparent viscosity and density of the emulsion layer.