Experimental Study on the Effect of Wax Crystal-Water Droplet Aggregations on Wax Deposition of Water-in-Oil Emulsions

Abstract

Wax deposition in waxy crude oil emulsions remains a flow assurance challenge in offshore production. Wax molecular diffusion driven by the positive temperature gradient from the liquid to the pipe wall has been widely studied and regarded as the primary wax deposition mechanism; however, a considerable wax deposit of emulsions at a zero-temperature difference was detected, in which condition molecular diffusion is not likely to occur, suggesting there were other mechanisms that contribute to wax deposition. In this work, the wax deposition mechanism of water-in-oil emulsions at a zero-temperature difference was explored by a cold finger apparatus regarding the role of wax crystals and water droplets on wax deposition. Through microscopy observation and component measurement of deposits, the wax deposition mechanism, namely, wax crystal-water droplet aggregation formation and adhesion behavior, was proposed. During wax deposition, wax crystals in the emulsion interacted with water droplets, organizing the basic structure of aggregation. The liquid oil phase was entrapped at the inner space of the aggregation and entrained by the wax crystal-water droplet structure to generate a wax deposit. The formation of wax crystal-water droplet aggregations depends on the experimental temperature, where a larger size of aggregations that formed at a lower temperature preferred to yield a thicker deposit under the same stirring speed, while for the same temperature operation conditions, the medium and low speed promoted the adhesion behavior of the aggregations to the cold finger resulting in an increase in deposits; however, the high stirring condition sloughed off the formed deposits, reducing the deposit mass. The findings denote that the presence of wax crystal-water droplet aggregations in emulsion should be considered when describing the wax deposition process at low temperatures.

Wax Deposition during the Transportation of Waxy Crude Oil: Mechanisms, Influencing Factors, Modeling, and Outlook

“…It was observed that increasing the temperature difference between the oil and the wall led to the formation of smaller and denser wax crystal particles at the deposit–oil interface, while increasing the shearing in the pipe flow reduced the number of wax crystal particles precipitated at the deposit–oil interface. Moreover, as shown in Figure , Ma et al. , pointed out that At zero temperature difference, water droplets alter the original structure of waxy oil, leading to wax crystals being linked to the water droplets through interfacial adsorption, or the water droplets being enveloped within a crystal network to form wax crystal-water droplet aggregates. Migration and adhesion of aggregates to the cold wall form the wax deposits of the emulsion.…”

“…Gelation deposition mechanism of water-in-oil emulsion presented by Ma et al , (a) Water droplets engineered to act as carriers for wax crystals to mitigate wax deposition in emulsions . (b) The gel temperature and viscosity altered as wax crystal-water droplet aggregates formed . (c) Mechanism of wax deposition in emulsions .…”

“…(b) The gel temperature and viscosity altered as wax crystal-water droplet aggregates formed . (c) Mechanism of wax deposition in emulsions . a) Reproduced with permission from ref .…”

“…Jennings and Weispfennig 72 discovered that when the coldfinger temperature was lowered while maintaining a constant crude oil temperature, an increase in the oil−wall temperature difference led to a steady rise in both wax deposit 66,67 (a) Water droplets engineered to act as carriers for wax crystals to mitigate wax deposition in emulsions. 67 (b) The gel temperature and viscosity altered as wax crystal-water droplet aggregates formed.…”

“…67 (b) The gel temperature and viscosity altered as wax crystal-water droplet aggregates formed. 66 mass and the mass of the C20+ component. However, the proportion of C20+ components gradually decreased with the rise in oil−wall temperature difference.…”