Settling of Wax Particles in Near-Gelling Systems Under Quiescent Conditions

摘要

During a shutdown of crude oil production from offshore fields, the trapped fluid inside the riser and flowlines starts cooling down, triggering the precipitation of the dissolved wax particles when the temperature drops below the Wax Appearance Temperature (WAT) of the mixture. This work aims to study the settling tendencies of precipitated wax under quiescent conditions when the final temperature of the crude oil is below its WAT but above or near its pour point. An experimental test facility was designed and constructed to simulate subsea cooling of crude oils under quiescent conditions. A series of experiments were carried out with different model oils and wax concentrations to study gravity settling. Moreover, modifications were made to an existing hindered particle settling model and its application to wax settling scenarios was assessed by comparing the results obtained thereof with the results obtained experimentally. The results obtained from the experimental runs show that gravity settling can be appreciable under certain operating conditions and fluid properties. It was observed that waxy model oils which have pour point (PP) temperatures above the surrounding ambient temperature tend to form gels as the fluid temperature decreases below the PP during the quiescent conditions. However, in cases where the pour point is close to or below the surrounding ambient temperature, precipitated wax particles may have a tendency to settle under gravity instead of gelling up. The results also portray the effect of wax concentration on the settling behavior. Understanding the behavior of precipitated wax particles in quiescent conditions under the influence of gravity in different systems of varying wax concentrations can help in identifying and remediating operational difficulties associated with wax precipitation during shutdown. The effect of gravity on wax particles in vertical pipelines during shutdown has not previously been studied much and the settling phenomenon in such situations is not well-known. Due to the effects of changing temperature, particle shape and other factors, gravity settling of wax particles in such systems is different from other systems where a particle is settling in a fluid under gravity. This work provides some input on certain key aspects of this phenomenon.