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 andrnflowlines starts cooling down, triggering the precipitation of the dissolved wax particles when therntemperature drops below the Wax Appearance Temperature (WAT) of the mixture. This work aims tornstudy the settling tendencies of precipitated wax under quiescent conditions when the final temperature ofrnthe crude oil is below its WAT but above or near its pour point.rnAn experimental test facility was designed and constructed to simulate subsea cooling of crude oilsrnunder quiescent conditions. A series of experiments were carried out with different model oils and waxrnconcentrations to study gravity settling. Moreover, modifications were made to an existing hinderedrnparticle settling model and its application to wax settling scenarios was assessed by comparing the resultsrnobtained thereof with the results obtained experimentally.rnThe results obtained from the experimental runs show that gravity settling can be appreciable underrncertain operating conditions and fluid properties. It was observed that waxy model oils which have pourrnpoint (PP) temperatures above the surrounding ambient temperature tend to form gels as the fluidrntemperature decreases below the PP during the quiescent conditions. However, in cases where the pourrnpoint is close to or below the surrounding ambient temperature, precipitated wax particles may have arntendency to settle under gravity instead of gelling up. The results also portray the effect of waxrnconcentration on the settling behavior. Understanding the behavior of precipitated wax particles inrnquiescent conditions under the influence of gravity in different systems of varying wax concentrations canrnhelp in identifying and remediating operational difficulties associated with wax precipitation duringrnshutdown.rnThe effect of gravity on wax particles in vertical pipelines during shutdown has not previously beenrnstudied much and the settling phenomenon in such situations is not well-known. Due to the effects ofrnchanging temperature, particle shape and other factors, gravity settling of wax particles in such systemsrnis different from other systems where a particle is settling in a fluid under gravity. This work providesrnsome input on certain key aspects of this phenomenon.