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This next example is on gas hydrates formation and flow assurance. This work looks at the gas hydrates formation process pictorially shown here. An initial water droplet will have a hydrate film grow upon it, thicken, and ultimately fully convert into a hydrate. These hydrates may agglomerate and plug a pipeline, causing flow assurance issues. In this work, methane hydrate formation dissociations were studied in the laboratory. This work was published by the Colorado School of the Mines in conjunction with Chevron. In these experiments, Conroe crude oil was placed in water, hydrates were formed under pressure of 77 bar and 4 °C, and subsequently dissociated at a temperature of 20 °C. Both the ParticleTrack with FBRM and ParticleView with PVM were placed inside this pressure cell. The motivation behind this was to understand the hydrate formation dissociation mechanisms under higher water cut conditions that is greater than 60 % water by volume, as is typically found in older or less profitable wells.
Here we have a sequence of PVM images taken at 4 °C. Initially, once these water droplets are in the continuous phase of oil, the hydrate film in image B, starts to form around the outside of these droplets, and in image C the hydrates start to agglomerate. Then, the temperature is raised to 20 °C, and these hydrates again begin to dissociate back down to individual flakes. Finally, the system returns to a new equilibrium with occluded oil droplets.
In the next example, FBRM was also used to measure flow assurance in a pipeline by IFP and Total in France. There were two experimental set ups, the first of which was an Archimedes Loop, which was temperature controlled from 273 – 283 K. In order to avoid hydrate crushing, no pumps were used, gas lift was used to regulate the flow, the system had an overall length of 36 m, and an internal diameter of approximately 1 cm. The second experimental set up involved the use of a Lyre Loop. The Lyre Loop was similarly controlled from 273 – 323 K. The flow was held constant, despite changes in solution viscosity, a Moineau pump was used to regulate this flow. This was a much larger system, with an overall length of 140 m, and an internal diameter of nearly 5 cm.
Looking at the Archimedes Loop, temperature and pressure are monitored from the onset of hydrate formation, through to stoppage of flow. Looking here at the temperature, pressure, and mean velocity information, we can couple these factors in order to generate a friction factor; if you look at the FBRM data taken concurrently, we see that there is an increase in the FBRM mean dimension at the same time the friction factor is increased. This is associated with the agglomeration in the system, FBRM can be used to directly monitor the rate and degree of agglomeration, and identify conditions where agglomeration occurs, and plugging is probable.
For the Lyre Loop, FBRM was used to characterize the breakage of hydrates under shear. The upper graph shows the temperature and friction factor of the Lyre Loop as a function of time. Initially there is an emulsion present, as shown by the FBRM chord length distribution in the lower left, which is unimodal, with most of the material being under 10 microns in dimension. As the friction factor increases, here at time 180 minutes, we see that the distribution is now bimodal. This indicates that not only do we have the initial emulsion, but we also have the formation of hydrates. This continues on at time 240 minutes. In the case of the Lyre Loop, a high level shear is generated in order to keep the constant velocity of the system. This shear results in the breakage of hydrates. We are able to track that very effectively with the FBRM, as illustrated by the difference between the two distributions measured at 240 and 320 minutes, where we see a clear reduction in the amount of material above 100 microns in dimension.