Large-Scale Experiments on Slug-Length Evolution in Long Pipes

We present a unique set of two- and three-phase slug-flow experiments conducted in a 766-m-long, 8-in. pipe at 45-bara pressure, using Exxsol™ D60 fluid (ExxonMobil Chemical, Houston, Texas, USA) as the oil phase and nitrogen as the gas phase. The first one-half of the pipe was horizontal, while the second one-half was inclined by 0.5°. A total of 10 narrow-beam gamma densitometers were mounted on the pipe to study flow evolution, and in particular slug-length development. The results show that the mean slug length initially increases with the distance from the inlet, but this increase slows down, and the mean slug length typically reaches a value between 20 and 50 diameters at the outlet. At low mixture velocities (<3 m/s), the slug-length distributions tend to be extremely wide, sometimes with standard deviations approaching 100%. The longest slugs that we observed were more than 250 pipe diameters (50 m). At higher mixture velocities (>3 m/s), the slug-length distributions are in general narrower. The effect of the water cut (WC) on the slug-length distribution is significant but complex, and it is difficult to establish any general trends regarding this relationship. Finally, it was observed that slug flow often requires a very long distance to develop. Specifically, in most of the slug-flow experiments, the flow regime 57 m downstream of the start of the horizontal section was not slug flow.