I'm studying oil production and found a fact that puzzled me. It states that fluid flow downstream of the wellhead must be supercritical in order not to disturb the flow upstream of it. From PetroWiki:
A wellhead choke controls the surface pressure and production rate from a well. Chokes usually are selected so that fluctuations in the line pressure downstream of the choke have no effect on the production rate. This requires that flow through the choke be at critical flow conditions.
So I learned that (super)critical flow is when the Froude number is $\ge1$, and, according to multiple sources, the Froude number is the speed of the flow divided by some characteristic speed, which varies from case to case:
- Thermopedia seems to suggest this is the speed of sound:
A choked plane forms at this location, and further reductions in downstream pressure have no effect on conditions upstream as the rarefaction waves travel at the local sound speed and are stalled at the choked plane.
- Wikipedia seems to suggest this is the group speed of some disturbance (like ripple):
Information travels at the wave velocity. This is the velocity at which waves travel outwards from a pebble thrown into a lake.
I tend to believe more in the latter, since I find it highly unlikely that oil travels at over 1500 m/s in the piping, but I'm confused.
So, what must be the speed of the oil in the piping? The speed of sound (~1500m/s), the speed of some kind of ripple (1-10 m/s -- much more reasonable) or something else entirely?