Please refer to the attached drawing! After watching some videos about the U-tube effect in drilling, a question came to me.
Imagine one fluid of heavy density being pushed down the drilling pipe by another lighter fluid, with the help of a pump (the drawing illustrating the concept shows 10 bar for the pump but this was actually randomly put). The heavy fluid at some point will start flowing up the annulus until reaching a certain height. When the pump can no longer do the work (a static condition is then reached), we close/seal the annulus with a "packer" (the crossed rectangles you see on both sides in the drawing).
When the seals are placed, we shut down the pump, the pressure which was then applied on top of the lighter fluid to push down the heavy fluid becomes $P_{atm}$ (say 1 bar).
This is where I need help. What happens at this point ? The heavy fluid will push the lighter one back up the drilling pipe (middle tube), but a void will be created under the seals, therefore pulling back the fluids, am I correct ?
So my question is, when the pressure below the seals reaches a value close to 0 bar, how much fluid can it support ?