We can likely use the behavior of one of the fluid columns to describe behavior regarding what is happening in another side of fluid column, if two fluid columns are connected at bottom. Basically, this situation is simply described in common oil filed name as “U Tube”.
In oil field especially drilling business, “U Tube” can be considered as a string of pipe (drill pipe and tubing) is in a wellbore and fluids are able to pass inside of string of pipe (drill pipe and tubing) and the annulus (area between wellbore and string of pipe). The figure below demonstrates “U Tube” in our drilling business.
Why is U-Tube very important?
It is very vital to keep a basic concept of U-Tube in mind.
If there are two different fluids between inside of string and annulus, fluids always flow from a higher pressure area to a lower pressure.
If the system is NOT closed, lighter fluid will be flown out and it will be stopped when system pressure is stabilized (see figure below).
If the system is closed, pressure must be the same at the bottom point where both sides of U-tube are connected. Therefore, drill pipe pressure and casing pressure (annulus pressure) will be responded based on fluid in each side and formation pressure at bottom hole (see figure below).
Please always remember that U-Tube concept can be widely applied in many drilling and workover application such as well control, cementing, etc.
Reference book: Well Control Books