Well Control - U-tube Concepts

U-tube Concepts

It is often helpful to visualize the well as a U-tube as in Figure beside. Column Y of the tube represents the annulus and column X represents the pipe (string) in the well. The bottom of the U-tube represents the bottom of the well. In most cases, there are fluids creating hydrostatic pressures in both the pipe and annulus. Atmospheric pressure can be omitted, since it works the same on both columns. If the fluid in both the pipe and annulus are of the same density, hydrostatic pressures would be equal and the fluid would be static on both sides of the tube. If the fluid in the annulus is heavier, it will exert more pressure downward and will flow into the string, displacing some of the lighter fluid out of the string causing a flow at surface. The fluid level will fall in the annulus, equalizing pressures. When there is a difference in the hydrostatic pressures, the fluid will try to reach balance point. This is called U-tubing, and it explains why there is often flow from the pipe when making connections. This is often evident when drilling fast because the effective density in the annulus is increased by cuttings.