BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
U-Tube Concept
The interrelation of formation pressure and hydrostatic pressure is more easily understood if
the well is thought of as being a large U-tube as shown in Figure12-11. The sum of the
pressures on the drillpipe side of the U-tube must be equal to the sum of the pressures on the
annulus side.
Sum of Pressure - Drillpipe Side
• Hydrostatic pressure = 5200 psi
• Shut-in drillpipe pressure (SIDP) = 0 psi
• Bottomhole pressure = 5200 psi + 0 psi = 5200 psi
Sum of Pressure - Casing Side
• Hydrostatic pressure = 5200 psi
• Shut-in casing pressure (SICP) = 0 psi
• Bottomhole pressure = 5200 psi + 0 psi = 5200 psi.
Figure 12 - 11 “U” Tube Static
With the pump off and hydrostatic pressure overbalancing formation pressure (0.052 x 10,000
ft x 10 lb m /gal = 5200 psi) both gauges equal zero. When fluid column pressures on both sides
of the U are equal to each other and equal to or greater than formation pressure, no formation
fluid can enter the wellbore. Both sides of the U stand full of fluid with zero surface pressure.
The purpose of the well control system is to circulate heavy fluid into the hole. The method of
circulation, then, is basic to the system of well control. It is not usually desirable to run the
fluid pump at full speed while circulating out a kick because the pressures required would
probably exceed the pressure limit of the pump. For this reason, a kick is usually circulated out
at about half normal pump speed. The pressure required to circulate the fluid through the U-
tube at some reduced pump rate is called the Reduced Circulating Pressure (RCP) (see Figure
12-12).
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-26