BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
This type of calculation can be difficult, time consuming, and will never be very accurate
because the type of gas, its solubility, and the actual configuration of the gas “bubble” is
unknown. The drillpipe pressure techniques described later do away with having to make this
type of calculation.
Figure 12 - 8 Rise of One Barrel of Gas With Controlled Expansion
Shut-In Drillpipe and Casing Pressures
Drillpipe and casing pressures are used in well control to determine working pressures. The
drillpipe pressure gauge or pump pressure gauge is in reality a bottom-hole pressure gauge.
After a well kick, when the pump is off and the well is shut-in, the drillpipe is a long gauge
stem that reaches to the bottom of the hole. The drillpipe pressure gauge reads the bottom-hole
pressure as seen from the gauge stem. If the drillpipe were empty, the surface gauge would
read bottom-hole pressure. But the drillpipe is not empty, so the gauge reading shows the
difference between bottom-hole pressure and the fluid column pressure in the drillpipe. So,
shut-in drillpipe pressure is the difference between the pressure exerted by the fluid column in
the drillpipe and the bottom-hole pressure. It is the kick pressure.
Casing pressure will normally always be higher than drillpipe pressure because the kick gas or
water will normally be lighter than the fluid. This makes the fluid column pressure in the
annulus less than the full column of fluid in the drillpipe. So, the annulus surface pressure will
be higher than the drillpipe pressure.
To determine the fluid weight needed to control a kick, the formation pressure must be known.
This may be obtained from the pressure on the drillpipe. Since fluid inside the drillpipe will not
be contaminated, the pressure on the drillpipe plus the hydrostatic pressure of the fluid column
equals formation pressure. Shut-in drillpipe pressure can be determined by:
• Reading directly from a gauge at the surface. If there is a back pressure valve in string then,
• Start pump slowly, continue until fluid moves or pump pressure increases suddenly.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-22