BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
in and the gas bubble circulated to the surface by holding the pit volume constant (preventing
the gas from expanding). For this example, the effect of temperature and compressibility is
ignored.
The bottom-hole pressure in the well is 5200 psi, and the gas volume is one bbl. If the gas is
circulated half way up the hole, there will be 2600 psi of fluid pressure above the gas bubble,
but the pressure in the gas bubble will still be 5200 psi according to the general gas law.
The surface annular pressure will be 2600 psi, which is the difference between the pressure in
the gas bubble and the fluid pressure above it. The bottom-hole pressure will be the pressure of
the fluid column, 5200 psi, plus the surface annular pressure, 2600 psi, for a total of 7800 psi.
The surface pressure and the wellbore pressure will continue to increase as the bubble is moved
up the hole without being allowed to expand. When it reaches the surface, the surface pressure
will be 5200 psi and the bottom-hole pressure 10,400 psi, or the equivalent of 20 lb m /gal fluid.
Long before the bubble gets to the surface, fracture pressures will be exceeded and an
underground blowout will occur.
In earlier years, this technique of Constant Pit Volume, or Barrel-In/Barrel-Out, was used to
kill wells. Although this method works for water flows, it almost always results in lost
circulation and an underground blowout when the formation fluid contains gas.
Rise of Gas with Uncontrolled Expansion
The opposite extreme of circulating the gas out of a well without holding any pressure on it is
equally bad. In the same circumstances, one bbl of gas is swabbed into the well, but the well is
not closed in and the pump is turned ON to circulate the bubble out of the hole.
When the gas is half-way up the hole, it has expanded to 2 bbl. When the gas was at the bottom
of the hole, the 1 bbl “column” of gas was only about 12½ ft long. Halfway up the hole, it is 25
ft long and three quarters the way up the hole, the gas has expanded to 32 bbl and occupies
about 400 ft of annulus. The 400 ft of gas has reduced the bottom-hole pressure by about 200
psi and more gas is probably entering the wellbore.
Circulating a gas bubble up without holding some surface pressure will unload the hole and
cause a blowout. The technique of trying to “out run” a well kick is risky because the bet is that
the formation has such low permeability that the well could hardly blow out. Trying to outrun a
well kick works occasionally because the kick may be water and, because the pumping rate is
increased, the bottom-hole pressure is increased by the amount of the annulus friction loss. It
still remains a risky endeavor and leads to more failures than successes.
Rise of Gas with Controlled Expansion
The proper way to handle a gas bubble in a well is to put enough pressure on the gas to keep
the bottom-hole pressure constant. This is the basic idea behind proper well control techniques.
Using the previous illustration, one bbl of gas has been swabbed into the hole but allowed to
expand in a controlled manor by using an adjustable choke at the surface. When the gas bubble
is half-way up the hole, the casing pressure would have to be 14 psi to keep the bottom-hole
pressure constant. When the gas is at 2500 ft, the casing pressure would have to be about 28 psi
(Figure 12-8). At 1200 ft, the casing pressure would have to be about 57 psi.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-21