BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
2. Another problem associated with tripping is swabbing. Swabbing is compounded
by bit or collar balling, the viscous drag of fluid on the pipe, and the speed at which
the pipe is pulled from the hole. Swabbed pressure reductions due to viscous drag
can be calculated using ADVANTAGE engineering software. Swab pressures
should be controlled by observing safe drilling practices. If swabbing problems are
anticipated, a maximum “safe” tripping speed should be calculated and adhered to
by the rig crew.
To detect kicks while tripping, an accurate measurement of the volume of fluid required to
keep the hole full must be made. This is best accomplished using a calibrated tank called a trip
tank. If the volume of fluid required to fill the hole is less than the displacement of the pipe
withdrawn from the hole, a kick is indicated and remedial steps should be taken.
Kicks While Tripping Into the Hole
When drilling with fluid gradients near the fracture gradient of exposed formations, surge
pressures caused by viscous drag of the fluid on the drillstring may result in lost circulation. If
lost circulation occurs, the decrease in pressure depends on how far the fluid level falls. If the
fluid level falls enough, the corresponding decrease in hydrostatic pressure may be sufficient to
allow formation fluids to enter the wellbore and cause a kick. A practical approach to
recognizing this problem is to observe pit level increase versus amount of drillpipe run into the
hole, and to observe the flowline on each stand run to insure that fluid returns are maintained.
Behavior of Gas in the Wellbore
To properly understand the problems of killing a kick, it is necessary to understand some of the
things that can and cannot be done successfully. Gas expansion and the gas laws are basic
concepts that should be understood to help prevent problems in controlling kicks.
The simple form of the general gas law states that gas expands as pressure on the gas is
reduced or that if gas is not allowed to expand, the pressure in the gas remains the same. This
statement is modified by the effect of the temperature and the type of gas, so that
mathematically the general gas law is,
where,
PV
1
T Z
1
1
1
=
P2V
T Z
P = gas pressure
V = gas volume
T = temperature
Z = compressibility factor
1 = the first set of conditions (bottom-hole conditions)
2 = the second set of conditions (up-hole conditions).
Rise of Gas without Expansion
A practical application of the gas laws can be illustrated by considering a 10,000 ft well with
10 lb m /gal fluid and 1 bbl of gas that has been swabbed in on a connection. The well was shut-
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-20
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