BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
rates, and flow checks. The behavior of each kick detection parameter and how it is affected by
gas kicks in oil fluids are discussed separately.
Pit Volumes
When a fluid enters the wellbore, the addition of this volume is observed at the surface.
Because instruments and techniques used to detect these increases have a limited amount of
sensitivity, it may take 10 or more bbl of formation fluid to be noticed at the surface. With gas
in water-base fluids, this happens fairly quickly after influx because the gas begins to migrate
since it is insoluble. In oil-base fluids, the gas is soluble and, therefore, will mix in and migrate
very little until almost to the surface. In oil-base fluids, it is important to be able to detect very
small pit gains quickly. This may require some modifications of pit level measurement
techniques.
Flow Rate Increases
Flow rate increases in oil-base fluids are much smaller than in water-base fluids for the same
reasons as cited earlier. In some cases, they have been about one-half as much. These smaller
changes may go undetected by the rig crew or “flow shows”. Special care needs to be taken
when fluid additions are made to the active system to insure all volume changes are accounted
for.
Flow Checks
Stopping the pumps and checking for flow shortly after taking a kick may not give a definite
indication that a kick has occurred. Even in water-base fluids, gas migration and expansion will
not be occurring immediately. A shutdown period as long as 10 minutes may be necessary
before flow can be detected.
Shut-In Pressures
Due to the compressibility of the gas and oil in the fluid, it may take longer for pressures to
stabilize. Normally, a 30- to 60-minute period is recommended after shutting the well in to
allow pressures to stabilize in water-base fluids. In an oil fluid, this may take 1.5 to 2 times
longer.
In conclusion, the ability of the rig crew to adjust to the differences of gas kicks in oil fluids
will determine how successfully the kick is controlled. The entire crew needs to be trained
concerning these differences and how they are to be handled.
Subsea Well Control
Drilling from floaters and semi-submersible rigs has always provided challenges in well
control situations, but drilling in water depths of 7000 ft and deeper has accentuated those
problems. A brief discussion of the main well control problems that may occur in deep water
drilling follows.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-38