BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
A uniform decrease in rate of penetration of shales normally occurs with depth. This decrease,
which results from increased compaction and density of the shales, can be related to the
differential between hydrostatic pressure and pore pressure. The magnitude of change in
differential pressure is the key to changes in penetration rates.
While drilling a transition zone, drilling fluid density should be maintained as low as possible
so that any change in pore pressure will be reflected in penetration rate. As pore pressure
increases, rate of penetration will increase. Any excessive overbalance in drilling fluid density
will decrease the differential pressure and partially mask the formation pressure increases.
As mentioned, the inability to control lithology presents a serious problem. Generally, a sudden
large increase in penetration rate may indicate a lithology change. To prove or disprove such a
conclusion, the other parameters of abnormal pressure detection must be carefully evaluated.
Rotary Torque
During a normal drilling operation, rotary torque gradually increases with depth due to the
effect of wall contact of the drillstring on the wellbore. Any abrupt changes from this gradual
trend indicates a twist-off of the drillstring, a locked cone on the bit, a washout in the
drillstring, or a change in formation pore pressure. The actual condition is usually determined
by an experienced driller.
Increased pore pressure causes larger amounts of shale cuttings to come into the wellbore and
the bit teeth will take larger bites into the formation. The increased amount of shale tends to
stick or impede bit rotation. Rotary torque will not be easy to interpret as an indicator where
hydrostatic pressure is greater than formation pressure in deviated holes or when drilling from
a floating vessel.
Changes in Drag on Connections
When drilling in a balanced or near-balanced situation, an increase in drag can occur while
making a connection in an abnormally pressured zone. This increase is caused by the extra
shale cuttings which enter the wellbore when abnormal pressure is encountered.
Because of the plastic nature of some pressured shales, as shown by Darley, the hole may close
in around drill collars and bit, and drag increases. Drag, much like rotary torque, can be
masked as an indicator while drilling deviated holes or drilling from a floating vessel.
Change in Pit Volume
An increase in pit volume (rigs now employ pit totalizers to monitor this variable) is an
indication of formation fluids entering the wellbore while drilling. Drilling ahead should be
interrupted. Mud returns should be monitored for the increasing presence of background gas.
Circulation may also be interrupted for a short period to observe for any flow from the annulus.
Caution should be exercised in this case, as the chance of becoming differentially stuck
increases if all pipe movement and circulation is interrupted.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-12