BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
DRILLING PARAMETERS USED TO INDICATE AN
ABNORMAL PRESSURE ZONE
Although attempts at predicting actual formation pressures are important it should be realized
that they are just predictions. While drilling, several indicators of abnormal zones must be
watched so that early detection is possible. The most common methods used to detect abnormal
pressure zones in their assumed order of occurrence are:
• Increased rate of penetration
• change in rotary torque
• change in drag on connections
• change in pit volume
• variation from normal “d” exponent and its modified d xc exponent
• gas content of drilling fluid
• variances in shape and size of shale cuttings
• shale density
• flowline temperatures
• chloride ion content of drilling fluid
• paleo information.
All the above indicators will not be present at any one time. Rig personnel, however, must be
able to recognize these indicators and interpret correctly those which are present on a particular
job. Experience has shown that when more than one indicator exists, a cross-checking system
which uses one indicator to verify others helps eliminate many potential drilling problems
related to abnormal pressures.
Increased Rate of Penetration
Rate of penetration is one of the most widely accepted methods of determining changes in pore
pressure. There are, however, variables other than pore pressure which affect rate of
penetration. They are,
• lithology changes
• hydraulics (bottom-hole cleaning)
• bit weight
• bit type
• bit condition
• rotary speed
• drilling fluid properties.
There is no way to control lithology. It is, therefore, paramount when using penetration rate as
a detection method to control the other governing variables as closely as possible.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-11