BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
• Change in normal porosity, usually an increase
• Change in formation fluid content and density
• Change in shale conductivity, usually an increase.
METHODS OF FORMATION PRESSURE PREDICTION
Because many formations have abnormally high fluid pressures, it is extremely important to
know about these pressures in planning and executing drilling operations. Prior to drilling,
there are two means of predicting the location of abnormal pressure zones – (1) seismic data
from the area to be drilled and, (2) electric log data from offset wells drilled near the proposed
location.
Seismic
Since the abnormally pressured zones have not compacted normally with depth, the velocity of
sound waves traveling through these formations is reduced. These reductions are measured,
plotted, and converted to an amount of abnormal pressure. Seismic interpretation can be
applied to the detection of two different types of abnormal pressured gas zones, shallow and
deep.
Shallow Gas
Formations containing gas tend to absorb sound waves. Formations above gas sands will be
highly reflective, whereas the gas sands will be absorptive. On a seismic profile, the gas sand
will show up as a “bright” spot. Shallow gas sands have been responsible for numerous
offshore blowouts, so their detection before drilling is important.
Deeper Gas
Abnormally pressured formations at deeper depths are more difficult to detect; the more
geological information available in an area, the greater accuracy in detecting these zones; the
more velocity profiles taken in an area, the better the definition of the normal compaction
trend. The seismic data yields curves representing sonic velocities of the formation to be
drilled.
Different lithologies have different velocity profiles. Therefore, a thorough knowledge of the
geology of an area aids in the interpretation of seismic information. The unit used to measure
the velocity is a value referred to as interval velocity. Interval velocity is the speed of sound
through an interval of formation and the reciprocal of interval transit time.
The interval transit times are usually plotted on logarithmic scale versus a linear depth scale
(see Figure 12-3). A normal line (±70 µ sec/ft @ 10,000 ft.) is drawn and extended into the
abnormal pressure interval and compared to the extrapolated shale line (±90 µ sec/ft @ 10,000
ft). Comparison between the observed value and the extrapolated value is a measure of the
amount of abnormal pressure. Common sources of error in velocity analysis include dipping
beds, faults, multiple reflections, curved ray paths, processing, and interpretation. An interval
transit time for limestone (±50 µ sec/ft @ 10,000 ft.) is shown for comparison purposes. The
seismic wave travels faster through the harder rocks and is slowed down by the presence of
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-4