Page 70 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 46

[ ] Well Design
1.4.4

Drilling in mountainous areas may require a drilling location to be
much higher than the water table in the area. Hydrostatic pressures in
the wellbore may be much higher than formation pore pressures until
casing is set fairly deep. Similarly, in arid areas (deserts), even without
being far above sea level, the water table level may be deep.
When formations are distorted over geological time, some of
the strata may be “stretched,” causing subnormal pressures. Other
parts of the same strata may become foreshortened, causing abnor-
mal pressures.
High downhole temperatures will cause pore fluid to expand. If the
fluid is not isolated and can move away then the fluid density and
hence hydrostatic pressures will reduce.
Transition zone. A transition zone occurs where the pore pressure
gradient changes with depth to a higher or lower-pressure regime.
Transition zones are important. If possible, potential transition zone
occurrences should be predicted before drilling into them.
If drilling goes too far into a transition zone of increasing gradient,
there is risk of a kick. However, if the casing is set too early above or in
the top of a transition zone, there may not be the kick tolerance need-
ed to drill deep enough in the next hole section. This is because fracture
gradient generally increases as pore pressure increases, so the deeper
casing is set into a transition zone, the stronger the casing shoe strength.
Where transition zones occur due to under-compacted shales, the
change in pressure gradient is likely to be gradual. This means that
pore pressures can be monitored while drilling and there is less risk of
drilling too far if the driller is vigilant.
If the transition takes place quickly there may not be much, or any,
warning before the well kicks. It is especially important to identify
potential overpressures where transitions may be rapid.
Detecting a transition zone while drilling is more difficult with a
higher overbalance of mud density over pore pressure.
Indicators of abnormal pore pressure at the well design
stage. Pore pressure and fracture pressure gradient prediction prior to
drilling the well will use information from a variety of sources. The
more field data there is available, the better the prediction should be.
Data sources include:

1. Seismic surveys showing subsurface structures, gas anomalies, pos-
sible pressure communicating paths (faults), and indications of
high pore pressure.

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