文档介绍：PORE PRESSURE AND FRAC
GRADIENT PREDICTION
OtiOptimum casi ng d esi gn
requires knowledge of pore
pressures and frac
ggaderadients
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Pore Pressure Prediction
Not all wells have abnormal
pressures
When abnormal pressures are
present, the frac gradient of the
last casinggg seat must be greater
than the pore pressure (plus trip
margg)pin and ECD) of the deepest
formation in the next hole
section
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Pore Pressure Prediction
Stress in rock
¾Overburden pressure is up to 1 psi/ft (23
kPa/m)
¾Formation pressure
− Normal is a water gradient of approximately
0. 465 psi/ft ( kPa /m )
− Abnormal is more
− Subnormal is less
¾Effective confining pressure
− Portion of rock pressure borne by rock matrix
(matrix stress)
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Pore Pressure Prediction
Tectonic activity may result in partial
support of overburden loads
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Pore Pressure Prediction
Various chemical and physical
interactions can result in
deviations from normal pressure
trends
RithftifRequires the formation of an
impermeable seal
Hard to predict
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Pore Pressure Prediction
Rapid deposition is the most
common cause of abnormal
pressures
Most pore pressure prediction
method s are b ased on th e rapid
deposition model
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Pore Pressure Prediction
Prediction methods work best in
continuous sand/shale
sequences such as the Gulf
Coast of the USA or North Sea
Shal e i s used t o predi ct pore
pressures and frac gradients
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Pore Pressure Prediction
Under rapid deposition, water
does not have a chance to leak
out of the rock
The water supports more of the
overbdburden pressure and dth