Geology Reference
In-Depth Information
Pressure
Two sandstone cores
8864 A=17.6% porosity
8293 B=29% porosity
Δ
P =
Pore (reservoir) pressure
external pressure minus internal pressure
Effective stress
15
Depth
Top overpressure
14
Overburden gradient
~1 psi/ft
Δ
P=6000
Δ
P=5000
Δ
P=3000
Δ
P=2000
Δ
P=2000
13
Hydrostatic gradient
~0.43psi/ft
8864 A
Δ
P=1000
Figure 5.34
Pressure
-
depth relations.
12
0
Δ
P=0
11
2000
Δ
P=6000
8293 B
10
4000
Δ
P=2000
Δ
P=1000
9
6000
123456789
Top of over-
pressures
External pressure PSI x 10
-3
8000
Figure 5.33
Laboratory measurements from two sandstone cores
showing velocity as a function of differential pressure (after Hicks
and Berry,
1956
). Note how the sands have very different pressure
sensitivities.
10,000
12,000
Figure 5.35
shows an example from the Gulf of
Mexico. The sonic has been plotted on a log scale,
revealing the
14,000
shales lying on a
straight line trend. With the onset of overpressure the
sonic values depart from the trend (i.e. increase in
slowness). The magnitude of the departure of the
sonic log from the
'
normally compacted
'
50
100
150
200
2.1
2.2
t
(sh)
(μ
s/ft)
ρ
b(sh)
(g/cc)
Figure 5.35
Sonic log example plotted on log scale to highlight
trend can be calibrated
to drilling mud weight and the degree of overpressure
(e.g. Shaker,
2003
).
'
normal
'
the
shale compaction trend and onset of overpressure, from
Hottmann and Johnson (
1965
). Copyright 1965 SPE; reproduced
with permission of SPE, further reproduction prohibited without
permission.
'
normal
'
5.3.6.2 Overpressure and amplitude signatures
Overpressuring will lower the effective pressure,
soften the rock and reduce the velocity compared to
a hydrostatically pressured rock. In some cases the top
of overpressure (also referred to as geopressure)is
defined by a strong negative amplitude response.
Top geopressure boundaries defined by normally
pressured
80
shale
overlying
overpressured
shale
