Geology Reference
In-Depth Information
a)
c)
AI
+
shale
'b'
'a'
Rc
Z
crossover
Sin
2
brine sand
'a'
'b'
-
b)
d)
Water
Shale a
Sand a
Shale b
AI
Sand b
Porosity
Quartz
Acoustic impedance
Figure 5.9
The effect of compaction on brine-filled sands and shales, (a) depth vs AI, (b) sand porosity vs AI, (c) AVO plot showing shale/sand
AVO responses, (d) acoustic impedance vs Poisson's ratio.
(i.e.
Κ
sat
), and hence the P wave velocity, can be
significantly affected by the replacement of relatively
incompressible brine with highly compressible gas or
condensate. The shear modulus of the rock is
unaffected by fluid replacement because liquids have
no rigidity. Since the density of the rock is decreased
when hydrocarbons replace brine (hydrocarbons are
invariably less dense than brine), the effect (following
Eq. (2.11)
in
Chapter 2
) is to increase the shear vel-
ocity slightly. Overall the effect of replacing brine with
hydrocarbon in a rock is to reduce both the acoustic
impedance and the Poisson
Clark (
1992
) and Batzle and Wang (
1992
) showed that
live oil (i.e. oil with dissolved gas at reservoir tempera-
ture and pressure) can have significantly lower moduli
and density than dead oil at surface temperature and
pressure. Gas generally has a bigger effect on seismic
amplitude than oil, though in deep basins light oils
and condensates may have an effect similar to that
of gas. Owing to the higher moduli of calcite and
dolomite compared to quartz, the magnitude of fluid
substitution effects tends to be significantly less for
carbonates compared to siliclastics.
Figure 5.10
shows a schematic illustration of the
effect of replacing brine with gas for the sand in
Fig. 5.9
. It is evident that the reduction in AI and
PR becomes less with decreasing porosity.
s ratio. It was originally
thought, based on the properties of stock tank oils,
that oil would have limited or no effect on seismic
amplitude (e.g. Gregory,
1977
). However, the work of
'
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