Geology Reference
In-Depth Information
laboratory measurements of compressibility if they
are available. However, laboratory PVT measure-
ments are acquired under isothermal rather than the
adiabatic conditions that characterise seismic wave
propagation in the petroleum reservoir. In some cases
the difference between the two bulk moduli may be as
much as a factor of 2 (Batzle and Wang,
1992
). When
estimating fluid compressibilities of condensates the
interpreter
Table 8.5.
Examples of dry clay elastic parameters (after Avseth
et al.,
2005
).
Clay mineral
K (Gpa)
μ
(Gpa)
Smectite
1705
7.5
lllite
39.4
11.7
Kaolinite
1.5
1.4
Kaolinite
37.9
14.8
should consult with the petroleum
Kaolinite
12
6
engineer.
The inputs required for the Batzle and Wang
(
1992
) equations are
Chlorite
95.3
11.4
reservoir temperature and pressure,
simply the weighted addition of the densities of the
various components:
ρ
fl
¼
brine salinity,
dissolved gas index for brine (from 0
¼
no
dissolved gas to 1
¼
gas-saturated),
S
w
ρ
w
+1
ð
S
w
Þρ
h
,
ð
8
:
28
Þ
density of gas relative to air,
where S
w
¼
water saturation,
ρ
w
¼
brine density and
oil API gravity
ρ
h
¼
hydrocarbon density.
For the bulk modulus mix, it is generally assumed
that Reuss
oil gas-oil-ratio (GOR).
'
(also called Wood
'
s) law applies:
Subsequent work by Han and Batzle (
2000a
,
b
) sug-
gests refinements to the original Batzle and Wang
(
1992
) equations. In particular it was determined that
the original Batzle and Wang (
1992
) equations over-
estimated the effect of GOR on the elastic properties
of oils.
As an illustration,
Fig. 8.21
shows some fluid
properties based on North Sea data. It shows that
gases are more compressible than oil and brine and
also that gas responds to increasing pressure and
temperature with an increase in density and fluid
modulus, whereas oil and brine show a slight decrease
in density and fluid modulus. It is important that the
interpreter determines appropriate basin-specific
constraints for the modelling of fluid parameters. It
is common for example to find relationships between
oil API and depth.
Figure 8.22a
shows an example
from the Central North Sea where oil API generally
increases with depth, principally as a result of bio-
degradation at shallow levels.
Figure 8.22b
shows how
the maximum amount of dissolved gas (a parameter
calculated as part of the Batzle and Wang (
1992
)
equations) increases with API and pressure and
temperature.
The effect of a small amount of gas on the acoustic
properties of sandstones was discussed in
Section
5.3.3
, potentially giving rise to bright spots that
cannot be distinguished from those related to com-
mercial gas (
Chapters 5
and
7
).
Figure 8.23
illustrates
that this phenomenon is related principally to the
1
K
fl
¼
K
w
+
1
S
w
S
w
K
h
,
ð
8
:
29
Þ
where K
fl
is the bulk modulus of a mixture consisting
of volume fraction S
w
of brine, with bulk modulus K
w
,
and hydrocarbon with bulk modulus K
h
. This equa-
tion describes homogeneous mixing, where the rela-
tive proportion of each fluid is the same in every pore
space. This is usually valid for modelling at seismic
frequencies in rocks that satisfy the Gassmann
assumptions, but other mixing schemes may be
needed to account for inhomogeneous (
'
'
patchy
) sat-
uration in some cases (
Section 8.4.4
).
The
fluid is simply the fluid present in the
formation, undisturbed by any invasion effect caused
by the drilling of the well. The properties of brine and
hydrocarbons at reservoir temperature and pressure
can be estimated using the Batzle and Wang (
1992
)
equations, which are generally satisfactory for gases
and oils in the range 15
'
virgin
'
40 API. For the sake of
brevity a detailed presentation of the equations will
be omitted here.
There is usually some uncertainty in the applica-
tion of these equations particularly with regard to
heavy oils and condensates. When the viscosity is
low, oils may support shear waves (Batzle et al.,
2004
) and there is the possibility of coupling between
rock frame and fluid. For very light hydrocarbons and
condensates
-
166
it may be preferable to use direct