Geoscience Reference
In-Depth Information
of gas (which will have a very low
K
h
) causes a large decrease in
K
fl
, resulting in
a large decrease in
V
p
.
(6) Calculate
ρ
fl
and
ρ
b
, the fluid and bulk densities for the new case, using
ρ
fl
=
ρ
w
S
w
+
(1
−
S
w
)
ρ
h
and
ρ
b
=
ρ
ma
(1
−
φ
)
+
φρ
fl
.
(7) Determine
K
sat
using
1
K
ma
2
K
d
−
K
sat
=
K
d
+
.
K
fl
+
1
−
φ
K
ma
−
K
d
K
ma
(8) Derive
V
s
using
µ
ρ
b
.
V
s
=
(9) Derive
V
p
from
K
sat
+
µ/
4
3
V
p
=
.
ρ
b
We now address some of the detailed issues in these calculations.
5.5.1.1 Calculating fluid parameters
Equations to calculate fluid properties have been given by Batzle & Wang
(1992).
Properties are dependent on pressure and temperature. For brines, the salinity has a
significant effect on the bulk modulus (high salt concentration imparts greater stiffness).
For oils, the properties depend on API gravity and gas-oil ratio. If data are available
from analysis of oil samples, then it may be possible to use directly measured values of
bulk modulus and density. (Note, however, that for our calculation we need the adiabatic
bulk modulus, and reservoir engineers usually measure an isothermal modulus, so a
correction needs to be applied.) For gas, the properties depend on the specific gravity,
which reflects the concentration of molecules heavier than methane. Oil-case values
are always intermediate between the brine and gas cases, but exactly where they fall
depends on the oil concerned: low API and GOR oils will be close to the brine case,
whereas high API and GOR oils will be close to the gas case.
Figure 5.19
shows some
representative curves for North Sea data.The interplay of temperature and pressure
effects means that the shapes of these curves are not intuitive, and for accurate work
the fluid properties need to be calculated from the Batzle & Wang equations.
