Geology Reference
In-Depth Information
a)
b)
Density (g/cc)
Fluid modulus (GPa)
0
0.5
1
1.5
0
1
2
3
0
0
500
500
1000
1500
1000
1500
2000
2500
3000
3500
4000
4500
5000
2000
2500
3000
3500
4000
4500
5000
Brine
38º API oil
Gas
Figure 8.21
Modelled relationships of density and fluid moduli as a function of depth, based on a generalised North Sea dataset.
a)
b)
API
0
20
40
60
0
300
250
500
200
5000ft
10000ft
15000ft
1000
150
1500
100
2000
50
0
2500
0
20
40
60
3000
Oil API
Figure 8.22
Fluid parameters from the Central North Sea; (a) API vs depth, (b) modelled effect of pressure and temperature on the maximum
GOR for oils with different API.
ϕ
¼
ρ
0
ρ
b
ρ
0
ρ
fl
homogenous mixing of water and gas at low tempera-
ture and pressure. The effect is less evident as the
temperature and pressure increase.
,
ð
8
:
30
Þ
where
ρ
0
is the matrix density,
ρ
fl
is the fluid density
and
ρ
b
is the measured bulk density. This is straight-
forward if the mineral and fluid parameters are
known and the density log is trustworthy. In real
rocks, however, only part of the fluid in the pore space
can move freely in and out of
8.2.4.3 Porosity
At first sight, porosity is a straightforward concept,
being the ratio of the pore space volume to the
volume of the rock, expressed as a decimal or percent-
age. It can thus be derived from fluid, mineral and
bulk densities:
the rock. Thus,
the porosity can be split into
'
(i.e. the connected porosity in which fluids can move)
'
effective porosity
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