Geology Reference
In-Depth Information
a)
b)
3200
3
2.5
3100
2
3000
1.5
2900
6.9MPa, 20C
34.5MPa, 40C
69MPa, 70C
103.5MPa, 90C
1
2800
0.5
2700
0
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
Water Saturation
Water Saturation
c)
d)
0.34
7400
0.32
7000
0.3
6600
0.28
0.26
6200
0.24
5800
0.22
5400
0.2
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.81
Water Saturation
Water Saturation
Figure 8.23
The effect of temperature and pressure on a gas-bearing sand. Graphs show the variation of (a) fluid modulus (using Reuss
mixing), (b) compressional velocity, (c) acoustic impedance and (d) Poisson
'
s ratio with changing water saturation. Constants used in models:
K
d
¼
8.8GPa,
μ ¼
5.7GPa, porosity
¼
0.22, gas gravity
¼
0.65.
and the
porosity. In clean sandstones, the
effective porosity is largely determined by grain size
and sorting; for example, it is possible for fine-grained
sandstone to have a total porosity of 15% but no
effective porosity owing to the way that the grains
are packed. The rock is effectively impermeable and it
wouldn
'
ineffective
'
1
t be appropriate to perform fluid substitution
on such a rock. In order to carry out fluid substitution
correctly it is important to consider the relationship
between porosity and saturation, particularly the irre-
ducible water saturation characteristic of the reservoir
(
Fig. 8.24
).
The effective porosity concept is also important in
shaley sands (
Chapter 5
). Most sandstones have some
shale as a component of the rock, where shale is
defined as the combination of clay minerals, silt and
chemically bound water (
Fig. 8.25
). Petrophysicists
will often work with effective porosity and effective
water saturation in shaley sands, owing to the ease
'
S
w
0
0
0
.4
Porosity
Figure 8.24
Typical form of the relationship between porosity and
water saturation in an unproduced sand reservoir. The curve defines
the irreducible water saturation. Note that with finer-grained
sandstones the trend shifts to the right.
168
