Geology Reference
In-Depth Information
a)
b)
V
sh
Phi
e
S
we
V
p
ρ
σ
0
1 0.5
0 1
0
1.95
2.95 2000 4000 0
0.5
V
sh
1
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
0.75
0.5
0.25
-1
0
0.1
0.2
0.3
Porosity
Figure 8.61
Fluid substitution in shaley sands - effective porosity approach with dry rock model. (a) Dry rock data from
Fig. 8.58
with a
dry rock model (dashed line) superimposed. Data points are coloured by shale volume, as indicated by the key on the right. (b) Fluid
substitution to gas using the dry rock model defined in (a): blue curves ¼ brine, red curves ¼ gas, depth marker spacing is 10 m. Note how the
effect of fluid substitution on the compressional velocity log varies with porosity and how the substituted Poisson's ratio log is similarly
well behaved (after Simm,
2007
).
1
M
lam
¼
M
sh
+
1
V
sh
V
sh
M
sand
estimate the end members and the technique
is relatively robust in the presence of small errors
in defining the end members.
Figure 8.62
illustrates
the calculation for a laminated interval with
50% V
sh
.
A more complex alternative to Katahara
ð
8
:
47
Þ
+
ρ
lam
¼ ρ
sh
V
sh
ρ
sand
1
ð
V
sh
Þ
ð
8
:
48
Þ
and
s
M
lam
ρ
lam
s lamin-
ated model has been presented by Skelt (
2004
). Skelt
'
s
method essentially decomposes the sonic and density
data into shale and sand components, then performs
fluid substitution in the sand component followed by
a re-combination.
'
V
p lam
¼
,
ð
8
:
49
Þ
where M
'
denotes laminated mix between shale and sand end
members, and V
sh
¼
¼
M modulus,
ρ ¼
bulk density,
'
lam
volume of shale.
Calculation of V
s
follows the same form with M
being replaced by
8.5.3 Low porosity and permeability
sandstones
Of the
. End member hydrocarbon
sand values are derived using end member wet sand
and Gassmann fluid substitution. Practical imple-
mentation of the technique on well logs requires
a reasonably accurate assessment of volume of
shale and also some idea of the parameters of shales
and wet sands. Trend curves derived from a colla-
tion of data from nearby wells can be used to
μ
reservoirs, fluid substitu-
tion may be a technique that could be applied to
'
'
unconventional
'
sands, (i.e. sandstones that require hydraulic
fracture stimulation in order for the reservoir to
produce). Tight sands are a heterogenous group of
tight
'
195
