Geology Reference
In-Depth Information
8
y = 0.3658Ln(x) + 5.4398
R
2
= 0.7732
K
dry
7
6
y = 1.3626Ln(x) + 1.0801
R
2
= 0.9841
5
G
dry
4
3
Bulk Modulus
Shear Modulus
Log. (Shear Modulus)
Log. (Bulk Modulus)
2
1
0
0
5
10
15
20
25
30
35
40
Effective pressure (MPa)
Figure 10.26
Fluid and shear modulus variation with effective pressure from laboratory measurements: Schiehallion Field, West of Britain
(after Meadows
et al
.,
2005
).
simply in terms of parameter changes in Gassmann
s
equation (
Chapter 8
), for example changing the fluid
parameters associated with saturation change from
virgin to residual saturation and accounting for the
pressure change on the fluid modulus via the Batzle
and Wang (
1992
) equations. It was realised, however,
that two important factors were overlooked; namely
the sensitivity of the dry rock frame to changes in
stress during production and the potential for hetero-
geneous saturation distributions.
In
Chapter 8
it was discussed how the stress sen-
sitivity of sandstones shows large variations and the
fact that in the absence of reservoir-specific labora-
tory data there is no obvious way of determining the
likely response for a given sandstone (e.g. MacBeth,
2004
; see also
Chapter 8
). Thus, acquiring laboratory
measurements from core data (e.g.
Fig. 10.26
)is
necessary to reduce the uncertainty. Dry rock modu-
lus changes tend to have greater importance in the
depletion of overpressured reservoirs compared to
consolidated reservoirs.
Producing a reservoir can lead to inhomogeneous
saturations throughout the pore space. This patchy
saturation effect (
Chapter 8
) has been described from
well log analysis (e.g. Dvorkin et al.,
1999
; Caspari
et al.,
2011
) and it is envisioned that this might occur
on a seismic scale (e.g. Sengupta and Mavko,
1998
).
The effect is most important in the case of gas, for
'
3100
Patchy saturation
3000
(a)
(b)
(c)
2900
Uniform saturation
2800
0
0.2
0.4
0.6
0.8
1
Oil saturation
Figure 10.27
Experimental results showing patchy saturation
effects related to gas injection into an oil reservoir. Cases (a),(b) and
(c) are different permeability scenarios with different layering
characteristics but essentially permeability improves from (a) to (c).
After Sengupta and Mavko,
2003
.
More subtle signals are likely to result from
pressure changes in consolidated oil reservoirs,
saturation changes in low API oil reservoirs under
waterflood,
saturation changes in carbonate oil reservoirs (e.g.
Wang et al.,
1991
).
Early feasibility modelling of time-lapse effects in
conventional oil
241
reservoirs
treated the problem
