Geoscience Reference
In-Depth Information
Examples of Solid Mineral Elastic Parameters
V
p
V
s
Bulk
Shear
Poisson's
Density
Modulus
Modulus
Ratio
Quartz
2.65
36.6
45
6.0376
4.1208
0.064
Chert
2.35
26
32
5.4
3.7
0.058
Calcite
2.71
76.8
32
6.6395
3.4363
0.317
Dolomite
2.87
94.9
45
7.3465
3.9597
0.295
Aragonite
2.94
47
39
5.8
3.6
0.187
Magnesite
3.01
114
68
8.2
4.75
0.247
Na-Felspar
2.62
55
28
5.9
3.3
0.272
K-Felspar
2.56
48
24
5.6
3.05
0.289
Ca-Felspar
2.73
85
38
7.05
3.75
0.303
Clays (approx)
2.68
41
17
4.9
2.5
0.324
Muscovite
2.82
52
31.5
5.8
3.35
0.25
Biotite
3
50
27.5
5.4
3
0.277
Halite
2.16
25.2
15.3
4.6
2.65
0.252
Anhydrite
3
66.5
34
6.15
3.4
0.28
Gypsum
2.31
58
30
6.75
3.7
0.285
Pyrite
5.02
158
149
8.4
5.45
0.137
Density in g/cm
3
, elastic moduli in GPa, velocities in km/s.
Fig. 5.20
5.5.1.3 Invasion effects
Unfortunately, the density and sonic logs recorded in the borehole may not record values
truly representative of the formation. The pressure in the borehole is usually kept higher
than the fluid pressure in the formation being drilled. This pressure differential forces
drilling fluid into permeable formations, replacing the original fluid in an
invaded zone
around the borehole. Many different types of drilling mud are in use. Water-based
muds may contain water of any salinity from very low to nearly saturated. Forcing
this water into an oil-bearing reservoir will of course change its density and seismic
velocity. Similarly, if an oil-based mud is used, it will change the properties of a brine-
filled reservoir in the invaded zone. The extent of the invaded zone depends on several
factors, including the permeability of the formation and the length of time that the
borehole is left exposed to circulating fluids. The effect on the sonic log also depends
on the source-receiver spacing; the wider the spacing, the more likely it is that the
tool will measure values in the formation beyond the invaded zone. It is possible to
use resistivity logs to estimate the oil saturation in an invaded zone, and then use steps
(5) and (6) of
section 5.5.1
to estimate the fluid properties in the invaded zone, given
the oil and water properties. Gassmann substitution can then be used to correct logs
back to what they would be if the formation fluid had not been disturbed. In practice,
this may not be a reliable approach, given that the logs may or may not be reading
in the invaded zone, depending on the distance that invasion has penetrated away from
the borehole. If different fluid zones of the same reservoir have been drilled, perhaps
with various mud types in different wells, then a useful check is possible. Starting from
