Environmental Engineering Reference
In-Depth Information
Figure 7.12.
Oedometric test with water injection: axial stress
versus volumetric strain [PAP 98]
Depth (m)
Material
Total porosity
In situ
water
saturation
Irreducible
saturation
2137.1-2137.2
shale
0.21-0.29
1
0.37-1.00
3122.5-3122.6
silty shale
0.08-0.10
1
~ 1.00
3402.4-3402.5
silty sandstone
0.09-0.11
0.38-0.45
0.22-0.34
Table 7.1.
Material parameters for laboratory injection and oedometric test samples
We now focus our attention on the Ravenna Terra field where a great deal of
data about subsidence is available, both during gas production and after the
shutdown of the wells. The situation in the Ravenna region during gas production
has already been dealt with in Chapter 5, where the spatial distribution of subsidence
from 1972 to 1977 is shown in Figure 5.25 together with the reservoir contour,
location of productive wells and the town of Ravenna. Since the area is close to the
sea and very sensitive to surface lowering with respect to the mean sea level, the
surface subsidence has been almost continuously monitored since 1949. We shall
consider in detail the data after the wells were closed in 1982 (after this period there
is only a negligibly small residual production). It will be shown that subsidence
models based on the effective stress principle alone may have serious difficulties in
explaining the behavior observed after the closedown of the wells.
Measurements for surface lowering were carried out for the periods 1982-1998,
1986-1998 and 1992-1998. The data have been first processed by the Department of
