Environmental Engineering Reference
In-Depth Information
There is direct and indirect evidence that the reservoir sands of this region show
the typical features of soils in the presence of capillary forces. They concern:
i) the increased stiffness of samples containing gas (i.e. unsaturated samples)
when compared to fully saturated ones;
ii) the occurrence of expansion strains in the early phases of gas extraction;
iii) the appearance of compression strains associated with water injection.
The experimental evidence of all three phenomena is described in detail in
[MEN 08] and will not be repeated here for the first two points.
We will, however, explain the collapsible behavior of the reservoir rock, which
is in line with what observed above. Saturation of a partially saturated sample in the
plastic range produces an additional volumetric strain: the LC curve moves to the
right (from LC4 to LC5 in Figure 7.9). A water injection test has been carried out on
the core from Angela well, located on the right hand-side of Figure 7.13 [PAP 98].
This sample, from a depth between 3402.4 and 3402.5 m, is made of silty sandstone
and had an
in situ
water saturation of 0.38-0.45 (see Table 7.1). It has been loaded in
an oedometer up to a geostatic vertical load of 35 MPa and then slowly saturated
(during 24 hours of water injection).
As shown in Figure 7.12, curve 1, the resulting additional vertical strain is over
0.004, a value far from being negligible when compared to total volumetric strain at
reservoir conditions. This is clear evidence that a soil model taking capillary effects
into account is applicable to the reservoir sands of the upper Adriatic basin. Water
injection tests have also been carried out on shale and silty shale samples, as
indicated in Table 7.1 and Figure 7.12 (curves 2 and 3). Curve 2 refers to a silty
shale sample with saturation 1.0 and porosity 0.09. Volumetric strain associated with
water injection is still present, but of limited value depending on the low porosity
value. Instead, sample 3 has an initial saturation of 0.91, porosity of 0.22 and a
lower geostatic compression. Again, there are some small strains associated with
water injection.
Generally speaking, these additional strains are much larger for the more
unsaturated specimen (curve 1) and for that sample the stress/strain curve after
injection does not follow the same trend as the curve before the water injection: the
overall tangent of this curve is less steep after saturation, i.e. the soil is weaker. We
consider curve 1 in the following sample because it appears to be the only one from
a reservoir containing gas [PAP 98].
