Environmental Engineering Reference
In-Depth Information
Since the 1990s, the pressure in the chalk reservoir has been stabilized by the
injection of a sea water volume equal to the volume of oil extracted. Nevertheless,
subsidence has continued at practically the same rate as before the sea water
injections
1
. This is due to modifications of the mechanical properties of chalk due to
interstitial fluid change (replacement of oil by water). Analysis of the sensitivity of
chalk to the nature of the saturating fluids is the aim of much ongoing research
[AND 95, DEL 96, HOM 98, LOR 98, MON 90, PAP 98, PIA 95, PIA 98, RIS 98,
SCH 96, SCH 98, SIM 99].
The analogies between the behaviors of chalk saturated by different fluids and of
unsaturated soils have lead to the concept of the “capillary effect” or “suction effect”
[DEL 96, RIS 98, SIM 99]. Indeed, in the case of unsaturated soils, the consequence
of a decrease in suction is an increase in compressibility and a decrease in the yield
stress (see Figure 7.7 for the Jossigny silt); while in the case of oil containing chalk,
the same phenomenon occurs when the saturation fluid changes from a non-wetting
fluid to a wetting one (see Figure 7.8).
0
Yield
1
2
3
4
s = 200 kPa
s = 400 kPa
s = 800 kPa
s = 1500 kPa
5
10
100
1000
Isotrope stress p (kPa)
Figure 7.7.
Isotropic compression curves of Jossigny silt
at four different suctions
1 From the beginning of the 21st century, the subsidence rate has slowed down, mainly
because of the change in chalk porosity of the reservoir areas affected by the phenomenon.
