Environmental Engineering Reference
In-Depth Information
7.4.3. Migration of oil in chalk after clay deposition (BC)
The suction S o continues to increase under a constant mean net stress. The stress
path now moves vertically upwards. SI moves towards SI3 and LC towards LC3.
Point C corresponds to the end of oil penetration into the chalk.
7 .4.4 . Depletion (CDE)
The decrease in oil pressure in the reservoir, p o , under a constant total stress, p ,
results in an increase in mean net stress, ( p - p o ). The variation of S o = p o - p w
during this phase also depends on the change in p w that is difficult to estimate due to
the lack of experimental data.
Nevertheless, the existing data on unsaturated soils show that suction decreases
during the compression tests at constant water content. This is the reason why the
corresponding stress path comes down from C to the right. In a first step (CD),
elastic settlements occur followed by plastic settlements when the LC curve is
reached, in D. The resulting increase in mean net stress moves the LC curve to LC4
to the end of depletion, in E, resulting in a hardening of the chalk and a decrease in
porosity.
7.4.5. Waterflooding (EF)
During the injection of water (waterflooding), the change in oil pressure p o and
thus mean net stress ( p - p o ) is not evident. Basically, we consider that after
depletion, p o is close to the hydrostatic pressure and that it will not be significantly
increased by the water injection. This assumes that the permeability is sufficiently
high. The mean net stress is likely to remain constant. As the water pressure
increases, suction S o decreases and the stress path corresponding to waterflooding is
a horizontal line with a downwards incline. This moves the plasticity curve LC to
LC5 (path EF), inducing the plastic settlements.
The waterflooding induces hardening, displacing LC from LC4 to LC5 (the
elastic zone is increased). This corresponds to a decrease in porosity induced by the
compaction due to collapse. In fact, according to the BBM model and the
experimental evidence, the volume reduction occurring along the EF path is equal to
that produced along the F'F path. This is because the initial and final positions of the
plastic curves are the same. This means that the mechanical effect of the decrease in
suction EF is equivalent to the effect of a loading in the saturated state along F'F.
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