Environmental Engineering Reference
In-Depth Information
Figure 7.9.
Evolution of stress and suction during exploitation
of the Ekofisk field [DEL 96]
7.4.1.
Deposition of chalk in sea water (OA)
During this phase, a slight compaction occurs followed by the diagenetic
forming of bonds between grains corresponding to a reinforcement process. At this
point, the chalk does not contain oil, so
S
o
remains equal to zero and the stress path
follows the net mean stress axis
p - p
o
. An initial elastic zone is defined on this axis
between points O and A.
7.4.2.
Migration of oil in chalk (AB)
In this phase, it is assumed that oil migration from the source rock to the
reservoir rock occurs during the deposition of the overlaying clays. As oil is
penetrating the chalk,
p
o
is higher than
p
a
, and suction S
o
increases. If deposition of
the overburden occurs at the same time, a simultaneous increase in the mean net
stress takes place. This stress path is represented by segment AB in Figure 7.9. At
this time, chalk is hardening because of the combined actions of the stress and
diagenetic bond formation and suction
S
o
increases. In the BBM model, the upper
limit of the elastic zone is a horizontal line called the SI plasticity line. The position
of this line on the
S
o
axis is defined by the highest suction encountered by the
sample. An upward motion of SI induces a rightward motion of LC. Therefore, in
agreement with experimental evidence in unsaturated soils, stress path AB moves
the plasticity curve LC towards the LC2 curve, on the right.
