Environmental Engineering Reference
In-Depth Information
case plastic strains (collapse) occur from 2 to 3. It is this collapse that may
correspond to the subsidence that continues over years.
7.4. Subsidence due to oil exploitation
Oil production is generally a source of pressure decrease in the fluids saturating
the reservoir rocks. This depletion leads to a compaction of the rocky reservoir.
The compaction of oil reservoirs and resulting subsidence is frequently observed,
and some examples are especially spectacular. Well-known examples include: in
Maracaibo hundreds of kilometers of dams had to be erected in order to girdle the
lake; in Long Beach harbor, California, the dams had to be heightened by more than
9 m; and in Wilmington field (also in California) subsidence has caused the collapse
of the Baldwin Hills dams.
In Europe, the North Sea oil fields exhibit subsidence reaching about 10 m. This
subsidence affects the sea floor on which the oil platforms are constructed. The
height of the platforms above the sea level is then reduced, which reduces the safety
with regard to storms.
In case of a accident on such huge structures, which sometimes have a daily
production of about 300,000 barrels (about 50,000 m
3
), we can imagine the extent of
the possible human, economic and environmental consequences.
The subsidence phenomenon related to oil extraction is especially well illustrated
by the example of the Ekofisk field [WIB 86, JON 87]. The Ekofisk field is located
in the middle of the North Sea. Its dimensions are about 7 km in an East-West
direction and about 10 km in a North-South direction. It has been exploited since
1970. Since 1985, subsidence of the sea floor has been detected, with a settling rate
of about 0.5 m per year.
This subsidence required the Ekofisk central complex to be lifted, at a cost of
some $480million; the construction of a concrete belt in order to protect the tank, for
more than $500million; and the modification of more than 120 wells, at a cost of
about $3-5million per well. As at the end of the 1990s the subsidence rate did not
appear to be slowing down, the whole of the complex was rebuilt.
From a geological point of view (see Figure 7.6), the Ekofisk field is composed
of some 3,000 m of tertiary overburden, mainly clayey, surrounding a set of chalk
layers belonging to Maastrichtian and Danian, constituting the oil reservoir.
