Environmental Engineering Reference
In-Depth Information
Figure 7.6.
Geological sketch of the Ekofisk field structure [MAS 80]
The underlying source-rock belongs to the Jurassic period. The whole seabed lies
on the saliferous strata of the Permian period. The thickness of the reservoir
formations is about 300 m, with a dome shape due to halokinesis in the Permian
layers.
There has been migration of oil from the Jurassic layers to the chalky overlaying
layers. The oil has stayed in these layers because of the presence of impervious
clayey layers from the tertiary overburden. The chalks constituting the reservoir are
exceptionally porous − up to 48% in Danian − with high initial pore pressure (about
43 MPa) leading to rather small effectives stresses of about 19 MPa. The vertical
load applied by the tertiary overburden is of some 62 MPa. Between the beginning
of exploitation and 1985, the decrease in pore pressure due to oil catchment
(depletion) was 22 MPa.
The chalk elastic behavior [AND 95, MON 90] is composed of a quasi-linear
phase, pseudoelastic, followed by a plastic stage with hardening; the transition
between the two phases corresponding to the “pore collapse”. Non-negligible creep
occurs during these phases. The value of the stresses of pore collapses is a function
of the porosity, silica content and nature of the saturating fluid (water of oil).
In these conditions, the depletion (a 22 MPa pressure decrease between 1970 and
1985) has caused the pore collapse of part of the reservoir, inducing its compaction.
In 1985, the compaction was about 6 m. The overlaying clay layers have passed the
major part of the compaction to the surface, causing sea floor subsidence of about
3.5 m in 1985.
