Environmental Engineering Reference
In-Depth Information
A three-dimensional rendering of the subsidence is shown in Figure 7.19, where
even the influence of the inner boundary around benchmark 563 can be seen. The
picture is clearly that of a local behavior, limited to the reservoir itself and the area
close by: the surface is lowered by the greatest amount close to the boundary and
immediately outside it, rather than over the central part of the reservoir. We observe
a reversed subsidence bowl. This is the opposite of what is usually observed and
obtained numerically during the production period.
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Figure 7.19.
Three-dimensional rendering of subsidence above and around
the reservoir for the period 1982-1998. Horizontal axes are labeled with geographical
coordinates expressed in m using Gauss-Boaga projection. The vertical axis shows the
surface lowering in m
We now try to provide an explanation of this peculiar behavior, which has not
been observed before. Note that we are in a period of increasing reservoir pressure,
see Figure 7.20, hence a Terzaghi-effective stress-based model would give a modest
rebound, while subsidence continues here. Inspection of evolution of the subsidence
pattern in the period observed shows that the reversed bowl is closing in towards the
reservoir center; see for instance the evolution of the 0.05 m isoline in Figures 7.16
to 7.18. This may be explained by the fact that the reservoir rock at the boundary is
the first to experience increasing saturation and hence becomes weaker and
undergoes structural collapse. The inner part of the reservoir remains partially
saturated for long time periods and maintains its stiffness. In fact, the central part
hardly experiences subsidence in the observed period. However, the saturation front
is moving inwards with time, which causes the pattern observed.
