Geoscience Reference
In-Depth Information
During conventional oil production, temperature changes in the reservoir are usually
fairly small. Injection of cold water (sometimes undertaken to maintain pressure and
improve sweep efficiency) may cause detectable changes. Where steam is injected into
a reservoir to improve recovery of viscous oils the temperature changes can be large,
particularly around the injector wells.
By combining the rock physics data with the output from a reservoir simulator, it
is possible to generate synthetic seismic models that show the changes to be expected
in the seismic response over time. This is important for planning a time-lapse survey.
Knowing the size of the changes will indicate whether time-lapse is feasible at all. A
dialogue is also needed with the reservoir engineers. To be useful, a time-lapse survey
needs to be available at a moment when it can influence reservoir management, e.g. in
deciding the location of an infill well. However, the size of time-lapse effects depends
on how much production has taken place. It may not be feasible to detect a time-lapse
signal during a very early stage of production, because there has not been enough
fluid movement or pressure change to be visible. Choosing when to shoot a time-lapse
survey requires collaboration between geophysicists and reservoir engineers. When
the survey has been acquired and interpreted, it is usual for the results to be different
from the synthetic seismic prediction derived from the reservoir model. The model
then needs to be modified so as to give a synthetic seismic response that matches the
time-lapse survey. Given the uncertainties in the model and in the seismic data, a range
of modifications may match the observed results. Again, dialogue is needed to arrive at
a result that can be used to guide reservoir interventions such as modifying oil offtake
or water injection patterns.
8.2
Seismic measurements
If we have a baseline and a repeat survey, what can we actually measure that will tell
us about the differences between them? There are several possibilities, as follow. In
any particular case it will be necessary to calculate the expected change for a given
reservoir, fluid fill and pressure change to see how big the seismic changes might be.
(a) TWT to a reflector. Above the reservoir, there should ideally be no TWT differences
between the baseline and repeat surveys. In the reservoir itself, seismic velocities
will change with pressure or changing fluid fill. The TWT thickness of the reservoir
will thus change with production. Seismic events below the reservoir will therefore
change in TWT between the two surveys. This effect can be quite large, ranging
from a few up to about 10 ms for a thick reservoir interval. In principle such a
time shift is easily detectable. Where seismic data have reasonably good signal to
noise ratio, it should be possible to pick a reflector to an accuracy of 1 or 2 ms.
Where we are dealing with a shift between one suite of reflectors (above the reser-
voir) and another (below the reservoir), then correlation methods should give even
