Time-lapse seismic - 3-D Seismic Interpretation - page 180

8

Time-lapse seismic

We saw in chapter 5 how seismic data can be used in favourable cases to infer the
nature of fluid fill (gas, oil or brine) in a reservoir. An application of this is to follow
the way that fluids move through a reservoir during production, by carrying out a
baseline seismic survey before production begins and then repeat surveys over the
production lifetime. Where 3-D surveys are repeated in this way, they are often referred
to as 4-D seismic , with the idea that time is the extra dimension over standard 3-D.
Differences in seismic amplitudes or travel-times between the surveys can reveal the
movement of fluid contacts (e.g. where produced oil has been replaced by brine) or
the extent of pressure changes that affect reservoir properties. As sketched in figs. 8.1
and 8.2, it is not always straightforward to separate out the causes of the changes. At a
producing well, the pressure drops, and if it drops far enough then gas will come out
of solution. This will result in a decrease in both P velocity and in density, resulting
in a drop in acoustic impedance. On the other hand, the drop in pore pressure causes
an increase in P velocity and density. At a water injector (fig. 8.2) , we replace oil by
water, which in itself would increase P velocity and density; however, the injection also
increases the pore pressure, which causes the reverse effect on P velocity and density.
By comparing a pre-production trace volume with a post-production volume, we can
at least see where the changes are happening, even if we do not completely understand
them initially. Figure 8.3 shows three snapshot sections of the reservoir sand in a UK
Palaeocene field. We see an initial brightening in the upper sand owing to gas coming
out of solution, followed by dimming which may be caused by gas being re-dissolved
or by water encroachment. This type of information can be used as a tool for reservoir
management, for example by monitoring the expansion of a gas cap, with the intention
of managing production so as to prevent it from reaching producing wells and thus
decreasing the oil production rate.
Reservoir models are built using well and seismic data. The well data have high
vertical resolution, but in many cases there are a limited number of wells available,
especially in a high drilling-cost environment such as deep-water offshore. Interpolation
between the wells depends on seismic data, which have limited vertical resolution even
after attempts have been made to improve it using the methods described in chapter 6 .
The result is that there may be considerable uncertainty over the possible barriers to

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