Geoscience Reference
In-Depth Information
A3.4
Time-lapse seismic
Time-lapse or 4-D seismic continues to grow in importance particularly in mature areas such as the
North Sea where most of the seismic surveys are shot for 4-D purposes. This is a stable and largely
predictable activity for the industry although in terms of square km it is still dwarfed by the volume
of seismic shot for exploration. At the time of writing (mid 2007), there has been considerable cost
inflation in terms of the daily rate paid for seismic surveys which means that the economics of some
4-D surveys are now less clear than they were. However, costs are expected to drop in the future as a
new generation of seismic boats increases the supply to the market.
One of the keys to improving 4-D results is acquisition repeatability. Getting the sources and
receivers in the same position from one survey to the next is of major benefit to the final results. This
has led to developments in seismic acquisition such as steerable streamers and improved automation
of boat steering. Another approach is to use fixed receivers that remain permanently in position in
cables on the ocean floor (Kommedal
et al.
,
2005)
.
This guarantees that the receivers stay in the same
location between surveys and also means that repeat surveys require only a small shooting vessel
rather than a large 3-D seismic boat capable of towing several lengthy receiver cables. Although the
initial expense is much larger for a permanent system, the cost of each repeat survey is substantially
reduced, the data quality is improved and the total costs over the life of the field may be reduced. It
also has the benefit that repeat surveys are available virtually on demand. Another option is to use
a node-based solution as discussed in A3.1; the receivers can be positioned accurately by the use
of a remotely operated submarine vehicle. The improved data quality available from Ocean Bottom
Cable means that many 4-D datasets are being acquired with this set-up rather than traditional towed
streamer.
Processing for 4-D has continued to develop; the quality of the 4-D difference sections has improved
and the technique is now applicable to fields where the expected rock property changes due to
production are relatively modest. According to Hoeber
et al.
(2005),
there are several key components
to processing different vintages of seismic to obtain a reliable 4-D signal by progressively 'equalizing'
them. Firstly, deterministic processes are used where applicable to perform operations such as zero-
phasing or tidal statics removal. Secondly, cross-equalization is enforced by using repeatability metrics
such as the normalized root mean square, NRMS, defined to be
NRMS
=
2
∗
RMS(A
−
B)
/
(RMS(A)
+
RMS(B))
,
where RMS(A) and RMS(B) are the root mean square amplitudes of the original and repeat sur-
veys. Thirdly, the use of simultaneous processes such as 4-D binning and co-filtering ensures that
repeatability is progressively enhanced. By following these steps the authors show that it is possible
to get good levels of repeatability even between surveys that were not originally designed with 4-D in
mind.
Although there are a number of case studies showing impressive-looking results from 4-D difference
sections, there are many fewer documented case studies showing how these data are being used to
manage the reservoir and quantifying the associated financial benefit. Many of the published studies
have shown that the main benefit from the 4-D has been to identify undrained segments of the field
and hence potential for reserves addition through additional drilling. A nice example has been given
by Gonzalez-Carballo
et al.
(2006)
where they listed the benefits of the 4-D as follows.
(1) Identifying the dynamic role of heterogeneities such as faults, high permeability streaks, etc.
(2) High resolution dynamic information such as gas bubble and water front evolution.
(3) Improved geological consistency in history matching reservoir simulations to production data.
