They were climbing over each other at the SEG's post conference workshop on 4D (time lapse) seismics hosted Bill Aylor of Amoco. An estimated 200 attendees stayed on an extra day braving the cold in Denver to hear a some impressive case histories covering this hot topic. The essential idea behind 4D seismics is to perform repeat 3D seismic surveys over a field to monitor changes within the reservoir due to the production of fluids. Clearly not all reservoirs will be amenable to these techniques and the best targets are shallow, unconsolidated reservoirs which can respond in a spectacular manner.
artefacts
Jim Robinson of Shell emphasised the technical considerations necessary to achieve valid differential seismics - minimising the risk of artefacts. Problems such as tides, weather, water-table changes may impact repeatability, and in some cases bottom cables or permanent detectors may be necessary. But it was the proselytising from Roger Anderson of Lamont Doherty that won over the audience. Anderson's thesis is that, just as 3D was a quantitative leap into a revolutionary technology, 4D will be the next step and that the attendees were to "remember this day" as being the advent of 4D as a mature technology. Some of the time lapse imaging presented at the SEG workshop is of such an immediate impact that even reservoir engineers have tipped their hats to the seismologists. For the first time the effects of water drive and gas cap expansion can be actually followed from the surface, promising to radically change the way certain types of reservoir are managed, and giving the 3D seismic business yet another shot in the arm.
spectacular
A beautifully illustrated article on 4D time lapse seismics appeared in the November edition of Petroleum Geoscience, the journal of the European Association of Geoscientists and Engineers. This paper, by Watts et al. on monitoring of the Magnus field illustrates the basics of the techniques involved, and shows how even older 3D seismic surveys can provide useful information. The important point in time lapse monitoring, as in seismic characterization of the reservoir in general is that the parameters observed should have some physical relationship with the changes in the reservoir.
data issues
Now why you may ask is an article about 4D time lapse seismics appearing in The Petroleum Data Manager? Two reasons. One, we already know that data volumes are growing exponentially, with around a terabyte of data acquired in a 3D survey. The impact of repeated 3D surveys means that we will have to manage multiple copies of these surveys in the future. The other reason relates to the current fad for compressing seismic data (see the editorial in this edition). If we are acquiring data with a view to comparing it with a future dataset recorded at the same location, then it would seem dangerous to apply some of the lossy compression algorithms to the data before storage. We can anticipate that, a few years down the road, we will be able to extract more bandwidth from the field data, and even perhaps visualize it with more powerful hardware. A 3D reservoir study today must include an element of planning for future re-use and should, in our opinion, be stored uncompressed, to allow for maximizing its value at some future date.
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