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Seawater velocity variations and real-time reservoir monitoring

Heriot-Watt Institute of Petroleum Engineering, Edinburgh, UK

Corresponding author: alex.bertrand@pet.hw.ac.uk

The first 20% of the full text of this article appears below.

As industry moves toward exploring and producing in deeper water areas, the impact of velocity variations in the water layer will become more critical. Acquisition crews are very much aware of this phenomenon. For example it is known that large salinity variations caused by salt seabed outcrop or fresh river water dumping can introduce velocity contrasts strong enough to create a reflection in the water column (such as the 3 s brine reflection in the Orca Basin, Gulf of Mexico) or can affect the balance of towed streamers. On the other hand, this problem requires more careful examination than is currently being undertaken by processing geophysicists and interpreters: Only the issue of velocity variations between infill and original acquisitions has been studied (Wombell, 1997 and MacKay and Fried, 2002).

In this work we address the issue of seawater velocity variations in the time-lapse analysis, which was highlighted in a paper on the challenges of deepwater by Barley (1999). We focus on permanent installations, as these are believed to provide highly repeatable 4D signatures. In the context of the instrumented oilfield, permanent installations will allow more frequent (every few months) "on demand" seismic data acquisitions. These will provide high resolution monitoring of subtle reservoir changes. It is anticipated that these changes could be distorted or even masked by seawater velocity changes.

In this paper, we first present background information on the speed of sound in water followed by some examples of seawater velocity variation analysis from the shallow Gulf of Mexico and West of Shetlands (north of the United Kingdom) using seismic tomography and oceanographic data. We then assess the potential impact of seawater velocity variations on real-time reservoir monitoring through synthetic seismic modeling.

	Generalities on water velocity

 
Speed of sound in water has been extensively studied by the acoustic and oceanographic communities through lab . . . [Full Text of this Article]