The Valhall field started production in 1982 and is expected to produce for another 25 years. Recently three new platforms were installed at the field to facilitate a water injection program and infill drilling.

The central area of the reservoir is difficult to image using conventional marine streamer data due to gas charges in the reservoir. The imaging challenges have been addressed by using seafloor recording of converted shear waves. In other parts of the field the seismic quality is very good and a conventional 4D seismic surveys acquired in 2002 provided impressive production induced 4D seismic images.

A permanent seismic array was installed across the field during the summer of 2003. The system allows for cost effective acquisition of frequent seismic surveys. The primary objective is to use the resulting 4D images to improve recovery and to provide technical assurance.

The permanent seismic recording system was selected based on its ability to acquire shear wave and expected improvement in repeatability to allow for monitoring of production and injection of low volumes.

In the period from September 2003 until April 2005, five time-lapse surveys have been acquired. High quality and repeatable data has allowed for identifying relevant 4D effects taken place over only a few months.

In this presentation we will discuss the Valhall permanent seismic system, present some results and discuss how these data are used to improve the value of the field.

The Valhall field

The Valhall field is located 290 km southwest of Stavanger, Norway, in 70 m water depth, figure 1. The reservoir is over-pressured Upper Cretaceous chalk. The field was discovered in May 1975 and production from the field started in 1982. After 23 years the field is still producing at plateau and is expected to produce economically for 23 more years, [1]. To date, around 500 million barrels of oil, out of the estimated 2, 7billion barrels in place, have been produced.

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