Permanent ocean-bottom seismic cables have been installed in the Valhall Field, offshore Norway. Five seismic surveys were acquired from the permanent system between October 2003 and April 2005, in addition to two towed-streamer surveys acquired in 1992 and 2002. The various 3D seismic data sets show strong time-lapse or 4D effects resulting from primary production. During the same time period an extensive drilling program has provided high-quality data on reservoir pressure, pressure gradients, and reservoir fluid distribution in the field.

This paper outlines how changes in seismic attributes were generated synthetically from a reservoir simulation model and compared against observed 4D seismic data in one area of the field. An objective function describing the misfit between the simulated and measured data was defined using both seismic and conventional production data. BP's Top-Down Reservoir Modelling (TDRM) technology1 was used to generate multiple reservoir descriptions that would minimize the objective function. This involved computer-assisted history matching with a genetic algorithm that varied more than 60 parameters in the reservoir simulator to improve the overall history match.

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