Integration of time-lapse seismic in the reservoir management of the Gullfaks field, northern North Sea

Time-lapse seismic data have improved the drainage understanding of the Gul lfaks oil field, and have resulted in several potential new infill drilling targets. The baseline survey was acquired in 1985, one year before product ion start-up. Since then, two more surveys have been acquired, one in 1995, and another one in 1996. The validity of several 4D observations is uncertain today, since the lates t survey was acquired three years ago. In order to reduce this uncertainty a new 3D seismic dataset has been acquired in 1999. However, there are also interpretational uncertainties associated with several observations, both due to data quality and the fact that the seismic data do not yield a direc t estimate of oil saturation. Key elements in the process of reducing the u ncertainties are cross-disciplinary verifications and detailed geophysical analysis and modelling, where different saturation profiles are input to th e model. In order to include the interpretational uncertainties in the proc ess of quantifying the amount of remaining oil for each of the observations , the volume estimates are presented as probability curves.