Seismic Reservoir Surveillance in the Alba Field, UK North Sea

Abstract

The feasibility of a time-lapse seismic response to production was identified early on in the life of the Alba field, UK North Sea. The reservoir rocks and fluid properties (oil and water), were conducive to a change in seismic properties in response to water encroachment. The acquisition of appropriate seismic data took longer to obtain, due in part to the vintage of the 3D baseline survey (acquired in 1989) and the focus on lithology identification in a subsequent repeat survey (acquired in 1998), shot using ocean bottom cable. The acquisition differences between the two surveys made it difficult to extract more than the simplest 4D response from the surveys.In 2002, the Britannia coventurers acquired a long offset survey in response to their subsurface requirements. This data provided an opportunity to revisit the overlying Alba 4D response, using a more recent baseline survey and monitor and more similar acquisition geometries.The intepretation of this more recent baseline and monitor pair showed that the amplitude of the direct hydrocarbon indicator (representing the oil water contact) was reduced by production. The contact was no longer a flat surface near active producers and the Direct Hydrocarbon Indicator (DHI) was seen to be tilted. This was probably in response to the production as the effect was not observed in the vicinity of abandoned producers. Comparison of the observed response to forward modeling of the seismic response from the reservoir simulator confirmed that the effect was water coning under the producers. The forward modeling also identified the possibility for amplitude changes at the top of the reservoir. These areas are being evaluated along with production and reservoir simulation to identify infill producer and injector locations.