4D seismic modelling on the Fangst reservoir onthe Heidrun Field: A quantitative approach of the fluid and saturation changes, and the relation to 4D-seismic

Abstract

Based on the pre production base surveys shot in 1986 and 1991, several repeated seismic surveys have been shot with the aim to monitor the fluid flow on the Heidrun Field offshore Mid-Norway. This is to better define targets for in-fill drilling to increase total oil recovery. Interpretations of such seismic time-lapse data are done withgreat success, but uncertainties upon what causes the reflections are still an issue. This thesis show modelled time-lapse seismic that represent production caused fluid changes in the Fangst reservoir on the Heidrun field. Time-lapse effects caused by time shift and pressure changes are neglected, because it is assumed that these effects are small compared to the fluid change.

Well information, theoretical assumptions and self constructed settings provides the basis to create synthetic models. These are tested and compared with real seismic data and the aim is to detect how 4D amplitude reflections interfere with each other in the formation. Synthetic gathers show great variation in reflectivity with different fluids in the formation and small AVO effects are seen.

With the settings made in this thesis, and compared with the inline 424 that was shot in 2008, it is likely to believe that the upper sand in the group, the Garn Formation, is filled with gas. The Ile Formation, in the lower part of thegroup, is interpreted to either be fully flooded by water or flooded by different variation of gas in the two lower sands. There are great uncertainties connected to the fluid saturation in this formation, because different assumptions supports different scenario. Using the models as a library can ease the interpretation of other areas,or motivate to make new models from of the experiences made during this work.