4D elastic inversion helps locate in-fill wells at Oseberg field

H. Rutledal, Norsk Hydro E&P, and J. Helgesen and H. Buran, both with CGG Norge, describe how, 4D elastic inversion for the Oseberg field was combined with modern 3D visualisation techniques to support the reservoir history matching process and detect areas of undrained reserves. Several successful in-fill wells have been planned and drilled based on the integrated analysis of inversion results, well data and reservoir models. Oseberg is a major oil and gas field operated by Norsk Hydro in the North Sea some 140 km off the coast of Norway. The reservoir comprises sandstones from the Middle Jurassic Brent Group in three eastward-dipping, tilted fault-blocks. Production at the Oseberg field started in 1988. The production plateau was reached in 1992, and production started to decline in 1996. Currently, close to 85% of the initial oil reserves have been produced. The challenge facing the operator Norsk Hydro is to maximise the economic value of the field by increasing oil recovery. Additional in-fill drilling, guided by 4D seismic interpretation, was identified as a way of achieving this. A monitor survey was acquired in 1999 and combined with a base survey from 1992. The two vintages were processed using a 4D sequence to maximise repeatability. In order to exploit the full potential of the 4D data, an elastic inversion of the 1999 vintage was carried out for reservoir characterisation purposes, using a 3D layer-based inversion (StrataVista). After successful testing, this elastic inversion was extended to the 1992 data for mapping of production effects.