The Stybarrow Field - a 4D Case Study

Abstract

The Stybarrow oil field was discovered in 2003 in the Exmouth Sub Basin, offshore Western Australia. Production started at Stybarrow in November 2007. A 4D seismic modelling study conducted early in the field's life indicated that Stybarrow would be a good candidate for reservoir monitoring using the 4D seismic technique. The modelling indicated that changes in reservoir pressure produced by the water injectors and changes in water saturation due to reservoir depletion should be observable on 4D seismic data. The first monitor survey at Stybarrow was recorded in November 2008, 12 months after the start of production and a second monitor in May, 2011. There were a number of geophysical challenges that needed to be overcome for the 4D seismic technique to be successful at Stybarrow. These included very strong azimuthal anisotropy, orthogonal acquisition directions and strong currents. Azimuthal anisotropy produced large artifacts in the data and degraded the strength of the 4D signal. Two methods were used to overcome the effect of azimuthal anisotropy. The first was a conditioning step based on azimuthal NMO prior to prestack time migration and the second method was orthorhombic prestack depth migration. Both methods produced good results. The results of the surveys were in agreement with the 4D modelling and a development well was drilled on the basis of the first monitor survey. The 4D surveys have proven to be an important tool at Stybarrow for facilitating optimal reservoir management.