Modelling the Impact of Overburden Stress Arching on 4D Seismic Amplitude Changes

Reservoir depletion can cause geomechanical changes in both the reservoir and in the overburden. When depletion pressures are large or when the reservoir rock is highly compressible, the increasing load on the reservoir rock during production may cause compaction. As the reservoir compacts and pulls away from the surface of the earth, a stress arch forms in the overburden. The stress arching reduces the load on the reservoir and results in a lower effective stress change in the reservoir than would be predicted from the reservoir pressures alone (shielding of the reservoir by the overburden). This effect is more pronounced for reservoirs with high aspect ratio (thickness to diameter ratio). These geomechanical changes result in both time-shift signals and impact on the amplitude changes between repeat seismic surveys. We show that it is important to include the geomechanical effects when modelling 4D seismic amplitude changes, as they can have a significant impact on the amplitudes observed. By incorporating these changes into a 4D simulation-to-seismic modelling workflow, we were able to explain an anomalous amplitude response observed in 4D seismic data acquired over the Atlantis field.