Revealing the Secrets of the Troll 4D Seismic Response - Case Study Using Compound Modelling

4D seismic data is extensively used in both planning and in geo-steering horizontal wells in the thin oil leg on Troll West. The 4D signal is complex due to a thin oil column, a mix of high and low permeable sands and gas saturation changes caused by pressure drop from production. Several seismic modelling studies have been conducted to understand the 4D response on the Troll field. On the 2003 monitor survey a deep 4D response was observed on the difference cubes, which has not been evaluated in earlier 4D modelling studies. To improve the interpretation of the time-laps seismic and to understand the reservoir dynamics, a more advanced seismic modelling study was initiated to take account for the increasing complexity of the 4D signal. The results from this study show that both low and high permeable sands are drained during production and that the deep 4D event is caused by gas out of solution due to pressure drop in the residual oil zone below the interpreted OWC. The modelling results have implications for both the geological and the reservoir model.