Estimation of Saturation Changes at the Edvard Grieg Field Using Simultaneous 4D Wave-Equation Based AVO Inversion

Time-lapse (4D) Wave-Equation-Based Amplitude-Vs-Offset (WEB-AVO) inversion of two vintages of seismic OBC data (2016 and 2018) over the Edvard Grieg field has been performed. Because of the non-linear relationship between seismic data and the earth properties to be derived, it is essential for 4D that we do not subtract the data directly, but that we subtract the inversion results of the two vintages. This gives us the opportunity to carry out simultaneous inversion, where we can apply constraints on the time-lapse property differences. The constraint was imposed such that the time-lapse property differences are sparse in depth. The results of the new inversion are compared with conventional calibration of NRMS time-lapse data differences, using reservoir simulation based on a pre-4D reservoir model. It turns out that the conventional method significantly over-estimates the saturation changes, whereas the WEB-AVO inversion predicts saturation changes in good agreement with the range of changes predicted by the 4D reservoir simulation. Also, the area over which the new inversion finds saturation changes, is smaller than what is predicted by the reservoir simulator. Given known uncertainties with the current dynamic reservoir model, the new 4D saturation map represents a more realistic image of the water swept areas.