4D study of fluid effects on seismic data in the Gullfaks Field, North Sea

Abstract

AbstractThe use of seismic data as a fluid indicator has been investigated on data from a typical North Sea Field (Gullfaks). Examples of qualitative seismic analysis on one single dataset as well as on repeated datasets are shown, clearly demonstrating the usefulness for reservoir description and reservoir management for this field. Quantitative comparisons between expected and measured changes between water‐ and oil‐saturated reservoir rocks are presented. On a single dataset, we found a seismic amplitude increase of 26% between the water zone and the oil zone for the dipping top reservoir horizon. This value was significantly lower than the expected value based on rock physics and simple two‐layer seismic modelling, which was 80%. The discrepancy is attributed to initial oil saturations of probably less than 100%, the relatively high‐noise level in the seismic data, and interference effects with reflectivity from the oil–water contact. For a horizon between a low‐permeable layer (Ness) and a high‐permeable layer (Etive) we find a good quantitative agreement between the observed amplitude increase on the time lapse seismic data (41%) and the predicted amplitude increase from fluid substitution of well logs (40%). The general observation made in this study is that the seismic amplitudes are in good correspondence with the rock physics theory. Furthermore, the seismic fluid effects are visible in good reservoir sands, while heterogeneous sands with higher clay content show a weaker fluid signal on the seismic data.