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

Abstract

The use of seismic data as a¯uid 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 ®eld. 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 signi®cantly 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 re¯ectivity from the oil± water contact. For a horizon between a low-permeable layer (Ness) and a high-permeable layer (Etive) we ®nd a good quantitative agreement between the observed amplitude increase on the time lapse seismic data (41%) and the predicted amplitude increase from¯uid 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¯uid effects are visible in good reservoir sands, while heterogeneous sands with higher clay content show a weaker¯uid signal on the seismic data.

On directional Metropolis–Hastings algorithms

“…We note a significant increase in the pressure wave velocity (Fig. 8, right) around 1900 msec, in correspondence with the interpretation given in Landrø and Strønen (2003). The variability in velocity is quite large, however.…”

“…, n. Figure 6 shows the seismic reflection amplitudes d for the three angles. A geophysical analysis of the data is presented in Landrø and Strønen (2003). Seismic data are of very high dimension, and for simplicity we only use a small part of the dataset in this example.…”

Time lapse (4D) and AVO analysis: A case study of Gullfaks field, Northern North Sea

“…This trend was seen throughout each of the formations for all the three wells. Modelling of the Tarbert Formation did not reproduce similar changes in V p compared to the results modelled by Landrø and Strønen (2003). This could be due to using a calculated porosity log as opposed to a constant 30% porosity value.…”

Long-term fluid expulsion revealed by carbonate crusts and pockmarks connected to subsurface gas anomalies and palaeo-channels in the central North Sea

“…Surface and subsurface expressions of gas seepage to the seabed are observed throughout the North Sea (e.g. Hovland and Sommerville 1985;Landrø and Strønen 2003;Schroot et al 2005;Forsberg et al 2007;Kilhams et al 2011). Indeed, gas flares, carbonate crusts and near-surface gas anomalies have been reported throughout the region (e.g.…”