A B S T R A C TCombined time-lapse reservoir simulation and seismic modelling has been performed on both 1D and 3D models of a channelized turbidite reservoir. The models have been built using core, log, laboratory and seismic data from the Nelson Field (central North Sea) as a template. Oil and water movement in the main channels, channel margins and interchannel regions is investigated, with a particular focus being the effect of poor net-to-gross. The analysis confirms that saturation effects dominate the response whilst stress-sensitivity effects play a minor role. The trough-peak signature in the seismic difference volumes formed by the sweep of the water can be continued and mapped slightly further than the channel margins. This characteristic 4D signature remains roughly intact, despite the complicated depositional architecture, and accurately delineates the area of moved fluid, but it needs additional calibration to combat the detrimental influence of the low net-to-gross. Signal strength is largely dependent on reservoir quality, but is also compromised by the net-to-gross, fluid distribution and, more critically, by the exact timing of the seismic survey. For example, a region of bypassed oil zone remains undetected as it forms early during the production. This work demonstrates that to understand fully the 4D signature at a quantitative level requires adequate knowledge of the fluid properties, but also, more critically, the geology.

Integrated analysis of 4D seismic data and petrophysical data is used to produce probabilistic fluid and lithology volumes for monitoring reservoir performance on the Nelson Field. Petrophysical analysis of log data shows distinct fields for oil sand, water sand, shale and heterolithic 'lithologies' in acoustic impedance -Poisson's ratio space. Elastic inversion techniques applied to conventional 4D AVO datasets convert the reflectivity data to acoustic impedance, shear impedance, Poisson's ratio and angle impedances. The elastic inversion datasets are used to quantify oil-water contact movements through volume sculpting techniques. Well-derived relationships are used to predict 3D volumes of oil sand probability from three different seismic survey vintages: 1990, 1997 and 2000. Changes in oil sand probability due to production are verified by comparison with repeat production logs. Integrated volume interpretation of 4D far offset inversion difference (oil-water contact (OWC) movement) and oil sand probability shows areas of unswept oil, highlighting infill opportunities. Early results from infill drilling have validated the method, realizing the potential economic benefits of 4D seismic technologies.

A B S T R A C TA new method for time-lapse signal separation and enhancement using singular-value decomposition is presented. Singular-value decomposition is used to separate a 4D signal into its constituent parts: common geology, time-lapse response and noise. Synthetic tests which demonstrate the advantages of the singular-value decomposition technique over traditional differencing methods are also presented. This signal separation and enhancement technique is used to map out both the original and moved oil-water contacts across the Nelson Field. The singular-value decomposition technique allows the oil-water contact to be mapped across regions which would have been missed using traditional differencing methods. In particular, areas toward the edges of the field are highlighted by the technique. The oil-water contact is observed to move upwards across the field, with the largest movements being associated, as anticipated, with natural production. The results obtained are broadly consistent with those predicted by the reservoir simulator model. Singular-value decomposition is demonstrated to be a useful tool for enhancing the time-lapse signal and for gaining confidence in areas where traditional differencing fails. I N T R O D U C T I O NThe differencing of seismic volumes to investigate time-lapse changes poses a variety of problems: simply subtracting one data set from another often has the effect of enhancing noise, with the true reservoir changes being swamped as a consequence. Numerous attempts to combat these problems have been made previously by, for example, Landrø et al. (1999), who computed the robust difference (effectively smoothing prior to differencing), which reduces the rms difference level

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