Seismic Reservoir Monitoring of a Producing Offshore Oil Field

A reservoir monitoring pilot study was conducted in the North Sea over Norsk Hydro's Oseberg field to confirm that the monitoring of hydrocarbon fluid movement was possible utilising repeat 3D seismic surveys. Two 3D surveys were acquired, separated in time by approximately 16 months, using the same recording vessel and equipment. These were processed using identical parameters, and the monitor survey was manoeuvred in 3D space to the base survey locations in order to allow the optimum comparison. Detailed interpretations of both surveys were made, and a high degree of similarity between the surveys was noted. The stratigraphic parameters of the geologic model and the related petrophysical parameters were used to create an acoustic model of the reservoir, which was refined by seismic inversion methods so that the refined geologic model best fitted both the initial geologic model and the seismic data. Reservoir simulations were then run on the refined model to estimate the fluid saturations corresponding to the dates of the two 3D surveys. These fluid saturations were used for comparison with the observed seismic differences. The comparisons showed a seismic anomaly between the two surveys at a location where the Gas/Oil contact was expected to move. The spatial extent and continuity of the anomaly were similar to those changes observed in the simulated saturations. The seismic data provided evidence of the configurations of fluid fronts between wells which were not directly observable from the well data.