Extended Elastic Impedance concept as a simple tool to monitor producing reservoirs

The Extended Elastic Impedance (EEI) concept has been used by the oil industry primarily for lithology and fluid prediction in exploration and development projects. We proposed a method of reservoir monitoring that identifies and maps changes in pressure and saturation in a producing reservoir by applying EEI to time-lapse seismic data. The method uses time-lapse seismic difference data rotated to specific EEI angles which have been optimised for the changes expected in a given reservoir. In one approach to the problem, one angle is found to be appropriate to identify predicted changes in saturation, using fluid-substitution models, while the other angle is found from rock-physics assumptions or laboratory measurements of fluid-pressure changes. This technique was tested using time-lapse seismic data for the Enfield oil field, in the North West Shelf, Australia, with estimates of optimal EEI rotation angles based on log data and Biot-Gassmann modelling for the fluid changes, and on rock physics models fit to measurements made on core samples for the pressure changes. Seismic reflectivity and inversion domains were used for comparison and analysis of the final rotated volumes. In addition to the approach described above, the optimal angles can be found from the scanning method that can be used as an express analysis in finding optimal angles for pressure and saturation changes in the producing reservoir.