RESERVOIR MONITORING USING 4D SEISMIC DATA AND ROCK-PHYSICS
Bernaez Pulgar, Alberto Jose Jose 1989-
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The Norne oil field (NF) in Norway is experiencing bypassed oil due to the water- flooding recovery strategy employed to maintain the initial pressures. Four time-lapse seismic surveys, used as a supportive tool on its field development plan, were provided for this research, and were qualitatively studied. Two out of these four surveys were quantitatively studied combining Rock-physics templates (RPT) and Time-lapse Pre-stack seismic inversion. The inverted volumes of acoustic impedance (AI) and Vp/Vs ratio were analyzed for the changes caused by the produced hydrocarbons and the injected water. A range of well-logs were used to build and calibrate an RPT that characterizes the reservoir sandstones of the NF. The analysis of the well-log data demonstrate that it is possible to discriminate between the brine- and oil-saturated sands in this field based on the seismic response to their elastic properties. Specifically, cross-plotting acoustic impedance against the ratio of compressional and shear velocities (Vp/Vs ratio) shows a clear separation between the saturations. The results of the inversion show variations in acoustic impedance and Vp/Vs ratio across two of the segments of the field. The areas around the producer wells show minor changes, contrasted with major variations around the injector wells. The comparison of the elastic properties inverted from successive 3D seismic surveys were interpreted with the customized rock-physics template to show the discrimination of the reservoir lithology and fluid dynamic changes across the different faulted blocks, revealing the impact of hydrocarbon production on the seismic response due to changes of reservoir mapping of fluid migration fronts. Combining time-lapse seismic inversion and RPTs captured the production-induced seismic changes when comparing the base- and monitor-inverted volumes. These changes were observed in the form of high acoustic impedance and a high Vp/Vs ratio around the injector wells, which gradually increase from the base to the monitor survey. These increases were interpreted as an effect of the water flooding recovery strategy applied to these reservoirs. Some areas of the field experienced higher water replacement than others, likely indicating that the areas with lower water replacement have experienced less hydrocarbon production.