4D Seismic Monitoring of Injector Performance and Reservoir Dynamics in a Heterogenous Reservoir

Abstract

Monitoring fluid movement in a heterogeneous water-flooded reservoir is key to optimizing water injection, detecting early water breakthrough, and locating bypassed oil. While well- and production-based reservoir monitoring techniques are commonly applied, they lack the inter-well spatial resolution required for an accurate assessment of the current water movement. As a key tool in spatial reservoir surveillance, 4D seismic surveys were acquired over a water-flooded field located offshore of the Niger Delta. The aim was to map fluid-flow paths and barriers, manage injection for breakthrough optimization, and optimize well placement. The application of 4D seismic techniques has tremendous potential to provide inter-well information on fluid dynamism over time, provided there is a clear link between the seismic observables and reservoir variables such as fluid, pressure, or temperature. Using physical properties of the rocks and fluids from input wells, a 2D seismic model was used to predict the amplitude response to changes in saturation. This involves estimation of the amplitude difference expected from incremental water saturation between pre-1987 and post-1996 wells. The study establishes a correlation between saturation and the two previously-acquired seismic (baseline and monitor) surveys, which were acquired over a nine-year interval. We interpreted the injected water flow path, the potential locations of bypassed oil, and compartmentalization. This was achieved by integrating the well production information, predicted seismic amplitudes at the wells, and the seismic attribute maps. The resulting analysis provided new insights into unexpected production behavior, such as the premature watering-out of updip wells relative to offset wells, and the anomalous pressure support across fault compartments. In accordance with the well performance, it was observed that the injected water follows preferential and heterogeneous flow directions, indicating a strong stratigraphic fluid-flow overprint on the reservoir not previously discernible from well data alone. Subsequently, early results from infill drilling and injection management show better well performance leading to improved oil recovery.