4-D Seismic Inversion - A Case Study Offshore Congo

The first 4D seismic survey in Congo was acquired over the Moho-Bilondo Field in January 2011, two years and a half after first oil. 4D was expected to help in tracking the movements of injected water inside complex turbiditic bodies and to follow the rise of the oil-water contact. 4D anomalies can be highlighted using seismic attributes such as envelope amplitude and RMS amplitude differences but these standard 4D attributes have a limited vertical resolution and difficulties were encountered to distinguish depletion from water injection. A simultaneous 4D inversion workflow was therefore applied to extend the seismic bandwidth and estimate quantitatively the changes in reservoir acoustic impedance. Spatially variant rock physics constraints were introduced to improve the imaging of the water pathway around key injector wells. Preliminary analysis of the inverted 4D attributes shows an improved image of the fluid movements inside the reservoir compared to conventional 4D attributes.