In naturally fractured reservoirs, permeability, and therefore production, can be controlled by fracture networks. The cumulative effect of these small aligned fractures is measurable through the azimuthal variation (anisotropy) of seismic attributes such as amplitude, velocity and impedance.
Often there is a direct relationship between the magnitude of azimuthal anisotropy, fracture intensity and permeability. This can be used to derive seismic-based fracture intensity estimates across fields. It is a unique and valuable resource for well planning and production management in naturally fractured reservoirs.
Seismic azimuthal amplitude anisotropy displayed in vector form on a reservoir horizon in the vicinity of a producing well (green). The vector colour and size indicates the magnitude of anisotropy. The display allows intuitive interpretation of the fracture intensity and orientation.
Wide-azimuth data with evenly populated azimuths and offsets are well suited for azimuthal anisotropy analysis and fracture characterization. Ocean-bottom acquisition can combine the best aspects of wide-azimuth and multi-component data to aid fracture studies. The PS (converted wave) data, recorded by multi-component systems, are particularly sensitive to azimuthal anisotropy and are an additional source of valuable information for fracture characterization.
In the following example, offshore Qatar, a wide-azimuth OBC dataset was acquired over a carbonate reservoir undergoing secondary recovery by water-flood. A seismic fracture characterization project was undertaken to map highly permeable fault zones and fracture corridors for well planning and reservoir management. The use of multi-component data has helped to reduce the uncertainty in the interpretation by providing independent estimates of seismic fracture intensity from the P and PS data for comparison.
Normalized azimuthal amplitude anisotropy intensity map with overlay of interpreted faults (black). Mud-loss points from wells (yellow) indicate permeable fracture zones. Outlined in red is the known conductive fracture corridor which correlates with high anisotropy intensity values, and outlined in green is an area of low fracture intensity.