We present a case study of geosteering extended reach drilling (ERD) wells into a reservoir using the results of anisotropic velocity model building with tomography and anisotropic depth imaging. Three ERD wells were drilled between September 2004 and June 2005 from a production platform on the Nkossa South structure (Congo) for a satellite field development. The design and the accurate geosteering of ERD wells in a structurally complex area is not an easy task. By geosteering we mean directing deviated wells into a reservoir and keeping them within the reservoir layer (Sheriff, 2002). Before drilling, the structural uncertainty has to be accounted for designing a robust trajectory. For this purpose, we need a depth image which is as accurate as possible. While drilling, because of the extended reach and due to the high deviation of the wells, log correlations are difficult. Therefore, the geosteering relies heavily on seismic information. The geological environment for the ERD drilling programme was too complex (lateral velocity variations, anisotropy) for standard seismic time-to-depth conversion techniques. On the other hand, the operational time frame did not allow for a PreSDM imaging project to be performed before the spud of the first well.

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