Challenges in Seismic Imaging and Reservoir Characterization of Presalt Oilfields in Offshore Brazil

摘要

Until very recently, the petroleum system of Brazilian presalt reservoirs could not count on good enough seismic imaging to support the seismic interpretation. In the 90’s, the decision of Petrobras to expand its exploration frontiers towards the sedimentary basins of Espírito Santo and Santos brought forth new efforts to improve the existing 2D seismic imaging. The favorable outcome of this campaign motivated the 2001 decision to acquire a 3D program covering potential exploratory leads. In 2005, Petrobras and its partners began exploratory drilling in presalt reservoirs, culminating in the discovery of the Lula- Iracema (2006) and Sapinhoá (2008) fields and other areas: Iara, Carioca, Júpiter, etc. In 2008, as a consequence of these results, Petrobras started oil production from the Brazilian presalt province. In 2008-2009 a high density 3D seismic program was acquired over these areas and state-of-the-art seismic processing technology was applied in Santos and Campos basin. In 2010-2011 a full-azimuth acquisition project took place east of Iara area (circular streamer acquisition), an orthogonal OBC project was acquired in Campos basin and also some borehole seismic projects (walkaway, walkaround and 3D VSP) were performed in Santos basin. The seismic processing flow evolved from building isotropic velocity models to anisotropic ones, with the use of in-depth imaging techniques such as TTI PSDM. An important processing challenge has been to improve the interbed multiples attenuation. Moreover, the seismic data processing and full-azimuth 3D VSP have been enhanced by focusing on improved imaging and characterization of Brazilian presalt reservoirs. Several processing methodologies were used, including in-house algorithms/workflows in addition to service companies’ solutions. The seismic characterization of the presalt reservoirs has made extensive use of inversion techniques, seismic facies and seismic attributes extraction from the inverted data with very good results. The results of elastic inversions are still a challenge. Another major challenge will be the implementation of 4D seismic technology for monitoring the behavior of fluid motion and pressure variation in the Brazilian presalt reservoirs.