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Coral Field: Case History in Oil Producing Albian Carbonates in Santos Basin, Brazil

Figueiredo, Luiz Fernando1
 Karam, Márcia1

1E&P, Queiroz Galvão Óleo & Gás, Rio de janeiro, Brazil.

In the South of Santos Basin, a consortium formed by Petrobras, Queiroz Galvão Óleo & Gás, Starfish Oil & Gas and Coplex do Brasil exploited from 2003 through 2008 an offshore carbonate oil reservoir in the Coral Field. Coral Field was discovered by Petrobras in 1990 September by the well 1-BSS-56, 170 km offshore Santa Catarina State, at water depth of 152 m. Carbonate oil reservoirs were encountered at 5000 m depth in a small horst structure of 8,0 km2. The total carbonate section comprised 538 m of clean oolithic/oncolithic high energy calcarenites interbedded with low energy shaly limestones, with only 87 m corresponding to oil satured porous rock. The reservoirs of Coral Field are albian carbonates of Guarujá Formation. The reservoirs show excelent lateral continuity and good vertical compartments controlled by the depositional energy. Pelloidal mudstones and wackestones form the internal seals. The reservoirs are oolithic/oncolithic calcarenites of high energy, interbedded with low energy sediments of open sea cicles which are seals for the units of the reservoir. The oil is very light, 43 API, with viscosities in reservoir condition of 0,24 cp to 0,41 cp. Original pressure is 600 kg/cm2 while the saturation pressure varies from 159 kg/cm2 to 194 kg/cm2 meaning that the reservoir is strongly undersatured. Temperature is 293 oF at -4976 m. Total oil in place amounted to 12 million m3 and proved reserve amounted to 2,7 million m3. During the entire project, asphaltene deposition in the well bore and in the neighbourhood of the perforations caused severe lost of productivity. Acidification jobs were common in order to restore productivity. The field production model was one of a highly undersatured reservoir submitted to a production mechanism of oil and water expansion. The reservoir rock and fluid properties conducted to very high productivity index of the wells, with the wells surging at the surface till the end of the project. Final recovery of 1,967 million m3 corresponded to 16% of 12 million m3 of original oil in place.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil