--> High Coverage OBM Images for Reservoir Characterization - Deep offshore Angola Case Study, by Sylvain Riba, Nathalie Bordas - Le Floch, and Frederico Domingos; #90037 (2005)

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High Coverage OBM Images for Reservoir Characterization - Deep offshore Angola Case Study

Sylvain Riba1, Nathalie Bordas - Le Floch2, and Frederico Domingos3
1 Total E & P Angola, Luanda, Angola
2 Schlumberger Angola, Luanda, Angola
3 SONANGOL DEX, Luanda, Angola

The resistivity imager was run over the 12" ¼ phase of an exploration well in deep offshore Angola. The well was slightly deviated (14° inclination maximum) and drilled in oil-base mud, with good borehole conditions for imagery. Acquired in wireline mode, the tool's logging speed was reduced below recommendations and pad pressures carefully monitored.

Speed-corrections, equalization, depth-matching and normalization were performed to provide enhanced images suitable for interpretation. Using a dual OBMI with a coverage of up to 42% of the wellbore, the borehole image allowed a structural and sedimentological analysis of cored and non-cored zones with high confidence. Geological features were picked manually on the images and classified into eight categories according to their nature. Two main structural zones were defined, based on changes in dip magnitude and azimuth. Drilling induced fractures and in-situ regional stress evaluation were also analysed.

Laminated sands were not discriminated by standard tools (nuclear, resistivity...) and, consequently, were not included in classical net sand count. A textural analysis of the resistivity image was performed to include these laminations into the net thickness. Automatic identification of thin beds zones was obtained with the resulting textural map. Then, using a normalised high-resolution shallow resistivity coming from the OBMI tool, a count of sandy laminations (versus shaly laminations) was achieved.

Thanks to the resistivity image, all the laminated sands were identified and taken into account in the net sand thickness, resulting in 25% increase in the net thickness.