Technical Paper: First Borehole Acoustic Reflection Survey mapping a deepwater turbidite sand

Society: SEG
Paper Number: 2369864
Presentation Date: 2006


The South Marlim oil field, discovered in 1987 offshore Campos Basin, Brazil, saw in the 1990's some deepwater production records being established and the development plans for the field should allow reaching a peak production of 420,000 boe/d in 2010. South Marlim is a turbidite reservoir of Upper Oligocene-Lower Miocene age. These reservoirs were initially thought to be homogeneous, widespread turbidite fans, but more recent studies based on later well data and 3D seismic surveys found that they can be rather complex and heterogeneous (Bruhn, 2001). In order to better define the turbidite reservoir geometry at one South Marlim's horizontal producer location, Petrobras decided to run a Borehole Acoustic Reflection Survey (BARS), the first time ever this was attempted in this type of environment. The Sonic ScannerTM tool from Schlumberger was drill-pipe conveyed (TLC) in order to collect the data. The processed images have a depth of investigation of 10 m and a vertical resolution of 0.3 m. The BARS technique mapped the main turbidite sand over 544m of horizontal well section, showing thickness variations between 4 and 10m. No other measurement provided this level of information. At the pilot-hole, the sand was 4 meter thick and the horizontal well was geosteered assuming constant sand thickness. A flat spot event was mapped one meter above the tip of the well, suggesting segregated gas in that reservoir area, which lies on a structural high. The well produced limited amounts of gas supporting the direct gas-oil contact indicator from the acoustic method. This is possibly the first fluid contact ever mapped by deep sonic imaging technique. The BARS images were consistent with the dips estimated from image logs, with the geosteering data and to a great extent, with the pilot-hole results. Their correlation with surface seismic is encouraging, despite the enormous scale differences. Petrobras plans to use the high-resolution sonic images in order to provide lateral constraints to seismic acoustic impedance inversion and as an input to geological modeling. The resulting improvements in the static model should in turn enhance considerably the prediction capability of the reservoir dynamics simulators.