This study demonstrated the utilization of seismic data and borehole geometry to determine the horizontal stress direction in Telisa Formation, Balam South Field Central Sumatra Basin, Indonesia. Telisa Formation is one of the oil reservoirs in the Balam South Field, where enhanced oil recovery project has been initiated in this shally-sand reservoir. In the reservoir of interest, seismic attributes gave strong indication of the presence of natural fractures. Acoustic impedance data showed relatively lower magnitude in the suspected natural fractures zone. The decrease of the acoustic impedance magnitude was interpreted as corresponding to the decrease of the acoustic seismic velocity in the suspected fractured zone.
Analysis of borehole shape from multi-arm caliper data in this field also supported the interpretation. The borehole shape is slightly elongated perpendicular to the interpreted maximum stress direction, giving indication of the presence of excess horizontal stress applied to the borehole from the maximum horizontal stress direction. Several other multi-arm caliper data were then acquired to confirm and calibrate the stress map for better interpretation result.
This study inferred the natural fractures orientation from seismic attributes to represent maximum horizontal stress direction. The interpreted map from the seismic data was then calibrated with the maximum horizontal stress orientation determined from the reconstructed borehole geometry. The final interpreted map of maximum horizontal stress direction was then used as a guideline for optimum hydraulic fracturing in the wells drilled in the waterflood project.
Balam South Field is located in Central Sumatra, Indonesia (Fig. 1). Telisa Formation in Balam South Field is one of the major oil reservoirs in this field with the original oil in place is in the order of 650 MMBO. Telisa Formation is characterized by sand-shale laminations over approximately 100 feet (33 meter) thick interval in Balam South (Fig. 2). The trapping mechanism of this reservoir is anticlines controlled by northwest and North-South faults as shown in (Fig. 3).