Borehole Acoustic Reflection Survey (BARS) data were acquired in the Brent formation in the Norwegian North Sea in an exploration well for Hydro. In total, an interval of approximately 300 m was logged. Initial near-wellbore formation images were obtained within 24 hours. The quality of the sonic waveform data was very good. However, significant uncertainties in the actual tool depth caused by rough weather conditions during the time of logging, affected to some extent the coherence and the range of the formation images. The high-resolution image shows an interface dipping at 5° and clearly visible for at least 45 ft away from the well bore. The dip of the interface is in agreement with the expected local geology at the well location. The high-resolution event can be correlated to a 1 m thin coal bed intersecting the borehole and indicated by the petrophysical logs. The coal bed is interpreted as the Top Etive Formation. The BARS images helped characterizing the local geology and the reservoir. In addition, the high-resolution sonic images augment the understanding of the seismic responses in the reservoir section.
The objective of BARS Survey is to use acoustic energy reflected by inhomogenieties in the formation to create a high-resolution image of the formation surrounding the borehole. This survey was done using the Sonic Scanner™ tool, which has 3 monopole transmitters and 13 receiver stations, each with 8 receivers at different azimuths around perimeter of the tool, for a total of 104 receivers. Using all monopole sources, the tool provides 312 waveforms at each depth position. Transmitted energy is reflected at the interfaces 1 and 2 and recorded at the receivers. The parts of the interfaces closest to the borehole will not be well imaged using large source-receiver offsets due to the large reflection angles involved. this effect is clearly evident for interface 1. For an object at a larger distance from the borehole, one should expect to see little difference in resolution between images obtained from data generated by the individual monopole sources. However, the use of all three sources provides a larger range of offsets and better imaging conditions. The BARS technique delivers acoustic images with 2-3 orders of magnitude higher resolution (and accordingly smaller range) compared to borehole seismic images. Potential applications for BARS are well placement relative to formation topography, reservoir structural analysis and characterization. High-resolution images around the well would also help identifying sub-seismic inter beds, faults or fractures. This paper describes the data processing using a BARS dataset recorded in January 2006 in the North Sea.
BARS data were acquired for Hydro in an exploration well in the Brent formation in the Norwegian North Sea. With well deviation up to 60°, the tool was conveyed on drill pipe using a Tough Logging Condition (TLC) technique. The BARS data were acquired in a single run along with dipole shear data. therefore not requiring an additional logging run for BARS.
