Examination of BHTV, FMS, and SHDT Images in Very Thinly Bedded Sands and Shales

Abstract

ABSTRACT Three logging tools can produce detailed images of the wellbore wall, based upon variations in rock properties: the Borehole Televiewer (BHTV), an acoustic device, and the Schlumberger Stratigraphie High-Resolution Dipmeter (SHDT) and Formation MicroScanner (FMS), electrical tools that measure relative formation resistivity. A Shell well was cored and then logged with the BHTV, SHDT, and FMS tools. The rocks consist, in part, of thinly interbedded sands and shales, with bed thicknesses down to 1 mm thick. The log and core data from this well were rigorously compared to the images generated by the BHTV, SHDT, and FMS tools. We conclude that the BHTV, SHDT, and FMS produced good images of the interbedded sands and shales down to a bed thickness of about 1.5 inches [38 mm]. Below 1.5 inches [38 mm], the SHDT image showed closer to the true number of very thin beds present. Overall, the BHTV amplitude data gave the best sense of actual sand:shale ratio; the FMS images were slightly less similar to the core, compared to the BHTV and SHDT. All three tools are influenced by very small changes in borehole size. Heavily cemented zones are easily detected by all three tools. None of the three tools produced an accurate image of slumped silty shale or homogenous-looking silty shale, possibly due to irregular borehole washouts.