Improved Borehole Imaging by Ultrasonics

Abstract

Abstract A new ultrasonic tool for borehole and casing imaging has been developed based on recent cementation imaging technology. A rotating ultrasonic transducer scans the borehole at a high sampling rate to provide detailed images of echo amplitude and radius. A 250 or 500 kHz focused transducer gives high resolution, penetration in heavy mud and low sensitivity to tool eccentering. The echoes are analyzed by a downhole digital signal processor to optimize the accuracy and reliability of the radius measurement. The measurements are corrected for eccentering, and the image color scales are dynamically adjusted for optimum sensitivity in real time by the surface computer. Comparisons with electrical imaging tools show the ultrasonic amplitude measurement tends to respond to lithology indirectly via changes in borehole radius or rugosity. Ultrasonic imaging is unique in making quantitative high-resolution measurements of borehole geometry that are useful for borehole stability analysis. Examples of automatic hole shape analysis are shown. The tool can also evaluate internal casing corrosion and detect holes. 1. Introduction Detailed images of the borehole can be produced by three common techniques: video cameras, microelectrical imagers and ultrasonic scanners. Video cameras operate only in clear liquids or gases. Electrical imagers cannot be used in oil-base mud. The ultrasonic method works in water-base and oilbase muds and is the only technique that provides a high-resolution caliper (borehole geometry) survey. The first- ultrasonic instrument, the borehole televiewer, was introduced over 25 years ago [1]. In the 1980s Shell and Amoco updated the design and in the last few years the technique has enjoyed a revival of interest with the introduction of new tools by service companies [2,3].