Borehole images obtained from microresistivity and ultrasonic imager tools form an important source of geological information. The interpretation of these images is often hampered by the presence of artefacts, arising from peculiarities of the logging tools, and/or unexpected borehole conditions. In this paper, the operations of microresistivity and ultrasonic imager tools are reviewed,
Summary 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. Introduction Detailed images of the borehole can be produced by three common techniques: video cameras, micro-electrical 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 oil-base 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. 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.
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].
Formation imaging using microelectrical arrays is an extremely powerful tool that enables the log analyst, geologist, geophysicist and petroleum engineer to better evaluate complex reservoirs. To date, electrical formation imaging tools with high spatial resolution can image only part of the borehole wall at any one time; anempts to increase borehole coverage have been achieved but with a small degradation of image resolution. A new formation imaging tool has been developed to provide real-time wellsite images of the borehole wall with unprecedented borehole coverage at a resolution of 0.2 in. The borehole coverage of this new tool is 80% in an 8-in. borehole, which represents an increase by a factor of two over previous generation tools. This increase has been achieved previous generation tools. This increase has been achieved while also improving the spatial resolution of the tool. This paper describes the applications of the new tool and the improvement in interpretation of the images. Four tools have been built and tested successfully in wells throughout the world. Examples are given of a fractured formation, a reverse fault, a braided sand and a horizontal well to demonstrate the improvement in the data interpretation. Introduction High-resolution electrical borehole imaging has proven to be an extremely powerful tool that enables the log analyst, geologist, geophysicist and petroleum engineer to better evaluate complex reservoirs. Borehole imaging has made it possible to obtain an in-situ description of the reservoir without having to resort to full-hole coring over the entire zone of interest. The first borehole tool capable of producing high-resolution electrical images, the Formation Micro Scanner*, of the borehole wall was introduced in 1986. This tool was an important extension of the well-known electrical dipmeter tool. The measurement was a pad-based, passively focused resistivity measurement. In fact, the first version of the Formation Micro Scanner tool provided the normal eight dipmeter measurements, coming from four pads, while at the same time supplementing the dipmeter data with electrical images derived from two of the four pads. The details of the array are discussed in reference 3. Briefly, each of the imaging arrays comprised 27 electrodes and had a lateral span of 7 cm, thus providing an image that covered about 20% of an 8-in. borehole wall. Each electrode had a diameter of 5 mm, resulting in a lateral resolution of about 5 mm, or 0.2 in. The high-quality images obtained from this tool allowed for the first time visualization of the true complexity of the reservoir on a wireline log without sensitivity to borehole shape, mud weight and solid content. Despite this significant advance, the limited coverage of the borehole wall meant multiple logging runs over the same depth interval in the well had to be made to image parts of the borehole wall that had not been previously parts of the borehole wall that had not been previously imaged. Success of this operation hinged on the tool rotating between the different logging runs, which was difficult to control. This shortcoming was resolved by introducing a second generation Formation Micro Scanner tool in 1988 that repartitioned the electrodes on four pads to provide an electrical image of about 40% of the borehole wall in an 8-in. borehole. This modification was possible only by giving up lateral resolution to gain possible only by giving up lateral resolution to gain coverage. The lateral resolution of the new Formation Micro Scanner tool was about 0.3 in. This small degradation in resolution was an acceptable tradeoff to gain the additional coverage. P. 653
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