Quantitative corrosion evaluation in wells using a multi-function ultrasonic imager

A multi-function ultrasonic imaging tool has been developed for well inspection. The tool is suspended at the end of a long armoured cable and operates by rotating a pulse-echo transducer in the well fluid. It was initially applied to cement evaluation with simultaneous casing inspection, using a resonance method (200-700 kHz). Subsequently, a focused 250/500 kHz transducer was provided for openhole imaging and detailed internal casing inspection. The latest functional addition to the instrument is a focused 2 MHz transducer with a spot size of 0.11", developed for high resolution internal and external corrosion imaging. The aim is to quantify small-scale defects such as pitting. The casing radius and thickness are measured from the arrival times of the internal and external casing echoes, using downhole digital processing. The casing is examined every 2/spl deg/ in azimuth and every 0.2" vertically in a helical scan as the armoured cable is reeled in. Laboratory experiments verified quantification of 0.16" diameter flat-bottom holes. Field test results demonstrated the detection of small-scale internal and external corrosion and damage.