Method and apparatus for acoustically investigating a casing and cement bond in a borehole

Abstract

Methods and apparatuses for acoustically investigating a casing in a borehole to derive the quality of a cement bond behind the casing and casing thickness are described. The techniques employ an acoustic pulse source having a frequency spectrum selected to stimulate a selected radial segment of the casing into a thickness resonance. The selected frequency spectrum enhances the reverberations between the inner and outer walls of the casing which traps the thickness reverberations with significant amplitudes for a duration depending upon the amount of acoustic energy leaked into adjacent media. The acoustic pulse causes acoustic returns which are formed by the reflections from interfaces between media of different acoustic impedances and acoustic energy leaked into the bore of the casing from the acoustic thickness reverberations stimulated within the casing walls. The acoustic returns are detected to generate a reflection signal which is processed to determine casing thickness or to evaluate the cement bond. The acoustic pulse has a frequency spectrum which is particularly effective in discriminating different cement bond conditions caused by small cement separations known as micro-annuli, around the casing. Several signal processing techniques and tools are described to provide accurate and high resolution cement bond evaluation and casing thickness determination by processing a portion of the reflection signal representative of the thickness reverberations.