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

A method for determining characteristics of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed from inside the casing at a radial segment thereof and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising the steps of

• deriving from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment;
• measiuring the energy in the selected reverberation segment and provide a bond signal indicative thereof to characterize the quality of the cement bond behind said radial segment of the casing;
• and
• measuring the frequency of components contributing to a peak value in the frequency domain of the reverberation segment and provide a thickness signal indicative thereof as the casing thickness at said radial segment for the evaluation of the cemented casing and the resolution of potential ambiguities in the cement bond evaluation at said radial segment.

An apparatus for determining the quality of the bond between cement and a casing cemented in a borehoele penetrating an earth formation from a reflection signal obtained from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising

• means responsive to said reflection signal for detecting an initial casing reflection from the casing and produce a signal indicative thereof;
• means actuated by the detected initial casing reflection signal for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls;
• and
• means for producing a bond signal indicative of the energy in the selected reverberation segment to characterize the quality of the cement bond behind said radial segment of the casing.

An apparatus for determining the thickness of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising

• means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls;
• means for generating a spectrum signal representative of the frequency spectrum of said reverberation segment;
• and
• means for determining the frequency of components in said spectrum signal contributing to a peak value thereof and producing a thickness signal representative thereof as the casing thickness.

An acoustic pulse echo apparatus for investigating a casing cemented in a borehole penetrating an earth formation comprising

• means for generating an acoustic pulse from inside the casing in a generally radial direction towards a selected radial segment of the casing wherein said acoustic pulse has a frequency spectrum selected to enhance entrapment of acoustic energy between the inner and outer casing walls at the radial segment for stimulation of reverberations therein and generating a reflection signal representative of acoustic returns from different layers of material in the path of the acoustic pulse with acoustic leakage from reverberations trapped inside said casing walls;
• means for selecting a reverberation segment of the reflection signal wherein said selected segment is substantially representative of said reverberation leakage in the acoustic returns;
• means for determining the energy in the selected reverberation segment and producing a bond signal indicative thereof to characterize the quality of the cement bond behind said radial segment of the casing;
• and
• means for determining from said reverberation segment the frequency of components contributing to a peak value in the frequency domain of said reverberation segment and generate a casing thickness signal representative thereof as the casing thickness at said radial segment for the evaluation of the cemented casing and the resolution of potential ambiguities in the cement bond evaluation at said radial segment.

An acoustic pulse echo apparatus for investigating the quality of the bond between cement and a casing located in a borehole penetrating an earth formation comprising

• means for generating from inside the casing an acoustic pulse towards a radial segment of the casing and the formation and producing a reflection signal representative of acoustic returns from the interaction of the acoustic pulse with different layers of material in the path of the acoustic pulse, said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate a thickness resonance between the inner and outer walls of the casing and with the acoustic wave frequencies further being selected to render micro-annuli representative of good cement bonds effectively transparent while enhancing reflections from annuli representative of bad cement bonds;
• means for selecting a reverberation segment of the reflection signal following an initial casing reflection wherein said reverberation segment is substantially representative of acoustic leakage from reverberations introduced in between the walls of the casing by said acoustic pulse;
• and
• means for measuring the energy in the reverberation segment of the reflection signal and produce a bond signal indicative thereof to characterize the quality of the cement bond.

An acoustic pulse echo apparatus for investigating the quality of the bond between cement and a casing located in a borehole penetrating an earth formation comprising

• means for generating from inside the casing a highly damped acoustic pulse towards the formation wherein said acoustic pulse has waves at frequencies selected to stimulate a thickness resonance inside the casing walls, said acoustic wave frequencies further being selected to render micro-annuli representative of good cement bonds effectively transparent while enhancing reflections from annuli representative of bad cement bonds;
• said acoustic pulse generating means being further responsive to acoustic returns produced by said acoustic pulse for producing a reflection signal representative thereof;
• said acoustic pulse producing means being at a predetermined minimum spacing from the casing to enable the detection of acoustic reverberations substantially free from secondary transmission interference;
• means responsive to said reflection signal for detecting an initial casing reflection from the casing;
• means actuated upon the detection of the initial casing reflection signal for selecting a reverberation segment following said initial casing reflection;
• and
• means for producing a bond signal indicative of the energy in the selected reverberation segment to characterize the quality of the cement bond.

An acoustic pulse echo apparatus for investigating the quality of the bond between cement and a casing located in a borehole penetrating an earth formation comprising

• means for generating from inside the casing an acoustic pulse towards a radial segment of the casing and produce a reflection signal representative of acoustic returns from different layers of material in the path of the acoustic pulse, said acoustic pulse being generated with a bandwidth selected to stimulate a thickness resonance between the inner and outer walls of the casing with substantially reduced reflections from hydraulically secure micro-annuli representative of good bonds and with significantly longer duration reverberations in the casing in the presence of annuli representative of bad cement bonds;
• means responsive to the reflection signal for generating a casing reflection signal indicative of the duration of an acoustic reflection from the casing;
• means responsive to the casing reflection signal for producing a reverberation segment selection signal to identify a reverberation segment of the reflection signal following the casing reflection;
• means enabled by the reverberation segment selection signal for measuring the energy in the reflection signal for the duration of the reverberation segment selection signal and produce a bond signal indicative of the quality of the bond between the casing and the cement located in the path of the acoustic pulse;
• means for producing a normalizing signal representative of a predetermined characteristic in the acoustic reflection from the casing;
• and
• means for combining said bond signal with the normalizing signal to produce a normalized bond signal representative of the quality of the cement bond.

An acoustic pulse echo method of investigating the quality of the bond between cement and a casing located in a borehole penetrating an earth formation comprising the steps of

• generating a pulse of acoustic energy towards the formation from inside the casing with the acoustic energy having a frequency spectrum which is selected to stimulate the casing into a thickness resonance to trap reverberations in the casing and having said frequency spectrum selected to generate acoustic waves at frequencies whose water wavelengths exceed the thickness of hydraulically secure micro-annuli by a factor sufficient to render said micro- annuli effectively transparent to said acoustic pulse;
• deriving a reflection signal representative of acoustic returns from different layers of material in the path of the acoustic pulse;
• and
• determining the energy in a reverberation segment of the derived reflection signal to the exclusion of a strong casing reflection in the reflection signal and substantially attributable to acoustic leakage from reverberations inside the casing as an indication of the quality of the bond between the casing and the cement located in the path of the acoustic pulse.

A method for acoustically investigating the quality of the bond between cement and a casing located in a borehole penetrating an earth formation with a pulse echo technique comprising the steps of

• generating an acoustic pulse inside the casing towards a selected radial segment of the casing and the formation to cause acoustic returns attributable to the acoustic interaction of the acoustic pulse with different layers of material in the path of the acoustic pulse, wherein said acoustic pulse has acoustic wave frequencies in a bandwidth selected to stimulate the casing into a thickness resonance to trap acoustic reverbertions inside the casing walls, with the acoustic wave frequencies further being selected to reduce reflections from micro-annuli representative of good cement bonds while enhancing reflections from annuli representative of bad cement bonds;
• detecting the acoustic returns to produce a reflection signal indicative thereof;
• selecting a casing segment from the reflection signal representative of a reflection from the casing;
• selecting a reverberation segment from the reflection signal representative of reflections occurring subsequent to said casing reflection and substantially representative of leakage returns from reverberations introduced in the casing by the acoustic pulse;
• and
• processing said selected segments to cooperatively produce a bond signal indicative of the quality of the cement bond.

An apparatus for acoustically investigating the quality of the bond between cement and a casing located in a borehole penetrating an earth formation with an acoustic pulse echo technique comprising

• means for producing an acoustic pulse having acoustic wave frequencies selected to stimulate the casing into a thickness resonance with enhanced entrapment of reverberations inside the casing and provide a reflection signal representative of acoustic returns caused by the acoustic pulse;
• means for extracting from the reflection signal a frequency segment selected to include casing thickness resonance frequencies and generate a bond signal representative thereof as indicative of the quality of the cement bond;
• means for extracting from the reflection signal a reference frequency segment and produce a reference signal indicative thereof;
• and
• means for combining the reference signal with the bond signal to provide a normalized bond signal indicative of the quality of the cement bond.

A method for acoustically evaluating the quality of the bond between cement and a casing in a borehole penetrating an earth formation comprising

• generating an acoustic pulse from inside the casing towards a radial segment of the casing wherein the acoustic pulse has a frequency bandwidth selected to stimulate a thickness resonance with acoustic reverberations inside the radial segment of the casing;
• detecting acoustic returns attributable to the interaction of the acoustic pulse with materials in the path of the acoustic pulse and produce a reflection signal indicative thereof;
• selecting a predetermined frequency band from the reflection signal wherein the selected frequency band includes casing thickness resonance frequencies and produce a bond signal representative thereof to indicate the quality of the cement bond;
• selecting a reference frequency band from the reflection signal and produce a reference signal indicative thereof;
• and
• combining the reference signal with the bond signal for normalization thereof.

An apparatus for investigating with an acoustic pulse a casing located in a borehole penetrating an earth formation comprising

• means for directing an acoustic pulse from inside the casing in a radial direction at a radial segment of the inner wall of the casing, wherein the acoustic pulse has acoustic wave frequencies selected to stimulate a thickness resonance inside the radial segment with enhanced entrapment of reverberations and providing a reflection signal representative of acoustic returns caused by the acoustic pulse;
• means for selecting from the reflection signal a portion which includes acoustic returns attributable to the acoustic reverberations inside the casing walls;
• means for generating a spectrum signal representative of the frequency spectrum of the selected portion;
• and
• means for determining the frequency of components in said spectrum signal contributing to a peak value thereof and producing a thickness signal representative thereof as the casing thickness.

A method for acoustically investigating a casing cemented in a borehole penetrating an earth formation comprising the steps of

• generating an acoustic pulse from inside the casing in a radial direction towards the formation wherein the acoustic pulse has a frequency bandwidth selected to stimulate a thickness resonance with acoustic reverberations inside the walls of a radial segment of the casing;
• detecting acoustic returns arising from the interaction of the acoustic pulse with materials in the path of the acoustic pulse and producing a reflection signal indicative thereof;
• selecting from the reflection signal a portion which includes acoustic returns produced by said acoustic reverberations inside the walls of the casing;
• forming a frequency spectrum of the selection portion;
• and
• determining the frequency of components which contribute to a maximum peak in the frequency spectrum of the selected portion and producing a signal representative thereof as an indication of the casing thickness.

An apparatus for investigating with an acoustic pulse a casing cemented in a borehoele penetrating an earth formation comprising

• means for generating a highly damped acoustic pulse from inside the casing in a radial direction towards a radial segment of the casing with said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate an acoustic resonance between the walls of the casing with acoustic reverberations and providing a reflection signal representative of acoustic returns caused by the acoustic pulse;
• means for generating digital samples of the reflection signal;
• means for selecting samples representative of said casing reverberations and occurring subsequent to samples representative of an initial casing reflection;
• means for generating a spectrum of the selected reverberation samples and form amplitude samples with associated frequency values;
• and
• means for determining a maximum amplitude sample and its associated frequency value as an indication of the thickness of the casing.

A method for investigating a casing cemented in a borehole penetrating an earth formation comprising the steps of

• generating a highly damped acoustic pulse from inside the casing in a radial direction towards a radial segment of the casing with said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate the casing into a thickness resonance with acoustic reverberations between the walls of the casing;
• detecting acoustic returns arising from the interaction of the acoustic pulse with materials in the path of the acoustic pulse and producing a reflection signal indicative thereof;
• converting the reflection signal to digital samples;
• forming a frequency spectrum of samples representative of casing reverberations occurring subsequent to samples representative of an initial acoustic reflection off the inner wall of the casing with the frequency spectrum composed of amplitude samples with associated frequency values;
• determining a peak amplitude sample in the frequency spectrum;
• and
• recording a thickness signal representative of the associated frequency value of the peak amplitude sample as an indication of casing thickness.

An apparatus for evaluating the cement bond of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising

• means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment;
• means for determining the energy in the selected reverberation segment and producing a bond signal indicative thereof to characterize the quality of the cement bond behind the casing;
• means for determining from a reverberation segment a casing thickness signal representative of the thickness of the casing at said radial segment;
• and
• means for normalizing said bond signal with said casing thickness signal to substantially remove the effect of casing thickness variations from the characterization of the quality of the cement bond at said radial segment.

A method for evaluating the cement bond of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising the steps of

• deriving from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment;
• measuring the energy in the selected reverberation segment and provide a bond signal indicative thereof to characterize the quality of the cement bond behind said radial segment;
• measuring the thickness of the casing effectively at said radial segment and provide a thickness signal indicative thereof;
• and
• effectively removing from said bond signal with said thickness signal, variations, which are substantially attributable to casing thickness changes.