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

Claims

I claim:

1. An apparatus 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 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 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.

2. The apparatus as set forth in claim 1 wherein said means for producing a bond signal further includes

means for determining the energy in a casing segment of the reflection signal representative of an acoustic reflection off an inner wall of the casing and provide a casing signal indicative thereof; and

means for normalizing said bond signal with said casing signal to correct for borehole conditions.

3. The apparatus as set forth in claim 2 wherein said casing thickness signal generating means further includes

means for producing a spectrum signal representative of the frequency spectrum of the reverberation segment; and

means for scanning said spectrum signal for said peak value to derive the frequency of said components associated with said peak value.

4. 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.

5. The method for determining casing characteristics as set forth in claim 4 and further including the step of

recording said bond and said thickness signals as a function of borehole depth to provide a composite log for the correlation of the cement bond quality with casing thickness.

6. The method for determining casing characteristics as set forth in claim 5 wherein said reflection signal is in a digitized form composed of samples and wherein said energy measuring step further comprises

summing absolute values of the samples representative of the reverberation segment as said bond signal; and

wherein said frequency measuring step further comprises

forming a fourier transformation of samples representative of the reverberation segment and composed of amplitude samples with associated frequency values;

scanning said amplitude samples for a peak thereof; and

selecting an associated frequency value of the peak sample as a measurement of the thickness of the casing.

7. The method for determining casing characteristics as set forth in claim 6 wherein said energy measuring step further comprises

summing absolute values of samples representative of the casing reflection produced by said acoustic pulse as an integral of said casing reflection; and

forming a quotient between the respectively summed reverberation segment samples and the casing reflection samples to produce a normalized bond signal.

8. An apparatus for determining the quality of the bond between cement and a casing cemented in a borehole 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 for selecting from the reflection signal to the exclusion of a strong casing reflection signal, a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment; and

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.

9. A method for determining the quality of the bond between cement and a casing cement in a borehole 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 the step of

measuring to the exclusion of a strong casing reflecting signal the energy in a reverberation segment of the reflection signal, wherein the reverberation segment is substantially representative of acoustic reverberations between the casing walls at said radial segment, and provide a bond signal indicative thereof to characterize the quality of the cement bond behind said radial segment of the casing.

10. The method for determining the quality of the cement bond as set forth in claim 9 wherein said reflection signal is in a digital form composed of samples and further including the step of

summing absolute values of samples representative of the energy of the casing reflection; and

dividing said bond signal by said sum for normalization thereof.

11. 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.

12. The apparatus for determining the quality of the cement bond as set forth in claim 11 wherein said reflection signal is in the form of digital samples and wherein said means for producing said bond signal includes means for producing a sum of the absolute values of reflection signal samples representative of said reverberation segment as said bond signal.

13. The apparatus for determining the quality of the cement bond as set forth in claim 12 wherein said bond signal producing means further includes

means for producing a sum of the absolute values of reflection samples representative of the initial casing reflection as a measure of the magnitude thereof; and

means for producing a quotient between said sums to normalize the bond signal.

14. The apparatus for determining the quality of the cement bond as set forth in claim 13 wherein said means for producing said casing reflection signal includes a threshold detector effectively responsive to the reflecting signal to determine when said reflection signal exceeds a level representative of the presence of said initial casing reflection.

15. The apparatus for determining the quality of the cement bond as set forth in claim 14 wherein said threshold detector is formed of

means for scanning samples of the reflection signal to determine the location of said initial casing reflection.

16. 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.

17. The apparatus for determining the casing thickness as set forth in claim 16 wherein the reflection signal is formed of digital samples and said spectrum generating means includes means for generating a fourier transform of samples representative of the reverberation segment as said spectrum signal.

18. A method 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 the steps of

generating a spectrum signal representative of the frequency spectrum of a reverberation segment of the reflection signal wherein said reverberation segment is substantially representative of acoustic reverberations between the casing walls at said radial segment; and

measuring the frequency of components in said spectrum signal contributing to a peak value thereof and providing a thickness signal representative of said measured frequency as indicative of the casing thickness at said radial segment.

19. The method for determining casing thickness as set forth in claim 18 wherein the reflection signal is formed of digital samples wherein said generating step produces a spectrum signal formed of samples indicative of amplitudes and associated frequency values and wherein said measuring step further includes

scanning said amplitude samples for said peak value and selecting the associated frequency of sample contributing to said peak as representative of the casing thickness.

20. 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.

21. The acoustic pulse echo apparatus for investigating a casing as set forth in claim 20 wherein the selecting means further includes means for detecting a casing segment in the reflection signal representative of an initial casing reflection produced by the acoustic pulse and generate a casing signal indicative thereof; and

means enabled by the casing signal for amplifying said reverberation segment following the initial casing reflection.

22. 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.

23. The cement bond quality investigating apparatus as claimed in claim 22 wherein said bond signal producing means further includes

means for producing a reverberation segment select signal commencing at a time commensurate with the arrival time of the reverberation segment of the reflection signal and continuing for a time commensurate with the duration of the portion of the reflection signal indicative of a bad bond between the casing and the cement; and

means controlled by the reverberation segment select signal and coupled to the reflection signal for selecting said reverberation segment from the reflection signal.

24. The cement bond quality investigating apparatus as claimed in claim 23 wherein said bond signal producing means still further includes

means for rectifying said selected predetermined segment; and

means for integrating said rectified predetermined segment effectively for the duration of said reverberation segment select signal.

25. The apparatus for investigating the quality of the cement bond in accordance with claim 22 wherein said reflection signal producing means is selectively located within the apparatus to establish a predetermined minimum spacing between the casing and the reflection signal producing means to produce a reflection signal with said reverberation segment substantially free from secondary transmission interference.

26. The apparatus for investigating the quality of the cement bond in accordance with claim 25 and further including

means responsive to the reflection signal for generating a casing reflection signal representative of a predetermined characteristic of said initial acoustic casing reflection; and

means for normalizing said bond signal with the casing reflection signal.

27. The apparatus for investigating the quality of the cement bond in accordance with claim 26 wherein said casing reflection signal generating means further includes means for measuring the amplitude of the casing reflection signal.

28. The apparatus for investigating the quality of the cement bond in accordance with claim 26 wherein said casing reflection signal generating means further includes means for effectively measuring the energy of the casing reflection signal.

29. The apparatus for investigating the quality of the cement bond in accordance with claim 26 wherein the means for generating the casing reflection signal further includes

a threshold detector responsive to the reflection signal for sensing a predetermined magnitude indicative of the arrival of said initial acoustic casing reflection and to produce an enabling signal representative thereof; and

means responsive to the reflection signal and enabling signal for selecting said casing reflection signal.

30. 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.

31. The apparatus for investigating the quality of the cement bond in accordance with claim 30 wherein said predetermined spacing between the casing and the reflection signal producing means is selected sufficiently large to enable the detection of acoustic returns having a magnitude above a predetermined level and substantially attributable to leakage from said reverberations inside the casing walls as a result of said acoustic pulse.

32. The apparatus for investigating the quality of the cement bond in accordance with claim 31 wherein said apparatus has a surface capable of generating secondary transmission interference by reflecting acoustic energy back towards the casing and wherein a predetermined minimum spacing, D, between the casing and said surface is determined in accordance with the relationship

D>N.sub.r L(C.sub.o /C.sub.1)

where L is the thickness of the casing, C.sub.o is the velocity of sound of the material enclosed by the casing, C.sub.1 is the velocity of sound inside the casing material and N.sub.r represents a substantial number of reverberations produced within the casing as a result of acoustic energy entrapment from the thickness resonance producing acoustic pulse and is determined by the relationship ##EQU4## where r.sub.o and r.sub.1 are respectively reflection coefficients between the material enclosed by the casing and the casing itself and between the casing and the material adjacent outside of the casing, and where x represents the predetermined level expressed as a fraction of the initial level of the reverberations.

33. 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.

34. 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.

35. The method of investigating the quality of the cement bond in accordance with claim 34 wherein said processing step still further includes the steps of

selecting a casing segment of the reflection signal representative of the casing reflection;

producing a casing signal indicative of a predetermined characteristic of the selected casing segment; and

applying the casing signal to normalize the determined energy in the reverberation segment relative to said predetermined characteristic of the casing reflection.

36. The method of investigating the quality of the cement bond in accordance with claim 35 wherein said casing signal producing step produces a casing signal effectively representative of the energy in the casing reflection.

37. The method of investigating the quality of the cement bond in accordance with claim 35 wherein said casing signal producing step produces a casing signal effectively representative of an amplitude of the casing reflection.

38. The method of investigating the quality of the cement bond in accordance with claim 34 wherein said detecting step is carried out at a predetermined distance from the casing to provide said reflection signal substantially free from secondary transmission interference.

39. 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.

40. The method of investigating the quality of the cement bond in accordance with claim 39 wherein said processing step further includes the steps of

measuring the energy in said segments; and

normalizing the measured energy of the reverberation segment with the measured energy in the casing segment to produce said bond signal.

41. The method for investigating the cement bond in accordance with claim 40 and further comprising the step of

preferentially amplifying the selected reverberation segment relative to the casing segment for enhanced accuracy in obtaining a measurement of the quality of the cement bond.

42. 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.

43. The apparatus for investigating the quality of the cement bond as set forth in claim 42 wherein the extracting means includes a pass band filter having its pass band aligned with the casing thickness resonance frequency.

44. The apparatus for investigating the quality of the cement bond as set forth in claim 43 wherein the pass band of the filter has a bandwidth generally less than about 15 per cent of the casing thickness resonance frequency.

45. 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;

det