Method to estimate cement acoustic wave speeds from data acquired by a cased hole ultrasonic cement evaluation tool

Patent metadata

Field	Value
Publication number	US-9784875-B2
Application number	US-201514612101-A
Country	US
Kind code	B2
Filing date	Feb 2, 2015
Priority date	Jan 31, 2014
Publication date	Oct 10, 2017
Grant date	Oct 10, 2017

Abstract

Embodiments of the disclosure may include systems and methods for estimating an acoustic property of an annulus in a cement evaluation system. In one embodiment, a casing arrival signal is acquired at acoustic receivers a cement evaluation tool. A spectral amplitude ratio is calculated based on the casing arrival signal. The spectral amplitude ratio is scanned to detect and identify discontinuities. If discontinuities are detected, the frequency at the discontinuity may be used to estimate a wavespeed of the annulus. If discontinuities are not detected, attenuation dispersions are calculated and estimated, and an estimated wavespeed and parameters are updated until the calculated and estimated attenuation dispersions match.

First claim

The claimed invention includes:

1. A method for estimating an acoustic property of an annulus in a cement evaluation system, the method comprising:
acquiring, at a plurality of acoustic receivers in the cement evaluation system, acoustic waveforms that have propagated through a casing of the cement evaluation system, the annulus, or both;
extracting, from the acoustic waveforms, a casing arrival signal comprising portions of the acoustic waveforms that have propagated through the casing, wherein the casing arrival signal substantially excludes portions of the acoustic waveforms arising from reflections from the annulus or reflections at an interface of the annulus and a formation surrounding the annulus;
calculating an attenuation dispersion, of the casing arrival signal based on portions of the casing arrival signal received at each receiver of the plurality of acoustic receivers, wherein the attenuation dispersion represents attenuation as a function of the frequency;
detecting a discontinuity in the attenuation dispersion;
when a discontinuity is detected, estimating a frequency at the detected discontinuity;
when a discontinuity is detected, calculating a phase velocity dispersion of the casing arrival signal; and
when the discontinuity is detected, estimating a wavespeed of the annulus based on the estimated frequency and the estimated phase velocity dispersions.

2. The method of claim 1 , wherein the plurality of acoustic receivers comprises a first receiver and a second receiver, and wherein calculating the attenuation dispersion comprises calculating a spectral amplitude ratio of the casing arrivals is based on a spectral amplitude equation:
ATT ( f )=20*log 10( abs (FFT(CasingArrival_FAR))/ abs (FFT(CasingArrival_NEAR)))
where CasingArrival_NEAR comprises a first portion of the casing arrival signal received at the first receiver, CasingArrival_FAR comprises a second portion of the casing arrival signal received at the second receiver, and FFT represents a fast Fourier transform calculation of a time-domain waveform.

3. The method of claim 1 , comprising preprocessing the casing signal to remove noise, and wherein calculating the attenuation dispersion comprises calculating a spectral amplitude ratio of the casing arrivals, wherein the plurality of acoustic receivers comprises at least a first receiver and a second receiver, and wherein the spectral amplitude ratio is the ratio of the spectral amplitude of a first portion of the casing arrival signal received at the first receiver and of a second portion of the casing arrival signal received at the second receiver.

4. The method of claim 3 , wherein detecting the discontinuity in the attenuation dispersion comprises determining whether the spectral amplitude ratio decreases below a threshold value.

5. The method of claim 3 , wherein estimating the frequency at the detected discontinuity comprises identifying a first frequency at which the spectral amplitude ratio decreases below the threshold value, wherein the first frequency is the estimated frequency.

6. The method of claim 1 , wherein calculating the attenuation dispersion comprises calculating a spectral amplitude ratio of the casing arrivals, and wherein detecting the discontinuity in the attenuation dispersion comprises determining whether a gradient of the spectral amplitude ratio increases above a threshold value.

7. The method of claim 5 , wherein estimating the frequency at the detected discontinuity comprises identifying a first frequency at which the gradient increases above the threshold value, wherein the first frequency is the estimated frequency.

8. The method of claim 1 , wherein estimating the wavespeed of the annulus based on the estimated frequency comprises:
calculating a phase difference based on the equation:
φ( f )=∠(FFT(CasingArrival_FAR)/FFT(CasingArrival_NEAR))

where φ(f) represents a phase difference related to flexural mode phase velocity dispersion, CasingArrival_NEAR comprises a first portion of the casing arrival signal received at the first receiver, CasingArrival_FAR comprises a second portion of the casing arrival signal received at the second receiver, and FFT represents a fast Fourier transform calculation of a time-domain waveform; and
wherein calculating the phase velocity dispersion is based on the equation:

v
⁡

(
f
)

=

Δ
⁢

⁢
z

t
0

-

ϕ
⁡

(
f
)

+

2
⁢
n
⁢

⁢
π

2
⁢
π
⁢

⁢
f

where v(f) represents a flexural mode phase velocity dispersion, Δz represents a spacing between the two acoustic receivers, t 0 represents a time shift applied before calculating the spectral amplitude ratio, and 2nπ is the phase ambiguity for any integer n.

9. The method of claim 1 , wherein calculating the phase velocity dispersion comprises using casing parameters including casing thickness, casing compressional and shear wavespeeds, casing density, and combinations thereof.

10. The method of claim 1 , wherein calculating the phase velocity dispersion comprises using a mode search algorithm.

11. The method of claim 1 , comprising identifying the estimated wavespeed as compressional or shear.

12. The method of claim 1 , comprising iteratively outputting cement properties based on the iteratively estimated wavespeeds and plotting a time-lapse graph of the iteratively output cement properties, wherein the time-lapse graph represents an evolution of properties of the annulus over time.

13. A method for estimating an acoustic property of an annulus in a cement evaluation system, the method comprising:
acquiring, at a plurality of acoustic receivers in the cement evaluation system, a pitch-catch signal comprising acoustic waveforms that have propagated through a casing adjacent to the annulus;
extracting a casing arrival signal from the pitch-catch signal, wherein the casing arrival signal comprises early-arriving portions of the pitch-catch signal, and wherein the casing arrival signal is substantially devoid of later-arriving portions arising from reflections from the annulus or reflections from an interface of the annulus and a formation around the annulus;
compensating each casing arrival signal for lateral beam spreading using a l

Assignees

• Schlumberger Technology Corp

Inventors

• Zeroug Smaine
• Yang Jiaqi
• Bose Sandip

Classifications

• G01V1/50 Primary

  Analysing data · CPC title

• E21B47/14

  using acoustic waves · CPC title

• E21B47/005

  Monitoring or checking of cementation quality or level · CPC title

• E21B47/0005

  Fixed Constructions · mapped topic