METHOD OF ACOUSTIC SURVEYING

• US 20140163889A1
• Filed: 07/13/2012
• Published: 06/12/2014
• Est. Priority Date: 07/13/2011
• Status: Active Grant

• Current Assignee: Silixa Limited
• Original Assignee: Silixa Limited
• Inventors: Finfer, Daniel, Johannessen, Kjetil
• Granted Patent: US 9,850,749 B2
• Time in Patent Office: Days
• US Class Current: 702/12
• CPC Class Codes: E21B 47/107 using acoustic means
• CPC Class Codes: E21B 47/135 using light waves, e.g. infrared or ultraviolet waves
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01V 1/226 Optoseismic systems

Abstract

The invention relates to the use of distributed optical fibre sensors for distributed acoustic sensing, and in particular, modal analysis of distributed acoustic data obtained in-well to monitoring well integrity. By determining one or more acoustic modes corresponding to distributed speed of sound measurements within the wellbore, and analysing variations in the distributed speed of sound measurement it is possible to derive information relating to a formation and/or fluid in the wellbore.

Claims

1. A method of surveying a wellbore comprising:

obtaining a distributed acoustic measurement within and corresponding to at least a portion of the wellbore;

processing the distributed acoustic signal to obtain a distributed speed of sound measurement within the wellbore and analysing variations in the distributed speed of sound measurement to derive information relating to a formation and/or fluid in the wellbore; and

determining an acoustic mode corresponding to the, each, or a distributed speed of sound measurement within the wellbore.

2. The method according to claim 1, wherein the analysis comprises analysing variations in the distributed speed of sound measurement as a function of position.

3. The method according to claim 1, wherein the analysis comprises analysing variations in the distributed speed of sound measurement as a function of time.

4. The method according to claim 1, wherein processing the distributed acoustic signal comprises determining a plurality of distributed speed of sound measurements within the wellbore as a function of position.

5. The method according claim 1, wherein processing the distributed acoustic signal comprises obtaining a plurality of distributed speed of sound measurements.

6. The method according claim 1, wherein the analysis comprises determining an acoustic amplitude corresponding to the, each, or a distributed speed of sound measurement.

7. The method according claim 1, wherein the analysis comprises determining relative amplitudes corresponding to different acoustic modes.

8. The method according claim 1, wherein the analysis comprises determining dispersion characteristics of the, each, or an acoustic mode.

9. The method according claim 1, wherein the analysis comprises determining an upper-frequency cut-off for the presence of modal phenomena.

10. The method according claim 1, wherein the analysis comprises inverting a wellbore model against the distributed speed of sound measurement in order to determine a value of one or more unknown parameters in the wellbore model.

11. The method according to claim 10, wherein the wellbore model is configured to receive as an input one or more speed of sound measurements and output one or more corresponding wellbore parameters;

wherein optionally the model is based on full 3-D elastodynamic equations and parameters of the well; and

wherein further optionally the one or more parameters comprise the hardness of the formation.

14. The method according to claim 10, wherein the wellbore model is configured to treat speed of sound as a known parameter and other model parameters as unknowns.

15. The method according to claim 1, wherein the analysis comprises identifying one or more features in the, each, or a distributed speed of sound measurement and attributing the one or more features to one or more corresponding events.

16. The method according to claim 15, wherein identifying one or more features comprises determining the presence and/or location of one or more discontinuities;

variations; and

/or relative variations between modes, in relation to speed of sound and/or amplitude corresponding to an acoustic signal.

17. The method according to claim 1, wherein the analysis comprises averaging the, each, or a distributed speed of sound measurement along at least a portion of the wellbore.

12-13. -13. (canceled)

18. A method of monitoring a formation, comprising:

obtaining a distributed acoustic measurement within and corresponding to at least a portion of the formation;

processing the distributed acoustic signal to obtain a distributed speed of sound measurement within the formation and analysing variations in the distributed speed of sound measurement to derive information relating to the formation; and

determining an acoustic mode corresponding to the, each, or a distributed speed of sound measurement within the formation.

19. The method according to claim 18, further comprising identifying the or each distributed speed of sound measurement that corresponds to an acoustic mode which penetrates the formation.

20. The method according to claim 18, wherein the method further comprises determining hardness of the formation.

21. A method of monitoring fluid flow within a wellbore, comprising:

obtaining a distributed acoustic measurement within and corresponding to at least a portion of the wellbore;

processing the distributed acoustic signal to obtain a distributed speed of sound measurement within the wellbore and analysing variations in the distributed speed of sound measurement to derive information relating to a formation and/or fluid in the wellbore; and

determining an acoustic mode corresponding to the, each, or a distributed speed of sound measurement within the wellbore.

22. The method according to claim 21, further comprising tracking eddies, detecting outgassing events, and/or detecting the presence and position of solids or particulate material in the wellbore.