Obtaining inelastic spectra free of detector born gamma rays induced by neutrons

Patent metadata

Field	Value
Publication number	US-11906690-B2
Application number	US-201916596871-A
Country	US
Kind code	B2
Filing date	Oct 9, 2019
Priority date	Oct 10, 2018
Publication date	Feb 20, 2024
Grant date	Feb 20, 2024

Abstract

In an embodiment, a method includes receiving a first measurement of gamma rays via a detector during a first period of time, receiving a second measurement of gamma rays via the detector during a second period of time, removing the second measurement from the first measurement to produce an inelastic spectrum, determining a spectral slope from the inelastic spectrum, determining a scaling factor based on the spectral slope, determining a spectral shape associated with the detector, determining a detector-induced spectrum by applying the scaling factor to the spectral shape, and removing the detector-induced spectrum from the inelastic spectrum to produce a clean inelastic spectrum.

First claim

The invention claimed is:

1. A method of measuring properties of a formation or well structure, comprising:
receiving a first measurement of gamma rays from the formation or well structure via a detector during a first period of time;
receiving a second measurement of gamma rays from the formation or well structure via the detector during a second period of time;
removing the second measurement from the first measurement to produce an inelastic spectrum;
determining a spectral slope from the inelastic spectrum;
determining a scaling factor based on the spectral slope;
determining a spectral shape associated with the detector;
determining a detector-induced spectrum by applying the scaling factor to the spectral shape; and
removing the detector-induced spectrum from the inelastic spectrum to produce a clean inelastic spectrum for use in evaluating properties of the formation or well structure.

2. The method of claim 1 , wherein the first period of time corresponds to a burst gate and the first measurement corresponds to a total spectrum.

3. The method of claim 1 , wherein the second period of time corresponds to a capture gate and the second measurement corresponds to capture gamma rays.

4. The method of claim 1 , further comprising:
determining a higher gamma count rate from a first segment of a gamma ray spectrum;
determining a lower gamma count rate from a second segment of the gamma ray spectrum;
comparing the higher count rate and the lower count rate to determine the spectral slope.

5. The method of claim 1 , further comprising:
obtaining a correlation between the spectral slope and the scaling factor.

6. The method of claim 1 , wherein the detector comprises a crystal, and the detector-induced spectrum is associated with characteristics of the crystal.

7. A computing device for measuring properties of a formation or well structure, comprising:
a microprocessor; and
memory including instructions that, when executed by the microprocessor, cause the computing device to:
receive a first measurement of gamma rays from the formation or well structure via a detector during a first period of time;
receive a second measurement of gamma rays from the formation or well structure via the detector during a second period of time;
remove the second measurement from the first measurement to produce an inelastic spectrum;
determine a spectral slope from the inelastic spectrum;
determine a scaling factor based on the spectral slope;
determine a spectral shape associated with the detector;
determine a detector-induced spectrum by applying the scaling factor to the spectral shape; and
remove the detector-induced spectrum from the inelastic spectrum to produce a clean inelastic spectrum for use in evaluating properties of the formation or well structure.

8. The computing device of claim 7 , wherein the first period of time corresponds to a burst gate and the first measurement corresponds to a total spectrum.

9. The computing device of claim 7 , wherein the second period of time corresponds to a capture gate and the second measurement corresponds to capture gamma rays.

10. The computing device of claim 7 , wherein the instructions, when executed by the microprocessor, further cause the computing device to:
determining a higher gamma count rate from a first segment of a gamma ray spectrum;
determining a lower gamma count rate from a second segment of the gamma ray spectrum; and
compare the higher count rate and the lower count rate to determine the spectral slope.

11. The computing device of claim 7 , wherein the instructions, when executed by the microprocessor, further cause the computing device to:
obtain a correlation between the spectral slope and the scaling factor.

12. The computing device of claim 7 , wherein the detector comprises a crystal, and the detector-induced spectrum is associated with characteristics of the crystal.

13. A system for conducting measurement operations for measuring properties of a formation or well structure, the system comprising:
a neutron measurement device forming at least a portion of a downhole tool string, the neutron measurement device operable to generate measurement data for detecting a wellbore characteristic, wherein the neutron measurement device comprises:
a neutron generation unit operable to emit neutrons toward a target;
a detector unit fixed relative to the neutron generator and operable to detect gamma rays from the target;
a microprocessor; and
memory including instructions that, when executed by the microprocessor, cause the system to:
receive a first measurement of gamma rays from the formation or well structure via a detector during a first period of time;
receive a second measurement of gamma rays from the formation or well structure via the detector during a second period of time;
remove the second measurement from the first measurement to produce an inelastic spectrum;
determine a spectral slope from the inelastic spectrum;
determine a scaling factor based on the spectral slope;
determine a spectral shape associated with the detector;
determine a detector-induced spectrum by applying the scaling factor to the spectral shape; and
remove the detector-induced spectrum from the inelastic spectrum to produce a clean inelastic spectrum for use in evaluating properties of the formation or well structure.

14. The system of claim 13 , wherein the first period of time corresponds to a burst gate and the first measurement corresponds to a total spectrum.

15. The system of claim 13 , wherein the second period of time corresponds to a capture gate and the second measurement corresponds to capture gamma rays.

16. The system of claim 13 , wherein the instructions, when executed by the microprocessor, further cause a computing device to:
determining a higher gamma count rate from a first segment of a gamma ray spectrum;
determining a lower gamma count rate from a second segment of the gamma ray spectrum; and
compare the higher count rate and the lower count rate to determine the spectral slope.

17. The system of claim 13 , wherein the instructions, when executed by the microprocessor, further cause a computing device to:
obtain a correlation between the spectral slope and the scaling factor.

18. The system of claim 13 , wherein the detector comprises a crystal, and the detector-induced spectrum is associated with characteristics of the crystal.

19. The system of claim 13 , wherein the neutron generation unit is a pulsed neutron generator.

20. The system of claim 13 , wherein the detector unit includes at least one gamma ray detector.

Assignees

• Baker Hughes Oilfield Operations Llc

Inventors

• Inanc Feyzi
• Anniyev Toyli
• Yuan Peng
• Vasilyev Maxim
• Chace David

Classifications

• G01V5/102

  the neutron source being of the pulsed type · CPC title

• G01V5/10

  using neutron sources {(neutron generating tubes H05H5/00; neutron sources using isotopes G21G4/00)} · CPC title

• G01V5/045 Primary

  Transmitting data to recording or processing apparatus; Recording data · CPC title

• E21B47/005

  Monitoring or checking of cementation quality or level · CPC title

• G01T3/06

  with scintillation detectors · CPC title