Neutron detection using a shielded gamma detector

Patent metadata

Field	Value
Publication number	US-9304214-B2
Application number	US-201113330211-A
Country	US
Kind code	B2
Filing date	Dec 19, 2011
Priority date	Jun 30, 2010
Publication date	Apr 5, 2016
Grant date	Apr 5, 2016

Abstract

A tool having a neutron source, a gamma ray detector, and a photomultiplier tube is provided. The gamma ray detector and the photomultiplier tube are at least partially surrounded by a layer of boron. The tool is used to make measurements, and the number of prompt gamma rays emitted by the boron is determined from the measurements. The number of neutrons detected may be inferred using the determined number of prompt gamma rays. The tool may also have a layer of neutron absorbing material different from boron or a layer of heavy metal at least partially surrounding the boron. The tool may be a logging tool used to delineate a porous formation and to determine its porosity. The tool may have a plurality of gamma ray detector/photomultiplier tube pairs and those pairs may be used to determine a formation hydrogen index and/or a borehole hydrogen index.

First claim

What is claimed is:

1. A method, comprising:
providing a tool having a neutron source, and a gamma ray detector, wherein the gamma ray detector is at least partially surrounded by a layer of boron;
making measurements using the tool, wherein said measurements comprise prompt gamma rays emitted by the boron and gamma rays emitted by surrounding environment external to the tool;
determining from the measurements the number of prompt gamma rays emitted by the boron; and
inferring the number of neutrons detected using the determined number of prompt gamma rays, wherein inferring the number of neutrons comprises relating a neutron/boron interaction rate to a count rate of the prompt gamma rays to obtain the inferred number of neutrons.

2. The method of claim 1 , wherein the tool is a logging tool.

3. The method of claim 2 , comprising delineating a porous formation and determining the porosity of the porous formation.

4. The method of claim 1 , wherein the prompt gamma rays have approximately 0.48 MeV of energy.

5. The method of claim 1 , wherein the prompt gamma ray is a decay product from a neutron/boron interaction.

6. The method of claim 5 , wherein the neutron is a thermal neutron or an epithermal neutron.

7. The method of claim 1 , comprising measuring gamma ray signals and neutron signals at the same time and at the same location.

8. The method of claim 7 , comprising determining one or more of a hydrogen index, a capture cross-section (Sigma), a gas content, and a gamma ray spectroscopy.

9. The method of claim 7 , comprising simultaneously optimizing the gamma ray detector for gas detection and for spectroscopy.

10. The method of claim 1 , comprising:
providing a layer of neutron absorbing material different from boron at least partially surrounding the layer of boron; and
detecting gamma rays only from epithermal neutron/boron interactions.

11. The method of claim 1 , comprising:
using the gamma ray detector to measure, in addition to measuring gamma rays produced from neutron inelastic scattering, a thermal neutron signal and an epithermal neutron signal during a neutron burst; and
removing the epithermal neutron signal and the thermal neutron signal from the measurement of the gamma rays to produce a less noisy measurement of the gamma rays.

12. The method of claim 11 , wherein the tool is a logging tool, and comprising differentiating a gas-filled porous formation from a water-filled porous formation.

13. The method of claim 1 , comprising regulating a gain of the gamma ray detector.

14. The method of claim 1 , comprising summing all the counts in a boron energy window to provide a higher count rate, and using the higher count rate to infer the number of neutrons detected.

15. The method of claim 1 , comprising:
measuring the multichannel scalar time spectrum of the boron peak; and
determining a formation capture cross-section and/or borehole capture cross-section.

16. The method of claim 15 , comprising correcting a gamma ray based capture cross-section for the depth of investigation and/or for environmental effects.

17. A method, comprising:
providing a logging tool having a neutron source and a plurality of gamma ray detectors, wherein at least two of the gamma ray detectors are at least partially surrounded by boron;
making gamma-ray count rate measurements using two or more of the gamma ray detectors that are at least partially surrounded by boron, wherein said measurements comprise prompt gamma rays emitted by the boron and gamma rays emitted by surrounding environment external to the tool;
discerning gamma-ray count rate boron peaks from the measurements;
determining one or more ratios of the discerned gamma-ray count rate boron peaks; and
determining a formation hydrogen index and/or a borehole fluid hydrogen index from the determined one or more ratios.

18. The method of claim 17 , comprising correcting a capture gamma ray based hydrogen index measurement for the depth of investigation and/or environmental effects.

19. The method of claim 17 , comprising normalizing one of the plurality of gamma ray detectors or self-normalizing the gamma ray detector from which the boron peak is measured.

20. A method, comprising:
providing a tool having a neutron source and a gamma ray detector wherein the gamma ray detector is at least partially surrounded by a layer of boron;
making measurements on a formation using the tool, wherein said measurements comprise prompt gamma rays emitted by the boron and gamma rays emitted by the formation;
discerning a boron peak contained in the measurements; and
determining a formation property or parameter from gamma rays emitted by the formation and the boron peak.

21. The method of claim 20 , wherein the formation property or parameter comprises porosity, formation sigma, or borehole fluid sigma, or any combination thereof.

22. The method of claim 21 , comprising using the determined formation property or parameter as a stand-alone porosity measurement or as a measurement complementing a gamma ray based measurement.

23. The method of claim 22 , wherein the using the formation property or parameter as a complementary measurement comprises providing corrections to a gamma ray based measurement for standoff, borehole fluid, borehole size, salinity, and/or lithology effects.

24. The method of claim 20 , wherein the neutron count rate is a function of only two times.

25. The method of claim 20 , comprising measuring a fast neutron flux from the neutron source using a neutron monitor detector.

26. The method of claim 25 , comprising normalizing a boron-10 gamma-ray count rate using the measured fast neutron flux.

27. The method of claim 20 , further comprising obtaining a neutron count rate from the boron peak as a function of time.

Assignees

• Zhou Tong
• Rose David
• Beekman Sicco
• Stoller Christian
• Schlumberger Technology Corp

Inventors

• Zhou Tong
• Rose David
• Beekman Sicco
• Stoller Christian

Classifications

• G01V5/104

  and detecting secondary Y-rays as well as reflected or back-scattered neutrons · CPC title

• G01T3/06 Primary

  with scintillation detectors · CPC title