Optimization of neutron-gamma tools for inelastic-gamma ray logging

Patent metadata

Field	Value
Publication number	US-9897719-B2
Application number	US-201013321933-A
Country	US
Kind code	B2
Filing date	May 21, 2010
Priority date	May 22, 2009
Publication date	Feb 20, 2018
Grant date	Feb 20, 2018

Abstract

Systems, methods, and devices for inelastic gamma-ray logging are provided. In one embodiment, such a method includes emitting neutrons into a subterranean formation from a downhole tool to produce inelastic gamma-rays, detecting a portion of the inelastic gamma-rays that scatter back to the downhole tool to obtain an inelastic gamma-ray signal, and determining a property of the subterranean formation based at least in part on the inelastic gamma-ray signal. The inelastic gamma-ray signal may be substantially free of epithermal and thermal neutron capture background.

First claim

What is claimed is:

1. A method comprising:
emitting neutrons into a subterranean formation from a downhole tool, such that inelastic gamma-rays are produced due to inelastic scattering in the subterranean formation and epithermal neutron capture gamma-rays are produced by epithermal neutron capture in the subterranean formation;
using a first scintillator to detect a portion of the inelastic gamma-rays and a portion of the epithermal neutron capture gamma-rays that scatter back to the downhole tool to obtain an inelastic gamma-ray signal; and
determining a property of the subterranean formation based at least in part on the inelastic gamma-ray signal from the first scintillator, wherein the inelastic gamma-ray signal is corrected to account for the portion of the epithermal neutron capture gamma-rays such that the inelastic gamma-ray signal is substantially free of epithermal neutron capture background due to epithermal neutron capture in the subterranean formation.

2. The method of claim 1 , wherein detecting the portion of the inelastic gamma-rays comprises detecting a count of the inelastic gamma-rays to obtain the inelastic gamma-ray signal.

3. The method of claim 1 , wherein the portion of the inelastic gamma-rays are detected using a scintillator crystal consisting essentially of one or more elements with a resonance integral of less than or equal to approximately 12 barns.

4. The method of claim 3 , wherein the portion of the inelastic gamma-rays are detected using a scintillator crystal comprising YAP, BGO, or a lanthanum-halide, or any combination thereof.

5. The method of claim 1 , wherein determining the property of the subterranean formation comprises determining, in a processor, a value of porosity of the subterranean formation, a value of gas saturation of the subterranean formation, or values of both porosity and gas saturation of the subterranean formation.

6. A downhole tool comprising:
a neutron source capable of emitting neutrons into a subterranean formation such that the neutrons inelastically scatter to produce inelastic gamma-rays;
a first gamma-ray detector comprising a first scintillator crystal capable of detecting the inelastic gamma-rays;
a first gamma-ray shield comprising one or more elements with a resonance integral of greater than 4 barns; and
a second gamma-ray shield disposed between the first gamma-ray detector and the first gamma-ray shield, wherein the second gamma-ray shield is configured to shield the first gamma-ray detector from epithermal neutron capture gamma-rays emitted by the first gamma-ray shield, wherein the second gamma-ray shield consists essentially of one or more elements with a resonance integral of less than approximately 4 barns.

7. The downhole tool of claim 6 , wherein the scintillator crystal consists essentially of one or more elements with a resonance integral of less than or equal to approximately 12 barns.

8. The downhole tool of claim 7 , wherein the scintillator crystal comprises YAP.

9. The downhole tool of claim 7 , wherein the scintillator crystal comprises a lanthanum halide material.

10. The downhole tool of claim 6 , wherein the scintillator crystal is contained in a scintillator housing consisting essentially of one or more elements having a resonance integral of less than or equal to approximately 1 barn.

11. The downhole tool of claim 6 , wherein the scintillator crystal is contained within a thermal neutron shield.

12. The downhole tool of claim 11 , wherein the thermal neutron shield comprises 6 Li.

13. The downhole tool of claim 6 , wherein the first gamma-ray shield comprises one or more elements having a resonance integral of greater than 100 barns.

14. The downhole tool of claim 13 , wherein one of the one or more elements having the resonance integral of greater than 100 barns comprises tungsten.

15. The downhole tool of claim 6 , comprising a second gamma-ray detector comprising a second scintillator crystal capable of detecting the inelastic gamma-rays, wherein the first gamma-ray shield is disposed between the first gamma-ray detector and the second gamma-ray detector; and
a second epithermal neutron capture gamma-ray shield disposed between the second gamma-ray detector and the first gamma-ray shield, wherein the second epithermal neutron capture gamma-ray shield is configured to shield the second gamma-ray detector from epithermal neutron capture gamma-rays emitted by the first gamma-ray shield, wherein the second epithermal neutron capture gamma-ray shield consists essentially of one or more elements with a resonance integral of less than approximately 4 barns.

16. The downhole tool of claim 6 , comprising a 3 He neutron detector having a thermal neutron shield, wherein the thermal neutron shield contains cadmium.

17. A system comprising:
a downhole tool comprising:
an electronic neutron source configured to emit a periodic burst of neutrons into a subterranean formation; and
a scintillation detector configured to detect a count of gamma-rays over time while and after the electronic neutron source emits the periodic burst of neutrons; and

data processing circuitry configured to receive the count and to determine a subset of the count that includes substantially only inelastic gamma-rays produced by inelastic scattering of the emitted neutrons with the subterranean formation based on when the gamma-rays were detected in relation to when the periodic burst of neutrons occurred, wherein the inelastic gamma-ray signal is substantially free of epithermal neutron capture background due to epithermal neutron capture in the subterranean formation.

18. The system of claim 17 , wherein the data processing circuitry is configured to determine the subset of the count based at least in part on an exponential function fitted to at least two time intervals of the count corresponding to times when the electronic neutron source is not emitting the periodic burst of neutrons.

19. The system of claim 17 , wherein the data processing circuitry is configured to determine the subset of the count based at least in part on an exponential function fitted to a time interval of the count corresponding to a time when the electronic neutron source is emitting the periodic burst of neutrons.

20. A method comprising:
receiving, in a processor, a first signal proportionate to a detected quantity of inelastic gamma-rays produced by inelastic scattering of neutrons in a subterranean formation;
receiving, in the processor, a second signal proportionate to a hydrogen index of the subterranean formation; and
determining, in the processor, a value of porosity of the subterranean formation, a value of gas saturation of the subterranean formation, or values of both porosity and gas saturation of the subterranean formation, based at least in part on the first signal and the second signal,
wherein the first signal comprises an inelastic gamma-ray count having substantially no epithermal neutron capture background due to epithermal neutron capture in the subterranean formation.

21. The method of claim 20 , wherein the first signal comprises a ratio of inelastic gamma-ray counts detected by a far scintillation detector and a near scintillation detector, wherein the near scintillation detector is disposed more closely to a source of the neutrons than the far scintillation detector.

22. The method of claim 20 , wherein the first signal comprises a logarithm of count rates of inelastic gamma-rays.

23. The

Assignees

• Stephenson Kenneth E
• Rose David
• Tkabladze Avtandil
• Stoller Christian
• Simonetti Donna
• Schlumberger Technology Corp

Inventors

• Stephenson Kenneth E
• Rose David
• Tkabladze Avtandil
• Stoller Christian
• Simonetti Donna

Classifications

• G01V5/104 Primary

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

• G01V5/101

  and detecting the secondary Y-rays produced in the surrounding layers of the bore hole · CPC title