Neutron-absorbing gamma ray window in a downhole tool

Patent metadata

Field	Value
Publication number	US-9897718-B2
Application number	US-201415104354-A
Country	US
Kind code	B2
Filing date	Dec 17, 2014
Priority date	Dec 23, 2013
Publication date	Feb 20, 2018
Grant date	Feb 20, 2018

Abstract

Methods and downhole tools involving neutron-absorbing gamma ray windows are provided. One such method involves emitting neutrons from a neutron source in a downhole tool in a well into a surrounding geological formation. This may produce formation gamma rays through interactions between the neutrons and elements of the geological formation. The formation gamma rays may be detected by a gamma ray detector when the gamma rays pass via a gamma ray window that includes a neutron-absorbing material disposed in a substrate material of the downhole tool. The gamma ray window may be both more transmissive of gamma rays than the substrate material and less transmissive of neutrons than a window without the neutron-absorbing material. This may decrease a neutron flux that would otherwise reach the gamma ray detector and the tool materials surrounding it and thus would otherwise lead to a background signal contaminating a signal corresponding to the detected formation gamma rays.

First claim

The invention claimed is:

1. A method comprising:
emitting neutrons from a neutron source in a downhole tool in a well into a surrounding geological formation to produce formation gamma rays through interactions between the neutrons and elements of the geological formation; and
detecting the formation gamma rays in a gamma ray detector via a gamma ray window disposed in a substrate material of the downhole tool, wherein the substrate material is a material of the tool housing, wherein the gamma ray window comprises a neutron-absorbing material and is both more transmissive of gamma rays than the substrate material of the downhole tool and less transmissive of neutrons than were the gamma ray window without the neutron-absorbing material, to decrease a neutron flux that would otherwise reach the gamma ray detector while enhancing a probability that the formation gamma rays reach the gamma ray detector.

2. The method of claim 1 , wherein the gamma rays are detected via the gamma ray window, wherein the gamma ray window comprises a plurality of component regions, at least one of which comprises the neutron-absorbing material and at least one of which is inert.

3. The method of claim 1 , comprising absorbing neutrons in the gamma ray window using the neutron-absorptive material in the gamma ray window that has a neutron absorption cross section of at least 10 barn.

4. The method of claim 1 , comprising reducing an energy of neutrons that enter the gamma ray window using a hydrogenous material in the gamma ray window to increase a likelihood that the neutrons are absorbed by the gamma ray window.

5. The method of claim 1 , comprising reflecting neutrons that enter the gamma ray window using a neutron-reflective material in the gamma ray window to decrease the neutron flux that would otherwise reach the gamma ray detector.

6. A downhole tool comprising:
a neutron source configured to emit neutrons into a geological formation to cause formation gamma rays to be created through inelastic scattering or neutron capture, or both;
one or more gamma ray detectors disposed within a substrate material of the downhole tool, wherein the substrate material is a material of the tool housing, wherein the one or more gamma ray detectors are configured to detect the formation gamma rays; and
one or more gamma ray windows disposed within the substrate material of downhole tool, wherein the one or more gamma ray windows comprise a neutron-absorbing material that is configured to permit more gamma rays to enter the one or more gamma ray detectors than would be permitted were the one or more gamma ray windows not disposed in the substrate material, while also permitting fewer neutrons to enter the one or more gamma ray detectors and the substrate material immediately adjacent to them than would be permitted were the one or more gamma ray windows without the neutron-absorbing material.

7. The downhole tool of claim 6 , wherein the neutron-absorbing material comprises Li, enriched 6 Li, B, enriched 10 B, Cd, Sm, or Gd, or any combination thereof.

8. The downhole tool of claim 6 , wherein at least one of the one or more gamma ray windows comprises an inert material mixed with the neutron-absorbing material, wherein the inert material comprises PEEK, PEKK, Al, silicone, or beryllium, or any combination thereof.

9. The downhole tool of claim 6 , wherein at least one of the one or more gamma ray windows comprises an outermost layer of inert material that covers an inner layer of the neutron-absorbing material and protects the inner layer of the neutron-absorbing material from exposure to environmental effects downhole.

10. The downhole tool of claim 9 , wherein the outermost layer of inert material comprises a sacrificial layer configured to be worn away without substantially changing the neutron-absorbing properties of the at least one gamma ray window.

11. The downhole tool of claim 9 , wherein the inner layer comprises an alloy comprising aluminum or lithium, or both, and the outermost layer comprises a nickel or a nickel alloy.

12. The downhole tool of claim 6 , wherein at least one of the one or more gamma ray windows comprises a 3 He detector or a 3 He gas volume, or both.

13. The downhole tool of claim 6 , wherein at least one of the one or more gamma ray windows comprises the neutron-absorbing material surrounded on all sides by an inert material.

14. The downhole tool of claim 13 , wherein the neutron-absorbing material comprises a powder or a gas.

15. The downhole tool of claim 6 , wherein at least one of the one or more gamma ray windows comprises alternating layers of inert structural material and the neutron-absorbing material.

Assignees

• Schlumberger Technology Corp

Inventors

• Mauborgne Marie Laure
• Mahjoub Ahmed Amine
• Berheide Markus
• Stoller Christian

Classifications

• G01V5/101 Primary

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