Neutron-gamma density through normalized inelastic ratio

Patent metadata

Field	Value
Publication number	US-10162079-B2
Application number	US-201715422782-A
Country	US
Kind code	B2
Filing date	Feb 2, 2017
Priority date	Nov 11, 2010
Publication date	Dec 25, 2018
Grant date	Dec 25, 2018

Abstract

Systems, methods, and devices for determining a neutron-gamma density (NGD) measurement of a subterranean formation that is accurate in both liquid- and gas-filled formations are provided. For example, a downhole tool for obtaining such an NGD measurement may include a neutron generator, a neutron detector, two gamma-ray detectors, and data processing circuitry. The neutron generator may emit neutrons into a formation, causing a fast neutron cloud to form. The neutron detector may detect a count of neutrons representing the extent of the neutron cloud. The gamma-ray detectors may detect counts of inelastic gamma-rays caused by neutrons that inelastically scatter off the formation. Since the extent of the fast neutron cloud may vary depending on whether the formation is liquid- or gas-filled, the data processing circuitry may determine the density of the formation based at least in part on the counts of inelastic gamma-rays normalized to the count of neutrons.

First claim

What is claimed is:

1. A downhole tool comprising:
a neutron generator configured to emit neutrons into a subterranean formation;
a neutron detector configured to detect a count of neutrons that return to the downhole tool after interacting with the subterranean formation;
a plurality of gamma-ray detectors configured to detect counts of inelastic gamma-rays that form when neutrons are inelastically scattered by the subterranean formation;
a neutron monitor detector configured to determine the flux of neutrons emitted by the neutron generator; and
data processing circuitry configured to determine a ratio of inelastic counts between at least two of the gamma ray detectors, normalize the count of neutrons by the flux of neutrons, and determine a density of the subterranean formation based at least in part on the ratio of inelastic gamma-rays and the normalized count of neutrons.

2. The downhole tool of claim 1 , wherein the neutron generator is configured to emit neutrons of at least 2 MeV.

3. The downhole tool of claim 1 , wherein the neutron detector is shielded from thermal neutrons so as to detect epithermal neutrons but not thermal neutrons.

4. The downhole tool of claim 1 , wherein the neutron monitor is configured to detect fast neutrons.

5. The downhole tool of claim 1 , wherein the neutron detector is configured to detect thermal neutrons.

6. The downhole tool of claim 1 , wherein the plurality of gamma-ray detectors comprises a first gamma-ray detector spaced a first distance from the neutron generator and a second gamma-ray detector spaced a second distance from the neutron generator.

7. The downhole tool of claim 1 , wherein the plurality of gamma-ray detectors are shielded from neutrons to reduce neutron capture background radiation.

8. A method comprising:
lowering a downhole tool into a subterranean formation through a borehole;
emitting a burst of neutrons of at least 2 MeV into the subterranean formation using a neutron generator in the downhole tool during a burst gate;
determining, by a neutron monitor, a flux of neutrons emitted by the neutron generator;
detecting, during the burst gate, two first counts of gamma-rays that include inelastic gamma-rays formed when the emitted neutrons inelastically scatter off the subterranean formation using two gamma-ray detectors spaced two respective distances from the neutron generator in the downhole tool;
detecting a first count of neutrons that return to the downhole tool after interacting with the subterranean formation using a neutron detector in the downhole tool;
normalizing the first count of neutrons by the flux of neutrons determined by the neutron monitor; and
determining a density measurement of the subterranean formation based at least in part on a ratio of the two first counts of gamma-rays and the normalized first count of neutrons using data processing circuitry associated with the downhole tool.

9. The method of claim 8 , wherein the burst gate is a period of time sufficient to allow the inelastic gamma-rays to be created through inelastic scattering off the subterranean formation but not to allow thermal neutrons to outnumber epithermal neutrons detected by the neutron detector during the burst gate.

10. The method of claim 8 , wherein the burst gate is a period of time between approximately 10 μs and 20 μs.

11. The method of claim 8 , wherein the density measurement is determined using the data processing circuitry, wherein the data processing circuitry is disposed within the downhole tool or at a remote location, or partially within the downhole tool and partially at the remote location.

12. The method of claim 8 , wherein the density measurement is determined based at least in part on the following relationship:
IRAT−c*log( 3 He−b),

where IRAT represents the ratio of the two first counts of gamma-rays, c and b represent normalization constants, and 3 He represents the first count of neutrons normalized by the flux a count of the neutron monitor.

13. The method of claim 8 , comprising detecting, after the burst gate, two second counts of gamma-rays that include a background of neutron capture gamma-rays and subtracting the two second counts of gamma-rays from the two first counts of gamma-rays to remove the background from the two first counts of gamma-rays, and the ratio is obtained by dividing the background subtracted net inelastic counts.

14. A downhole tool comprising:
a neutron generator configured to emit neutrons into a subterranean formation to form a fast neutron cloud, wherein an extent of the fast neutron cloud varies at least in part on an extent to which the subterranean formation is filled with liquid or gas;
a plurality of gamma-ray detectors configured to detect counts of inelastic gamma-rays caused by inelastic scattering when the neutrons of the fast neutron cloud inelastically scatter off the formation;
a neutron detector configured to measure the extent of the fast neutron cloud;
a neutron monitor configured to determine a flux of neutrons emitted by the neutron generator; and
data processing circuitry configured to determine a density of the subterranean formation regardless of whether the subterranean formation is liquid-filled or gas-filled based at least in part on a ratio of the counts of inelastic gamma-rays, optionally subtracted of background gamma rays, and the measurement of the extent of the fast neutron cloud normalized by the flux of neutrons.

15. The downhole tool of claim 14 , wherein the neutron detector is configured to measure the extent of the fast neutron cloud by detecting a count of epithermal neutrons that return to the downhole tool from the burst of neutrons emitted into the subterranean formation normalized by the flux of neutrons.

16. The downhole tool of claim 14 , wherein the neutron detector is configured to measure the extent of the fast neutron cloud by detecting a count of thermal, fast, and epithermal neutrons that return to the downhole tool from the burst of neutrons emitted into the subterranean formation.

17. The downhole tool of claim 14 , wherein the density measurement is determined based at least in part on the following relationship:
IRAT−c*log( 3 He−b),

where IRAT represents the ratio of the two counts of inelastic gamma-rays, 3 He represents the count of neutrons, and c and b represent constants selected to cause the ratio of inelastic gamma-rays to vary linearly with density regardless of whether the subterranean formation is liquid-filled or gas-filled.

18. The downhole tool of claim 14 , wherein the gamma ray detector is shielded from thermal neutrons by surrounding it by a thermal neutron absorber.

19. The downhole tool of claim 18 , wherein the neutron absorber does not emit gamma rays as a consequence of the absorption of a neutron.

20. The downhole tool of claim 19 , wherein the neutron absorber contains 6 Li.

Assignees

• Schlumberger Technology Corp

Inventors

• Tkabladze Avtandil
• Evans Michael Lynn
• Stephenson Kenneth Edward

Classifications

• G01V5/104 Primary

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

• G01V5/105

  the neutron source being of the pulsed type · CPC title

• G01V5/108 Primary

  the neutron source being of the pulsed type · CPC title

• G01V5/107

  and detecting reflected or back-scattered neutrons · CPC title