Downhole gamma-ray detection

• US 7,081,616 B2
• Filed: 12/12/2003
• Issued: 07/25/2006
• Est. Priority Date: 12/12/2003
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Grau, Jim, Stoller, Christian, Plasek, Ronald
• Primary Examiner(s): Luebke, Renee
• Assistant Examiner(s): ZETTL, MARY E
• Application Number:
  US10/734,727
• Publication Number:
  US 20050127282A1
• Time in Patent Office: 956 Days
• Field of Search: 250/262, 250/253, 250/370.09, 250/256
• US Class Current: 250/262
• CPC Class Codes: G01T 1/40 Stabilisation of spectrometers
• CPC Class Codes: G01V 5/101 and detecting the secondary Y-rays produced in the surrounding layers of the bore hole

Abstract

A method for evaluating a natural gamma-ray activity within a borehole, comprises the steps of:

• • stabilizing the gain according to a method for stabilizing a gain of a gamma-ray detector for use in a downhole logging tool;
  • determining an interval count rate, the interval count rate corresponding to gamma-rays having an energy within a predetermined correction interval;
  • calculating a correction count rate from the determined interval count rate; and
  • using the correction count rate to evaluate the natural gamma-ray activity.
  • The method for stabilizing the gain of the gamma-ray detector for use in the downhole logging tool, comprises the steps of:
  • processing a backscatter peak of a full gamma spectrum such that the backscatter peak constitutes a reference peak;
  • determining a first rate, the first rate corresponding to gamma-rays having an energy within a first predetermined energy interval;
  • determining a second rate, the second rate corresponding to gamma-rays having an energy within a second predetermined energy interval; and
  • the first predetermined energy interval and the second predetermined energy interval straddle the backscatter peak.

Claims

1. A method for stabilizing a gain of a gamma-ray detector for use in a downhole logging tool, wherein the method comprises the steps of:

processing a backscatter peak of a full gamma spectrum such that the backscatter peak constitutes a reference peak;

determining a first rate, the first rate corresponding to gamma-rays having an energy within a first predetermined energy interval;

determining a second rate, the second rate corresponding to gamma-rays having an energy within a second predetermined energy interval; and

wherein the first predetermined energy interval and the second predetermined energy interval straddle the backscatter peak.

2. The method of claim 1, the method further comprising:

adjusting the gain such that a ratio of the first rate and the second rate substantially equals a predetermined value.

3. The method of claim 1, the method further comprising adjusting the gain such that a difference of the first rate and the second rate multiplied by a predetermined positive coefficient substantially equals zero.

4. The method of claim 1, the method further comprising:

measuring a centroid position of a detected backscatter peak;

adjusting the gain such that the measured centroid position equals a reference centroid position.

5. The method according to claim 1, wherein the detector is intended to detect natural gamma-rays from a formation surrounding a borehole.

6. The method according to claim 5, wherein a gamma-ray inducing source is located in a neighborhood of the gamma-ray detector.

7. The method according to claim 1, wherein the detector is intended to detect neutron-induced gamma-rays.

8. A system for stabilizing a gain of a gamma-ray detector for use in a downhole logging tool, the system comprising:

the gamma-ray detector to detect gamma-rays;

discriminating means allowing to compare the energy of the detected gamma-rays to at least three regulation thresholds, the three regulation thresholds being located in an energy neighborhood of a backscatter peak of a full gamma spectrum, the backscatter peak constituting a reference peak, the discriminating means determining a first rate and a second rate, the first rate and the second rate corresponding to gamma-rays having an energy respectively within a first predetermined energy interval and a second predetermined energy interval, the first predetermined energy interval and the second predetermined energy interval straddling the backscatter peak; and

adjusting means to adjust the gain of the gamma-ray detector.

9. The system of claim 8, further comprising:

calculating means to calculate a ratio of the first rate and the second rate and to compare the ratio to a predetermined value.

10. The system of claim 8, wherein:

the discriminating means allowing to compare the energy of the detected gamma-rays to a relatively high number of regulation thresholds so as to obtain a complete spectrum;

the system further comprises calculating means to calculate a centroid position of a detected backscatter peak of the complete spectrum and to compare the calculated centroid position to a reference centroid position.

11. The system of claim 8, wherein

the gamma-ray detector is located in a drilling tool;

the gamma-ray detector is intended to detect natural gamma-rays from a formation surrounding a drilled borehole.

12. A method for evaluating a natural gamma-ray activity within a borehole, the method comprising:

stabilizing a gain of a gamma-ray detector by processing a backscatter peak of a full gamma spectrum such that the backscatter peak constitutes a reference peak by;

determining a first rate, the first rate corresponding to gamma-rays having an energy within a first predetermined energy interval, determining a second rate, the second rate corresponding to gamma-rays having an energy within a second predetermined energy interval, the first predetermined energy interval and the second predetermined energy interval straddling the backscatter peak; and

adjusting the gain to such that a value of a ratio of the first rate and the second rate substantially equals a predetermined value;

determining an interval count rate, the interval count rate corresponding to gamma-rays having an energy within a predetermined correction interval;

calculating a correction count rate from the determined interval count rate; and

using the correction count rate to evaluate the natural gamma-ray activity.

13. The method according to claim 12, wherein the predetermined correction interval is semi infinite above a predetermined correction threshold.

14. The method according to claim 13, further comprising:

measuring a total gamma count rate, the total gamma count rate corresponding to gamma-rays detected by the gamma-ray detector;

subtracting the correction count rate from the total gamma count rate to evaluate the natural gamma-ray activity.

15. The method according to claim 14, wherein the correction count rate is proportional to the determined interval count rate.

16. The method according to claim 12, wherein

a gamma-ray inducing source is located downhole in a neighborhood of the system;

and

the gamma-ray inducing source is an high energy neutron generator.

17. The method according to claim 16, wherein the evaluating of the natural gamma-ray activity is performed during a drilling of the borehole.

18. The method according to claim 17, wherein the neutron-induced gamma-rays are due to an activation of oxygen atoms located within a drilling mud.

19. A system for evaluating a natural gamma-ray activity within a borehole, the system comprising:

a gamma-ray detector located downhole to detect gamma-rays;

at least one discriminator to allow to determine an interval count rate, the interval count rate corresponding to gamma-rays having an energy within a predetermined correction interval, the at least one discriminator allowing to determine and compare a first rate and a second rate, the first rate and the second rate corresponding to gamma-rays having an energy respectively within a first predetermined energy interval and a second predetermined energy interval, the first predetermined energy interval and the second predetermined energy interval straddling a backscatter peak of a full gamma spectrum, the backscatter peak constituting a reference peak;

adjusting means to adjust a gain of the gamma-ray detector according to a result of the comparing;

calculating means to calculate a ratio of the first rate and the second rate and to compare the ratio to a predetermined value; and

processing means to calculate a correction count rate from the determined interval count rate, the correction count rate being used to evaluate the natural gamma-ray activity.

20. The system according to claim 19, wherein the predetermined correction interval is semi infinite above a predetermined correction threshold.

21. The system according to claim 19, wherein

a gamma-ray inducing source is located downhole near the system;

the gamma-ray inducing source is an high energy neutron generator.

22. The system according to claim 21, wherein the detector is located in a drilling tool.

23. The system according to claim 22, wherein the detector detects neutron-induced gamma-rays, the neutron-induced gamma-rays being due to an activation of oxygen atoms located in a drilling mud by high energy neutrons.

24. The system according to claims 19, further comprising:

a shield located at a rear side of a crystal of the gamma-ray detector to reduce the detecting of gamma-rays coming from the rear side.

25. The system according to claims 24, further comprising a collar surrounding the crystal, the collar having a recess on a front side of the crystal to improve a transmission of gamma-rays coming from the front side.