Compensated sigma from measurements made by a pulsed neutron instrument

Patent metadata

Field	Value
Publication number	US-9798037-B2
Application number	US-201314424396-A
Country	US
Kind code	B2
Filing date	Sep 5, 2013
Priority date	Sep 5, 2012
Publication date	Oct 24, 2017
Grant date	Oct 24, 2017

Abstract

A method for determining a formation thermal neutron decay rate from measurements of radiation resulting from at least one burst of high energy neutrons into formations surrounding a wellbore includes determining a first apparent neutron decay rate in a time window beginning at a first selected time after an end of the at least one burst, a second apparent decay rate from a time window beginning at a second selected time after the burst and a third apparent decay rate from a third selected time after the burst. The second time is later than the first time. A thermal neutron capture cross section of fluid in the wellbore is determined. A decay rate correction factor is determined based on the first and second apparent decay rates and a parameter indicative of the wellbore capture cross-section. The correction factor is applied to the third apparent decay rate to determine the formation thermal neutron decay rate.

First claim

What is claimed is:

1. A method, comprising:
moving a pulsed neutron logging instrument in a wellbore;
imparting at least one controlled duration burst of high energy neutrons into formations surrounding the wellbore;
measuring at least one of thermal neutrons and capture gamma rays resulting from said imparting;
in a processor, determining a first apparent decay rate of thermal neutrons in a time window beginning at a first selected time after an end of the at least one burst;
in the processor, determining a second apparent decay rate of thermal neutrons in a time window beginning at a second selected time after an end of the at least one burst, the second selected time being later than the first selected time;
in the processor, determining a third apparent decay rate of thermal neutrons in a time window beginning at a third selected time after an end of the at least one burst;
in the processor, determining a decay rate correction factor based on the first and second decay rates and a parameter indicative of the wellbore thermal neutron capture cross section;
in the processor applying the decay rate correction factor to the third apparent decay rate to determine the formation thermal neutron decay rate, wherein the correction factor is based on an arithmetic difference between the first and second decay rates; and
determining when a crossover condition exists and inverting a sign of the apparent decay rate difference when the crossover condition exists.

2. The method of claim 1 further comprising determining a formation thermal neutron capture cross section from the formation thermal neutron decay rate.

3. The method of claim 1 wherein the radiation detector comprises at least one of a gamma ray detector and a thermal neutron detector.

4. The method of claim 1 wherein the determining when a crossover condition exists comprises determining a ratio of an apparent thermal neutron decay rate based on the first radiation detector and a corresponding thermal neutron decay rate from measurements made by the second radiation detector which has a different spacing from a position of imparting the burst of neutrons than the first radiation detector, the crossover condition determined to exist when the ratio exceeds a selected threshold.

5. The method of claim 1 wherein the determining when a crossover condition exists is done by comparing an apparent formation sigma with the wellbore thermal neutron capture cross section.

6. The method of claim 1 further comprising adaptively filtering the determined decay rate correction factor.

7. The method of claim 1 wherein the parameter indicative of wellbore fluid thermal neutron capture cross section is determined by making a thermal neutron decay rate measurement of the fluid in the wellbore.

8. The method of claim 1 wherein the parameter indicative of wellbore fluid thermal neutron capture cross section is determined by measuring a resistivity of the fluid.

9. The method of claim 1 wherein the third selected time is the same as the first or second selected time.

10. A method for well logging, comprising:
moving a well logging instrument along an interior of a wellbore drilled through subsurface formations, the instrument including a pulsed neutron source and at least one radiation detector disposed at an axially spaced apart position from the pulsed neutron source;
operating the pulsed neutron source to emit at least one controlled duration burst of neutrons into the wellbore and the formations;
detecting radiation related to thermal neutron population using the radiation detector after an end of the at least one burst;
in a processor, determining a first apparent decay rate of thermal neutrons in a time window beginning at a first selected time after an end of the at least one burst;
in the processor, determining a second apparent decay rate of thermal neutrons in a time window beginning at a second selected time after an end of the at least one burst, the second selected time being later than the first selected time;
in the processor, determining a third apparent decay rate of thermal neutrons in a time window beginning at a third selected time after an end of the at least one burst;
in the processor, determining a thermal neutron capture cross section of fluid in the wellbore;
in the processor, determining a decay rate correction factor based on the first and second apparent decay rates and the wellbore capture cross-section;
in the processor applying the correction factor to the third apparent decay rate to determine the formation thermal neutron decay rate, and
adaptively filtering the determined decay rate correction factor.

11. The method of claim 10 further comprising determining a formation thermal neutron capture cross section from the formation thermal neutron decay rate.

12. The method of claim 10 wherein the radiation detector comprises at least one of a gamma ray detector and a thermal neutron detector.

13. The method of claim 10 wherein the correction factor is based on an arithmetic difference between the first and second apparent decay rates.

14. The method of claim 13 further comprising determining when a crossover condition exists and inverting a sign of the apparent decay rate difference when the crossover condition exists.

15. The method of claim 14 wherein the determining when a crossover condition exists comprises determining a ratio of the second apparent thermal neutron decay rate and a corresponding thermal neutron decay rate from measurements made by a radiation detector having a different spacing from a position of imparting the burst of neutrons than a radiation detector used to determine the second apparent thermal neutron decay rate, the crossover condition determined to exist when the ratio exceeds a selected threshold.

16. The method of claim 14 wherein the determining when a crossover condition exists by comparing an apparent formation sigma with the wellbore thermal neutron capture cross section.

17. The method of claim 10 wherein the wellbore fluid capture cross section is determined by making a thermal neutron decay rate measurement of the fluid in the wellbore.

18. The method of claim 10 wherein the wellbore fluid capture cross section is determined by measuring a resistivity of the fluid.

19. The method of claim 10 wherein the third selected time is the same as the second selected time.

20. A method, comprising:
moving a pulsed neutron logging instrument in a wellbore;
imparting at least one controlled duration burst of high energy neutrons into formations surrounding the wellbore;
measuring at least one of thermal neutrons and capture gamma rays resulting from said imparting;
in a processor, determining a first apparent decay rate of thermal neutrons in a time window beginning at a first selected time after an end of the at least one burst;
in the processor, determining a second apparent decay rate of thermal neutrons in a time window beginning at a second selected time after an end of the at least one burst, the second selected time being later than the first selected time;
in the processor, determining a third apparent decay rate of thermal neutrons in a time window beginning at a third selected time after an end of the at least one burst;
in the processor, determining a decay rate correction factor based on the first and second decay rates and a parameter indicative of the wellbore thermal neutron capture cross section; and
in the processor applying the decay rate correction factor to the third app

Assignees

• Schlumberger Technology Corp

Inventors

• Zhou Tong
• Stoller Christian

Classifications

• G01V5/108 Primary

  the neutron source being of the pulsed type · CPC title

• G01V5/101 Primary

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

• G01V5/105

  the neutron source being of the pulsed type · CPC title

• G01V5/102

  the neutron source being of the pulsed type · CPC title