Neutron time of flight wellbore logging

Patent metadata

Field	Value
Publication number	US-11209569-B2
Application number	US-201916460867-A
Country	US
Kind code	B2
Filing date	Jul 2, 2019
Priority date	Jul 2, 2019
Publication date	Dec 28, 2021
Grant date	Dec 28, 2021

Abstract

Methods and tools for determining one or more parameters of an earth formation using time-of-flight (TOF) measurements of fast neutrons through the formation are disclosed. The disclosed tools feature a neutron source capable of emitting a population of fast neutrons having a distribution of neutron energies and one or more neutron detectors. The TOF of the fast neutrons travelling from the neutron source to the detector(s) and traversing a portion of the formation is measured and binned as a function of TOF (which is a function of neutron energy). By determining which neutron energies are attenuated by the intervening formation, the composition of the intervening formation is determined.

First claim

What is claimed is:

1. A method of measuring at least one parameter of an earth formation, the method comprising:
deploying a logging tool in a wellbore traversing the formation, wherein the logging tool comprises: a neutron source configured to emit fast neutrons, and at least one neutron detector,
using the neutron source to emit fast neutrons into the formation,
using the at least one neutron detector to detect neutrons arriving at the detector,
measuring a time-of-flight (TOF) of fast neutrons traversing from the neutron source to the at least one neutron detector through a portion of the earth formation, and
using the measured TOF to determine the at least one parameter.

2. The method of claim 1 , wherein the neutron source is configured to emit a population of neutrons having a distribution of neutron energies.

3. The method of claim 1 , wherein the measured TOF is indicative of a neutron energy of the fast neutrons traversing through the portion of the earth formation.

4. The method of claim 3 , wherein using the measured TOF to determine the at least one parameter comprises determining an amount of at least one material present in the earth formation based on neutron energies of fast neutrons that are transmitted through or attenuated by the formation.

5. The method of claim 4 , wherein the at least one material is selected from the group consisting of hydrocarbons and water.

6. The method of claim 1 , wherein the at least one parameter is fluid saturation.

7. The method of claim 1 , wherein measuring the TOF of fast neutrons traversing from the neutron source to the at least one neutron detector through a portion of the earth formation comprises:
providing a start signal coincident with the neutron source emitting fast neutrons,
providing a detect signal coincident with the at least one detector detecting fast neutrons, and
determining the TOF as a difference between the detect signal and the start signal.

8. The method of claim 7 , wherein the neutron source comprises a neutron generator and a scintillator material configured such that at least a portion of neutrons generated by the neutron generator causes the scintillator material to emit light, and wherein the start signal is triggered by the emitted light.

9. The method of claim 1 , wherein the TOF is 5 nanoseconds to 500 nanoseconds.

10. The method of claim 1 , further comprising determining an attenuation of traversing a sample of wellbore fluid collected from the wellbore.

11. The method of claim 1 , wherein measuring the time-of-flight (TOF) of fast neutrons traversing from the neutron source to the at least one neutron detector through a portion of the earth formation comprises:
emitting a population of fast neutrons at the neutron source, wherein the population of fast neutrons comprises neutrons having a distribution of neutron energies,
providing a start signal coincident with the emitting of the population of fast neutrons,
triggering the one or more neutron detectors to begin detecting fast neutrons based on the start signal, and
binning neutron detection counts of the at least one detector as a function of time elapsed between the start signal and the counts.

12. The method of claim 11 , further comprising normalizing the neutron detection counts with respect to a source neutron count comprising a count of the fast neutrons emitted by the neutron source.

13. The method of claim 12 , further comprising determining a neutron attenuation for selected bins.

14. The method of claim 13 , further comprising determining a composition of fluid contained within pores of the formation based on the neutron attenuation.

15. A logging tool configured to be conveyed in a wellbore traversing an earth formation, the logging tool comprising:
a neutron source configured to emit fast neutrons,
at least one neutron detector configured to detect neutrons arriving at the detector,
a trigger circuitry configured to trigger a start signal coincident with the neutron source emitting the fast neutrons, and
a timing circuitry configured to measure a time-of-flight (TOF) of the fast neutrons between the neutron source and the at least one detector.

16. The logging tool of claim 15 , the neutron source is configured to emit a population of fast neutrons having a distribution of neutron energies.

17. The logging tool of claim 16 , wherein the neutron source comprises a neutron generator and a scintillator material configured such that at least a portion of neutrons generated by the neutron generator interacts with the scintillator material to provide the population of fast neutrons having a distribution of neutron energies.

18. The logging tool of claim 17 , wherein the interaction of the at least a portion of neutrons with the scintillator material causes the scintillator material to emit light.

19. The logging tool of claim 18 , wherein the trigger circuitry is configured to trigger the start signal based on the emitted light.

20. The logging tool of claim 19 , wherein the timing circuitry comprises at least one time-to-digital converter (TDC) configured to receive the start signal as a first input and a detection signal from the at least one neutron detector as a second input.

Assignees

• Weatherford Tech Holdings Llc

Inventors

• Mccleskey Matthew
• Mekic Natasa

Classifications

• G01T3/06

  with scintillation detectors · CPC title

• G01V5/108

  the neutron source being of the pulsed type · CPC title

• G01V5/102

  the neutron source being of the pulsed type · CPC title

• G01V5/107 Primary

  and detecting reflected or back-scattered neutrons · CPC title

• G01V5/105 Primary

  the neutron source being of the pulsed type · CPC title