Dark current correction in scinitillator detectors for downhole nuclear applications

Patent metadata

Field	Value
Publication number	US-9766366-B2
Application number	US-201515108612-A
Country	US
Kind code	B2
Filing date	Sep 14, 2015
Priority date	Sep 14, 2015
Publication date	Sep 19, 2017
Grant date	Sep 19, 2017

Abstract

A radiation logging tool is provided that includes a scintillator detector for use on a wellbore tool string to characterize earth formations. The scintillator detector has a shutter to allow for the collection of data differentiating between incident radiation, such as backscatter signal, and system noise, such as dark current, vibration noise, electronics thermal noise, and electrostatic noise. The radiation logging tool provides for a method of calibrating and measuring incident radiation by the removal of system noise. The shutter is positioned between the photosensor and scintillation member of the scintillator detector, and is able to switch between open and closed states while the scintillation detector is deployed. Measurements of signal noise can be used to calibrate the sampling signal of incident radiation on the scintillator detector.

First claim

That which is claimed is:

1. A system for measuring and processing nuclear radiation data collected within a wellbore, comprising:
a tool string; and
a radiation logging tool mounted to the tool string, the radiation logging tool including (i) a radiation emitter operable to emit a nuclear signal into earth strata and (ii) a scintillator detector to collect backscatter signals from earth strata, the scintillator detector including a shutter operable to actuate between an open configuration and a closed configuration, wherein the shutter in the open configuration allows the scintillator detector to collect signal data based on the backscatter signals, and the shutter in the closed configuration allows the scintillator detector to collect system noise data to remove system noise from the signal data.

2. The system according to claim 1 , further comprising a local processing unit comprising a non-transitory, computer-readable medium having instructions that are executable for removing system noise from the signal data to output corrected signal data, the system noise including dark current.

3. The system according to claim 2 , wherein the local processing unit includes instructions for calculating a correction value based on the system noise and instructions for using the correction value to remove the system noise from the signal data.

4. The system according to claim 2 , further comprising a remote control unit, electronically coupled to the local processing unit, the remote control unit being operable to receive signal data from the local processing unit.

5. The system according to claim 2 , wherein the scintillator detector further comprises a check source coupled to the scintillator detector for emitting a reference signal that is collected by the scintillator detector.

6. The system according to claim 5 , wherein the local processing unit includes instructions for calculating a photosensor drift of the scintillator detector based on a check source reference signal, and for adjusting a power delivered to the scintillator detector based on the calculated photosensor drift.

7. The system according to claim 5 , wherein the local processing unit includes instructions for calculating a photosensor drift of the scintillator detector based on a check source reference signal, for calculating an adjustment to the system noise based on the calculated photosensor drift, and for calculating a correction value based on the adjustment to the system noise for removing from the signal data.

8. A radiation logging tool, comprising:
a radiation emitter;
a scintillator detector comprising:
a scintillation member operable to receive nuclear radiation signals from or through an earth formation and to generate photons in response to receiving the nuclear radiation signals;
a photosensor for receiving the photons from the scintillation member based on the nuclear radiation signals and for converting the photons to output signals; and
a shutter (i) operable between an open configuration and a closed configuration to remove system noise from the output signals and (ii) positioned between the scintillation member and the photosensor.

9. The radiation logging tool according to claim 8 , wherein the photosensor is a photomultiplier tube, wherein the system noise includes dark current.

10. The radiation logging tool according to claim 8 , wherein the shutter is a liquid crystal shutter, a mechanical shutter, or an optical shutter.

11. The radiation logging tool according to claim 8 , wherein the shutter is operable to actuate between the open configuration, that allows the scintillator detector to collect signal data based on backscatter signals, and the closed configuration, that allows the scintillator detector to collect system noise data to remove from the collected signal data, by a shutter control voltage.

12. The radiation logging tool according to claim 8 , further comprising a shielding element positioned between the radiation emitter and the scintillator detector.

13. The radiation logging tool according to claim 8 , further comprising a check source located adjacent to the scintillation member.

14. The radiation logging tool according to claim 8 , further comprising a check source located within the scintillation member.

15. The radiation logging tool according to claim 12 , further comprising:
a secondary shutter operable between an open configuration and a closed configuration, coupled to the scintillation member;
a shield case that surrounds a secondary scintillation member, except for a region of the secondary scintillation member coupled to the secondary shutter, wherein a check source is located within the secondary scintillation member; and
an optical fiber that optically couples the secondary shutter and the secondary scintillation member.

16. The radiation logging tool according to claim 15 , further comprising a lens optically coupled to and positioned between the secondary scintillation member and the optical fiber.

17. A method of operating a radiation logging tool in a wellbore, comprising:
emitting a nuclear signal with a radiation emitter into earth strata;
actuating a shutter element, coupled to and positioned between a photosensor and a scintillator detector, between an open configuration and a closed configuration;
with the shutter element in the open configuration, logging sample signal data based on incident signal with the photosensor;
with the shutter element in the closed configuration, logging dark current with the photosensor;
calculating a correction value based on the dark current logging; and
applying the correction value to the sample signal data to remove a component of dark current from the sample signal data.

18. The method according to claim 17 , further comprising logging reference signal data concurrent with sample signal data where the shutter element of the scintillator detector in an open configuration.

19. The method according to claim 17 , further comprising logging vibration noise and electrostatic noise concurrent with dark current logging where the shutter element of the scintillator detector is in the closed configuration, and calculating the correction value based on the dark current, vibration noise, and electrostatic noise logging data.

20. The method according to claim 17 , further comprising generating a data set indicative of signals received external to the scintillator detector and transmitting that data set to a remote control unit.

Assignees

• Halliburton Energy Services Inc

Inventors

• Vehra Imran Sharif

Classifications

• E21B47/12

  Means for transmitting measuring-signals or control signals from the well to the surface, or from the surface to the well, e.g. for logging while drilling · CPC title

• E21B49/00

  Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells · CPC title

• G01T1/2026

  Well-type detectors · CPC title

• G01N2223/505

  scintillation · CPC title

• G01V5/08 Primary

  using primary nuclear radiation sources or X-rays {(, e.g. for inducing radioactivity; investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays, neutrons G01N23/00)} · CPC title