Borehole measurements using a fast and high energy resolution gamma ray detector assembly

Abstract

A gamma ray detector assembly for a borehole logging system that requires the measure of gamma radiation with optimized gamma ray energy resolution and with fast emission times required to obtain meaningful measurements in high radiation fields. The detector assembly comprises a lanthanum bromide (labr3) scintillation crystal and a digital spectrometer that cooperates with the crystal to maximize pulse processing throughput by digital filtering and digital pile-up inspection of the pulses. The detector assembly is capable of digital pulse measurement and digital pile-up inspection with dead-time less than 600 nanoseconds per event. pulse height can be accurately measured (corrected for pile-up effects) for 2 pulses separated by as little as 150 nanoseconds. Although the invention is applicable to virtually any borehole logging methodology that uses the measure of gamma radiation in harsh borehole conditions, the invention is particularly applicable to carbon/oxygen logging.

Description

assemblyproviding provide an alternative to traditional open-hole logging such as through casing density and neutron porosity logging, and gas detection. As a result, various design trade-offs are used in optimizing these specific applications. For example, the formation porosity is a measure of the spatial distribution of radiation and requires certain axial detector assembly spacings from the source. Carbon/oxygen (C/O) logging is a spectral energy measurement and requires high count-rates at detector assemblies axially spaced close to the neutron source.

FIG. 1 illustrates a multipurpose pulsed neutron logging instrument 10 disposed within a borehole 32 penetrating an earth formation 40. The borehole is cased with casing 33, and the casing-borehole annulus is filled with a grouting material such as cement. Subsection 11 houses an array of detector assemblies as well as a pulsed neutron generator 12. More specifically, there are four detector assemblies each comprising a LaBr3 detector crystal and a digital spectrometer for filtering and pulse inspection. These detector assemblies are referred to as the proximal detector assembly 14, the near detector assembly 16, the far detector assembly 20, and long detector assembly 22. These detector assemblies are disposed at increasing axial spacings from the neutron generator 12, as their names imply. Between the near detector assembly 16 and the far detector assembly 20 is disposed a fast neutron detector 18 that measures the fast neutron output flux and pulse shape of the neutron generator 12. This array was originally disclosed in the publication “Improvements in a Through-Casing Pulsed Neutron Density Log” paper SPE 71742, 2001 SPE Annual Conference Proceedings, which is herein entered into this disclosure by reference. The use of detector assemblies LaBr3 crystal and the previously referenced digital spectrometer have been added to the array to improve the C/O results.

The subsection 11 is operationally connected to an instrument subsection 24. The instrument subsection houses control circuits and power circuits to operate and control the elements of the subsection 11. A telemetry subsection 26 is operationally connected to the instrument section 24. A suitable connector 28 connects the multipurpose logging tool 10 to a lower end of a preferably multiconductor logging cable 30. The upper end of the logging cable 30 terminates at a draw works, which is well known in the art and is represented conceptually at 34.

Still referring to FIG. 1, detector assembly response data are telemetered from the tool 10 to the surface 39 of the earth where they are received by an uphole telemetry unit (not shown) preferably disposed within surface equipment 36. These data are processed in a surface processor (not shown) within the surface equipment 36 to yield a log 38 of one or more parameters of interest. Alternately, data can be partially of or completely processed in a downhole processor within the instrument section 24 and telemetered via the telemetry subsection 26 to the surface equipment 36. Control parameters can also be telemeterd telemetered from the surface equipment 36 to the tool 10 via the telemetry system and wireline cable 30.

Again referring to FIG. 1, the tool 10 is designed to go through tubing (not shown), has an outside diameter of 1.69 inches (4.29 centimeters), and is rated for operations at 20 thousand pounds per square inch (Kpsi) pressure and at a maximum temperature of about 325 degrees Fahrenheit (° F.).

Although shown embodied in a wireline logging tool, the detector assembly 11 can also be embodied in other borehole instruments. These instruments include pump-down (“memory”) instruments conveyed by drilling fluid flow, instruments conveyed by coiled tubing, instruments conveyed by a drill string, and instruments conveyed by a “slick line”.

The LaBr₃ Detector Crystal

In 2006 the LaBr3(Ce) crystal was introduced in a logging package by Saint Gobain (www.saint-gobain.com) under the trademark BriLanCe380™. In Table 1, the physical parameters for this crystal are compared with properties of other scintillation crystals used in prior art well logging detector assemblies. The scintillation crystals are NaI, BGO, GSO in addition to LaBr3. The crystal properties are light output in percent, energy resolution in percent, crystal density in grams per cubic centimeter, effective atomic number, and scintillation decay time or “emission time” in microseconds.

TABLE 1
Physical properties of LaBr3 and other scintillators used in
well logging (*designates 3 inch (diameter) by 3 inch (length) crystals.
Resolutions are for 137Cs gamma radiation at 0.662 MeV)
CRYSTAL PROPERTY	NaI	BGO	GSO	LaBr3
Light output (%)	100	12	18	165
Energy Resolution (%)	7*	9.3	8	2.9*
Density (g/cc)	3.67	7.13	6.71	5.08
Effective atomic number	50	74	59	47
Temperature coeff. (%/C.)	−0.3	−1.5	−0.3	−0.05
Decay time (μsec)	230	300	60 & 600	16

Again referring to Table 1, the outstanding features of LaBr3 are the peak resolution, temperature response and emission time. In the gamma radiation energy ranges or “windows” used in one C/O logging method, good peak resolution is important to assure accurate energy calibrations. More advanced C/O logging methods use spectral fitting techniques such as Library Least Squares for formation lithology identification or C/O determination. This approach exploits the good energy resolution of LaBr3 to add more uniqueness to library spectrum elements. The temperature response assures good resolution and stable measurement across the temperature range encountered in the borehole environment.

Similar to NaI, LaBr3 exhibits a thermal neutron activation background. More specifically, the bromine in LaBr3 has a relatively large thermal neutron activation cross section with the induced isotopes being gamma ray emitters. Preliminary test results indicate that the bromine activation that appears on the tail of the decay is about twice as strong as the iodine activation in NaI. There are two activation daughters. The first is 82Br that decays with a half life of 1.47 days. The second and more troublesome is 80Br which has two decay modes with half lives of 17.68 minutes and 4.4 hours. This neutron activation background signal can be minimized by thermal neutron shielding of the LaBr3 crystal.

The Digital Spectrometer and Pulse Selection System

To achieve the optimal scintillation pulse throughput for the detector assembly, the detector assembly uses a digital spectrometer designed by XIA LLC that is disclosed in detail in the previously referenced U.S. Pat. No. 6,590,957 B1. The digital spectrometer has been configured to obtain detector assembly specifications discussed in subsequent sections of this disclosure.

FIG. 2 illustrates major elements of each gamma ray detector assembly 45. A LaBr3 crystal 46 is optically coupled to a photomultiplier tube 47. Output pules from the photomultiplier tube 47 pass through a preamplifier 48 and into the digital spectrometer 49. The pulse processor of the digital spectrometer 49 receives the “raw” detector data and uses digital filtering and digital inspection techniques to process these data by pulse height and time, and to discard “pile-up” pulses which are events that are ruined by pulse pile-up. All gamma ray events down to 100 KeV are processed in order to preserve resolution.

Referring again to FIG. 1, this means that the proximal detector assembly 14 and near detector assembly 16 can be exposed to a gamma radiation field greater than one million pulses per second during a pulse from the neutron generator 12. This intense “instantaneous” count rate is typical for neutron generator output and detector assembly axial spacings for the logging tool 10 depicted in FIG. 1. Throughput tests have established a processing dead-time of approximately 0.8 microseconds. This translates to a theoretical maximum of 480,000 events (in terms of instantaneous count rates) that can be effectively processed by the detector assembly 45 depicted in FIG. 2.

Results Using the Detector Assembly

In practice, the response of the detector assembly 45 is limited by filtering of electronics noise and digital sampling rates. It has been demonstrated that the assembly 45 is capable of pulse measurement and pile-up inspection with dead-time of less than 600 nS/event. Pulse height can be accurately resolved and measured (corrected for pile-up pulse effects) for 2 pulses separated by as little as 150 nS.

It is instructive to express specifications of the LaBr3 crystal 46 cooperating with the digital spectrometer 49 in terms of well logging precision. In the context of C/O logging precision, the logging tool 10 was operated in high-porosity carbonate calibration standards with oil and water in the pore space and fresh water in the borehole. The neutron source was operated at a pulse repetition rate of about 5 kiloHertz (KHz) with each burst having a duration of about 30 microseconds. The “window ratio” C/O technique was used. Count rates refer to those recorded by the near detector assembly 16. The carbon count rate C represents first group of pulses recorded in the carbon energy window ranging from about 3.0 to about 4.7 MeV. The oxygen count rate O represents a second group of pulses recorded in the oxygen energy window ranging from about 4.7 MeV to about 6.4 MeV. A typical two foot (0.61 meters) logging sample at 6 feet (1.82 meters) per minute represents 20 seconds. Operating at 80 percent of the maximum throughput, the counts collected by the digital spectrometer 49 are approximately 52,000 and 30,000 for the carbon and oxygen windows, respectively. Given the C/O ratio for the standard with fresh water in the pore space is 1.73, the following expression for the standard deviation of the C/O ratio in this carbonate as:

$\begin{array}{cc}{\sigma }_{C/O}=\sqrt{{\left(\frac{C}{O}\right)}^2\left(\frac{1}{C}+\frac{1}{O}\right)}& \left(1\right)\end{array}$

The deviation of the C/O ratio is 0.012, and enfolding the dynamic range between these standards, the deviation is 7.7 saturation units (s.u.), which is an apparent improvement over prior art assemblies.

The above disclosure is to be regarded as illustrative and not restrictive, and the invention is limited only by the claims that follow.

Claims

1. A borehole instrument comprising a neutron generator axially spaced from a gamma ray detector assembly, said assembly comprising:

a labr3 scintillation crystal; and

a digital spectrometer cooperating with said labr3 scintillation crystal and configured to

measure pulses from said scintillation crystal,

digitally filter said pulses from said scintillation crystal,

digitally inspect the filtered pulses to detect pile-up pulses,

periodically forming an estimate of an average ratio of said measured pulses per non-pile-up pulse, and

reject said pile-up pulses by combining said measured pulses with said ratio;

wherein;

said neutron generator emits a plurality of bursts of neutrons the duration of each burst of said plurality of bursts being about 30 microseconds and said plurality of bursts being emitted at a repetition rate of about 5 KHz; and

said detector assembly is operated during said bursts.

2. The borehole instrument of claim 1 wherein said detector assembly measures pulses and digitally filters pulses and digitally inspects pulses and rejects said pile-up pulses with dead-time less than 600 nanoseconds per event.

3. The borehole instrument of claim 1 wherein said detector assembly measures and resolves heights of two said pulses separated by about 150 nanoseconds.

4. The borehole instrument of claim 1 wherein said detector assembly operates at about 325 degrees Fahrenheit.

5. The borehole instrument of claim 1 wherein said borehole instrument is conveyed by a wireline or a drill string or coiled tubing or a slick line or drilling fluid flow.

6. The borehole instrument of claim 1 wherein a first group said pulses is collected in a gamma ray energy range of about 3.0 MeV to about 4.7 MeV and a second group is collected in a gamma ray energy range of about 4.7 MeV to about 6.4 MeV.

7. A borehole instrument comprising a neutron generator axially spaced from a gamma ray detector assembly, said assembly comprising:

a labr3 scintillation crystal; and

a digital spectrometer cooperating with said labr3 scintillation crystal and configured to

measure pulses from said scintillation crystal,

digitally filter said pulses from said scintillation crystal,

digitally inspect the filtered pulses to detect pile-up pulses, and

reject said pile-up pulses;

wherein

said neutron generator emits a plurality of bursts of neutrons the duration of each burst of said plurality of bursts being about 30 microseconds and said plurality of bursts being emitted at a repetition rate of about 5 KHz; and said detector assembly is operated during said bursts;

a first group said pulses is collected in a gamma ray energy range of about 3.0 MeV to about 4.7 MeV and a second group is collected in a gamma ray energy range of about 4.7 MeV to about 6.4 MeV;

a C/O ratio is formed from the ratio of said first group to said second group;

said C/O ratio is indicative of the amount of carbon to oxygen within environs in which said borehole instrument operates; and

determining said C/O ratio to a precision of about 0.012 standard deviation for a 20 second sample for a single detector.

8. The borehole instrument of claim 7 wherein said detector assembly measures pulses and digitally filters pulses and digitally inspects pulses and rejects said pile-up pulses with dead-time less than 600 nanoseconds per event.

9. The borehole instrument of claim 7 wherein said detector assembly measures and resolves heights of two said pulses separated by about 150 nanoseconds.

10. The borehole instrument of claim 7 wherein said detector assembly operates at about 325 degrees Fahrenheit.

11. The borehole instrument of claim 7 wherein said borehole instrument is conveyed by a wireline or a drill string or coiled tubing or a slick line or drilling fluid flow.

12. A method for measuring radiation in a borehole, the method comprising:

disposing a gamma ray detector assembly within said borehole wherein said assembly comprises a LaBr₃ scintillation crystal; and a digital spectrometer cooperating with said labr3 scintillation crystal and configured to measure pulses from said scintillation crystal, digitally filter said pulses from said scintillation crystal, digitally inspect the filtered pulses to detect pile-up pulses, periodically forming an estimate of an average ratio of said measured pulses per non-pile up pulse, and reject said pile-up pulses by combining said measure of pulses with said ratio;

disposing a neutron generator within said borehole axially spaced from the detector assembly;

generating a plurality of bursts of neutrons with the neutron generator wherein the duration of each said burst is about 30 microseconds and said plurality of bursts is emitted at a repetition rate of about 5 KHz; and

operating the detector assembly during each burst of said plurality of bursts.

13. The method of claim 12 further comprising, with said detector assembly, measuring and digitally filtering and digitally inspecting and rejecting said pile-up pulses with dead-time less than 600 nanoseconds.

14. The method of claim 12 further comprising, with said detector assembly, accurately measuring heights of two said pulses separated by about 150 nanoseconds.

15. The method of claim 12 further comprising operating said detector assembly at about 325 degrees Fahrenheit.

16. The method of claim 12 wherein:

said detector assembly and said neutron generator are disposed within a borehole instrument that is conveyed in said borehole by a wireline or a drill string or coiled tubing or a slick line or drilling fluid flow.

17. The method of claim 12 further comprising collecting a first group said pulses in a gamma ray energy range of about 3.0 MeV to about 4.7 Mev and collecting a second group of said pulses in a gamma ray energy range of about 4.7 MeV to about 6.4 MeV.

18. A method for measuring radiation in a borehole, the method comprising:

disposing a gamma ray detector assembly within said borehole wherein said assembly comprises a LaBr₃ scintillation crystal; and a digital spectrometer cooperating with said labr3 scintillation crystal and configured to measure pulses from said scintillation crystal, digitally filter said pulses from said scintillation crystal, digitally inspect the filtered pulses to detect pile-up pulses, and reject said pile-up pulses;

disposing a neutron generator within said borehole axially spaced from the detector assembly;

generating a plurality of bursts of neutrons with the neutron generator wherein the duration of each said burst is about 30 microseconds and said plurality of bursts is emitted at a repetition rate of about 5 KHz;

operating the detector assembly during each burst of said plurality of bursts;

collecting a first group said pulses in a gamma ray energy range of about 3.0 MeV to about 4.7 MeV and collecting a second group of said pulses in a gamma ray energy range of about 4.7 MeV to about 6.4 MeV;

forming a C/O ratio from a ratio of said first group to said second group;

from said C/O ratio determining an indicator of the amount of carbon to oxygen within environs in which said borehole instrument is operating; and

determining said C/O ratio to a precision of about 0.012 standard deviation for a 20 second sample for a single detector.

19. The method of claim 18 further comprising, with said detector assembly, measuring and digitally filtering and digitally inspecting and rejecting said pile-up pulses with dead-time less than 600 nanoseconds.

20. The method of claim 18 further comprising, with said detector assembly, accurately measuring heights of two said pulses separated by about 150 nanoseconds.

21. The method of claim 18 further comprising operating said detector assembly at about 325 degrees Fahrenheit.

22. The method of claim 18 wherein:

said detector assembly and said neutron generator are disposed within a borehole instrument that is conveyed in said borehole by a wireline or a drill string or coiled tubing or a slick line or drilling fluid flow.

23. The method of claim 18 further comprising collecting a first group said pulses in a gamma ray energy range of about 3.0 MeV to about 4.7 MeV and collecting a second group of said pulses in a gamma ray energy range of about 4.7 MeV to about 6.4 MeV.

24. A well logging tool comprising:

a pulsed neutron generator adapted to produce bursts of high energy neutrons that induce formation of gamma rays in a borehole environment; and

a gamma ray detector assembly that comprises a lanthanum bromide scintillation crystal that emits light pulses in response to the gamma rays and a digital spectrometer cooperating with the lanthanum bromide scintillation crystal and configured to measure the light pulses, digitally inspect the light pulses to detect pile-up pulses, and by combining the measured pulses with an average ratio of measured pulses per non-pile up pulse, reject any pile-up pulses detected;

wherein the well logging tool is adapted to detect one or more of inelastic scatter gamma radiation and thermal capture gamma radiation.

25. The well logging tool of claim 24, wherein the gamma ray detector assembly is adapted to digitally measure and inspect the pulses and reject any pile-up pulses with a processing dead-time of less than about 0.8 microseconds.

26. The well logging tool of claim 24, wherein the gamma ray detector assembly is adapted to digitally measure and inspect the pulses and reject any pile-up pulses with a dead-time of less than 600 nanoseconds per event.

27. The well logging tool of claim 24, wherein the gamma ray detector assembly is adapted to measure and resolve heights of two pulses separated by at least 150 nanoseconds.

28. The well logging tool of claim 24, wherein the gamma ray detector assembly further comprises a photomultiplier tube optically coupled to the lanthanum bromide scintillation crystal.

29. The well logging tool of claim 28, wherein the gamma ray detector assembly further comprises a preamplifier connecting the photomultiplier and the digital spectrometer.

30. The well logging tool of claim 24, comprising a plurality of gamma ray detector assemblies.

31. The well logging tool of claim 30, comprising four gamma ray detector assemblies.

32. The well logging tool of claim 24, further comprising a fast neutron detector that is adapted to measure a fast neutron output flux and a pulse shape of the neutron bursts from the pulsed neutron generator.

33. The well logging tool of claim 24, further comprising thermal neutron shielding of the lanthanum bromide scintillation crystal.

34. The well logging tool of claim 24, further comprising a processor that is adapted to at least partially process data generated by the detector assembly while the tool is disposed in the borehole environment.

35. The well logging tool of claim 24, wherein the tool processes all gamma ray events down to 100 KeV.

36. A well logging tool comprising:

a pulsed neutron generator adapted to produce bursts of high energy neutrons that induce formation of gamma rays in a borehole environment;

a plurality of gamma ray detector assemblies that are axially spaced apart from the neutron generator, wherein each gamma ray detector assembly comprises a lanthanum bromide scintillation crystal that emits light pulses in response to the gamma rays and a digital spectrometer cooperating with the lanthanum bromide scintillation crystal and configured to measure the light pulses, digitally inspect the light pulses to detect pile-up pulses, and by combining the measured pulses with an average ratio of measured pulses per non-pile up pulse reject any pile-up pulses detected; and

a fast neutron detector that is adapted to measure a fast neutron output flux and a pulse shape of the neutron bursts from the pulsed neutron generator;

wherein the plurality gamma ray detector assemblies is adapted to detect one or both of inelastic scatter gamma radiation and thermal capture gamma radiation.

37. The well logging tool of claim 36, comprising four gamma ray detector assemblies.

38. A method for well logging, the method comprising:

deploying a pulsed neutron logging instrument in a borehole in an earth formation, the pulsed neutron logging instrument comprising a high energy pulse neutron generator and a gamma ray detector assembly that includes a lanthanum bromide scintillation crystal and a digital spectrometer cooperating with the lanthanum bromide scintillation crystal;

irradiating the earth formation with a plurality of bursts of high energy neutrons from the pulse neutron generator thereby inducing gamma radiation in the formation;

detecting the gamma radiation induced in the formation as light pulses emitted by the lanthanum bromide scintillation crystal;

utilizing the digital spectrometer to measure the light pulses from the lanthanum bromide scintillation crystal and digitally inspect the light pulses to detect pile-up pulses and by combining the measured pulses with an average ratio of measured pulses per non-pile up pulse reject the pile-up pulses; and

determining parameters of the earth formation from the detected gamma radiation.

39. The method of claim 38, wherein the earth formation parameters determined are selected from the group consisting of: carbon/oxygen measurements, sigma, behind casing water flow, density, porosity, gas detection, and formation lithology.

40. The method of claim 38, wherein the pulsed neutron logging instrument determines an inelastic scatter spectrum measured from gamma rays detected during the high energy neutron bursts from the pulse neutron generator.

41. The method of claim 38, wherein the pulsed neutron logging instrument determines a capture radiation spectrum measured from gamma rays detected between the high energy neutron bursts from the pulse neutron generator.

42. The method of claim 38, wherein the pulsed neutron logging instrument determines a thermal neutron cross section (“sigma”) of the borehole environment.

43. The method of claim 38, wherein the pulsed neutron logging instrument further comprises a fast neutron detector that measures a fast neutron output flux and a pulse shape of the neutron bursts from the pulsed neutron generator.

44. The method of claim 38, wherein the gamma ray detector assembly is adapted to digitally measure and inspect the pulses and reject any pile-up pulses with a dead-time of less than 600 nanoseconds per event.

45. The method of claim 38, wherein the gamma ray detector assembly is adapted to measure and resolve heights of two pulses separated by at least 150 nanoseconds.

46. The method of claim 38, further comprising:

collecting a first group of pulses in a gamma ray energy range of about 3.0 MeV to about 4.7 MeV and collecting a second group of pulses in a gamma ray energy range of about 4.7 MeV to about 6.4 MeV, forming a C/O ratio from a ratio of the first group to the second group; and determining an indicator of the amount of carbon to oxygen within earth formation from the C/O ratio.

47. The method of claim 38, wherein gamma rays down to an energy of 100 KeV are processed.