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.

What is claimed is: 
     
       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 LaBr 3  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 LaBr 3  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.