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:
BACKGROUND 
     Borehole well logging systems that emit bursts of high energy (of the order of 14 million electron volts (MeV)) neutrons are routinely used in geophysical exploration, recovery and monitoring operations. These systems are typically used in cased boreholes, although some uncased or “open hole” applications are known in the art. As examples, pulsed neutron logging systems are used to measure formation density in cased boreholes, to determine formation lithology, to detect gas within formation pore space, and to identify and to optionally measure the flow of water behind casing. 
     The earliest commercial pulsed neutron logging system was to delineate saline formation liquid from non-saline liquid, which was assumed to be liquid hydrocarbon. Chlorine in saline water has a relatively large thermal neutron absorption cross section, while carbon and hydrogen in hydrocarbons have relatively small thermal neutron cross sections. The decay rate of thermal neutrons is measured between bursts of fast neutrons by measuring capture gamma radiation as a function of time. This decay rate is, therefore, indicative of the thermal neutron capture cross section of the borehole environs. This quantity is commonly referred to as “sigma”. Based upon the large difference thermal neutron absorption cross section of saline water and liquid hydrocarbon, sigma combined with other measurements such as formation porosity is used to obtain a hydrocarbon saturation value for the formation. Again, this saturation value is based upon the assumption that any non-saline pore fluid is hydrocarbon. 
     All formation waters are not saline. A measure of sigma cannot, therefore, be used to delineate unequivocally between fresh formation water and liquid hydrocarbon. The “carbon/oxygen” or “C/O” logging system was developed to delineate between fresh water and hydrocarbon. The methodology of the C/O logging system is based upon a measure of a ratio of carbon to oxygen content of the borehole environs. This ratio can be used to delineate between fresh formation water and liquid hydrocarbon, because hydrocarbon contains carbon but no oxygen, and fresh water contains oxygen but no carbon. The system, like its “sigma” logging system counterpart, uses a pulsed source of 14 MeV neutrons. The system uses a measure of inelastic scatter gamma radiation (rather than thermal capture gamma radiation) to obtain desired results. Inelastic scatter cross sections are sufficiently large, and the emitted inelastic scatter radiation is sufficiently different in energy to permit the measure of an inelastic gamma radiation ratio indicative of the C/O ratio of the borehole environs. Inelastic scatter reactions are many orders of magnitude faster than the thermal capture process used in sigma logging. As a result, the inelastic scatter radiation measurement must be made during the neutron burst. This results in a very intense “instantaneous” gamma radiation field at the detector assembly. The received radiation is amplified as pulses of collected light and the height of the pulse is related to incident gamma-ray energy. Accurate measurement of the pulse height is corrupted by pulse pile-up (i.e. where one pulse is superimposed on another) resulting from the intense instantaneous radiation. The rejection of pile-up events yields a very low “observed” inelastic count rate from which the C/O information is derived. Stated another way, the observed statistical precision of C/O logging is typically poor even though the “instantaneous” inelastic scatter radiation flux during the burst is quite large. It is of the utmost importance, therefor, to use a fast gamma ray detection system and to minimize pulse pile-up during a measure interval to maximize the statistical precision and the accuracy of measured radiation attributable to carbon and to oxygen inelastic scattering. 
     SUMMARY OF THE INVENTION 
     The invention is directed to 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 that exhibits outstanding peak resolution and emission time compared to other types of scintillation crystals. For carbon/oxygen logging, another advantage of LaBr3 and other higher density halides such as LaI (U.S. Pat. No. 7,084,403) and LuI (U.S. Pat. No. 7,329,874), is that the lack of oxygen in the crystal matrix offers an incremental boost in C/O sensitivity per unit volume, compared to fast, higher density detectors such as LYSO, GSO, LUAP that contain oxygen. 
     In intense gamma radiation fields, speed of detector pulse processing and pulse throughput are important considering the discussion in the BACKGROUND section of this disclosure. To maximize processing throughput and to maintain high resolution requires fast measurement and pile-up inspection of the pulses. A digital spectrometer has been designed based on digital filtering and digital pulse pile-up inspection that offers speed and energy resolution improvements over tradition analog measurement and inspection techniques. Concepts of this digital spectrometer are disclosed in U.S. Pat. No. 6,590,957 B1, which is here in entered into this disclosure by reference. The topology of the system in the referenced disclosure has been redesigned and configured for high-temperature operations with a LaBr3 detector crystal for use in well logging. In practice, the detector assembly comprising the digital spectrometer and the LaBr3 crystal is limited by filtering of electronics noise and digital sampling rates. The detector assembly is capable of pulse measurement and digital pile-up inspection with dead-time less than 600 nanoseconds per detection event (nS/event). Pulse height (thus energy resolution) can be accurately measured (corrected for pile-up effects) for 2 pulses separated by as little as 150 nanoseconds (nS). This detector assembly performance is facilitated by the combination of the LaBr3 detector crystal and the digital filtering and digital pile-up inspection spectrometer of the referenced disclosure. 
     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 C/O logging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The manner in which the above recited features and advantages, briefly summarized above, are obtained can be understood in detail by reference to the embodiments illustrated in the appended drawings. 
         FIG. 1  illustrates a multipurpose pulsed neutron logging instrument comprising four gamma ray detector assembly and disposed within a borehole penetrating an earth formation; and 
         FIG. 2  illustrates major elements of a gamma ray detector assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be disclosed as a multipurpose well logging system that is adaptable to measure previously mentioned geophysical logging applications. It should be understood that the invention could be equally embodied as a “stand-alone” system designed to measure a single parameter of interest such as a carbon/oxygen ratio. 
     The multipurpose pulsed-neutron system must be versatile enough to cover many different cased-hole applications including reservoir evaluation using sigma measurements, reservoir evaluation using carbon/oxygen (C/O) measurements, and behind casing water flow. The system must further providing 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 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 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 LaBr 3  Detector Crystal 
     In 2006 the LaBr3 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 LaBr 3  and other scintillators used in 
               
               
                 well logging (*designates 3 inch (diameter) by 3 inch (length) crystals. 
               
               
                 Resolutions are for  137 Cs gamma radiation at 0.662 MeV) 
               
             
          
           
               
                 CRYSTAL PROPERTY 
                 NaI 
                 BGO 
                 GSO 
                 LaBr 3   
               
               
                   
               
             
          
           
               
                 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 &amp; 
                 16 
               
               
                   
                   
                   
                 600 
               
               
                   
               
             
          
         
       
     
     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: 
     
       
         
           
             
               
                 
                   
                     σ 
                     
                       C 
                       / 
                       O 
                     
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             C 
                             O 
                           
                           ) 
                         
                         2 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             1 
                             C 
                           
                           + 
                           
                             1 
                             O 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     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.