Abstract:
A method for determining a petrophysical property of a formation includes detecting gamma rays at two different spaced apart positions from a position of emitting neutrons into the formation at an energy level sufficient to induce inelastic scatting gamma rays. The neutrons are emitted in a plurality of bursts of neutrons into the formation, the bursts each having a first selected duration. Each burst is followed by a wait time having a second selected duration, the gamma rays detected during each of the bursts and each of the wait times. A ratio of numbers of gamma rays detected during the bursts is determined (burst ratio). A ratio of numbers of gamma rays detected during the wait times is determined (capture ratio). The burst ratio is used to correct the capture ratio. The petrophysical property is determined from the corrected capture ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not applicable. 
       BACKGROUND 
       [0004]    This disclosure is related to the field of neutron well logging measurements for determining petrophysical properties of subsurface formations traversed by a wellbore. More specifically, the disclosure relates to using various neutron cross section values determined from neutron measurements to determine one or more petrophysical parameters of such formations. 
         [0005]    Various neutron based measurements have been used to evaluate characteristics of subsurface formations from a wellbore since at least the 1950s. Neutrons can interact with subsurface formations in different ways. They can be scattered elastically, which means kinetic energy and momentum are conserved; they can be scattered inelastically, which means certain nuclei go into an excited state while kinetic energy is lost; they can also be captured by a nucleus to form a new nucleus; it is also possible that the neutron interaction causes a nuclear reaction resulting in the emission of one or more nucleons from the target nucleus. The probability of a neutron interacting with a nucleus is measured by the respective interaction cross section, which is a function of many parameters, such as incident neutron energy, outgoing neutron energy (if a neutron emerges from the interaction), scattering angle, interaction type and interactive nucleus type, among others. Thus, neutrons can enable measurement of many different formation properties due to the variety and complexity of their interactions. 
         [0006]    An important wellbore neutron measurement known in the art is the thermal neutron die-away measurement. This is a measure of how fast thermal neutrons disappear. If the rate of disappearance (“decay”) is approximated by an exponential function then the decay exponent (“decay constant”) can be used to directly determine the formation thermal neutron capture cross section. In the oil and gas industry the macroscopic neutron capture cross section of the formation is called “sigma”. Typically this cross section is measured in capture units (c.u.), where 1 capture unit is equal to 1000 cm′. 
         [0007]    Another important wellbore neutron measurement known in the art is the neutron porosity measurement. The basic principle of such measurement is to impart high energy neutrons (typically several MeV depending on the source type) into the formation and measure the thermal (or epithermal) neutron flux at a certain distance from the source. The detector can be either a neutron detector or a gamma ray detector (measuring neutron induced gamma rays as an indirect measurement of the neutron flux). This measurement is very sensitive to the hydrogen content in the formation because hydrogen is the most effective neutron moderator among all elements. High hydrogen content can slow down neutrons to thermal energy (0.025 eV at room temperature) before they can travel very far. Thus, HI (Hydrogen Index) and porosity (fresh water filled) may be used to interpret the measurement. A limitation of the neutron porosity measurement is that it is accurate only for water filled, clean (clay free) single lithology (such as sandstone, limestone and dolomite) formations. Some other environmental conditions need special treatment, such as gas-filled porosity, shale, and complex lithology. In addition, the thermal neutron porosity measurement is sensitive to temperature and to borehole and formation salinity. 
         [0008]    Slowing-Down Length (Ls) is a parameter that describes how far a fast neutron travels on average before it is slowed down to thermal energy. It has been used in the past to interpret the neutron porosity measurement as well. The tool response can be predicted accurately, but the limitation is that Ls does not follow a volumetric mixing law. Thus, this technique is not widely used by petrophysicists. 
       SUMMARY 
       [0009]    A method according to one aspect relates to a method for determining a petrophysical property of a formation. The method includes detecting gamma rays at two different spaced apart positions from a position of emitting neutrons into the formation at an energy level sufficient to induce inelastic scatting gamma rays. The neutrons are emitted in a plurality of bursts of neutrons into the formation, the bursts each having a first selected duration. Each burst is followed by a wait time having a second selected duration, the gamma rays detected during each of the bursts and each of the wait times. A ratio of numbers of gamma rays detected during the bursts is determined (burst ratio). A ratio of numbers of gamma rays detected during the wait times is determined (capture ratio). The burst ratio is used to correct the capture ratio. The petrophysical property is determined from the corrected capture ratio. 
         [0010]    Other aspects and advantages will be apparent from the description and claims that follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  shows an example well logging instrument conveyed through a wellbore by an electrical cable (“wireline”). 
           [0012]      FIG. 1B  shows an example logging while drilling instrument on a drill string. 
           [0013]      FIG. 1C  shows an example computer system that may be used in some implementations. 
           [0014]      FIG. 1D  shows a schematic representation of an example multiple detector neutron well logging instrument. 
           [0015]      FIG. 2  shows an example embodiment of detection timing that may be used in some embodiments. 
           [0016]      FIG. 3  shows a graph of compensated detector count rate ratio with respect to thermal neutron elastic scattering cross section for a number of different formation conditions and in the same wellbore conditions. 
           [0017]      FIG. 4  shows a graph of a difference between the logarithm of detector count rate ratio during neutron bursts and the logarithm of detector count rate ratio between bursts with respect to the thermal neutron elastic scattering cross section (in cm −1 ). 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1A  shows an example well logging instrument  30 . The measurement components of the instrument  30  may be disposed in a housing  111  shaped and sealed to be moved along the interior of a wellbore  32 . The instrument housing  111  may contain at least one energy source  115 , e.g., a neutron source such as electrically operated pulsed neutron source (hereinafter “source”), and one or more detectors  116 ,  117  each disposed at different axial spacings from the source  115 . The source  115  may emit neutron radiation. Shielding (not shown) may be disposed in the housing  111  between the source  115  and the detectors  116 ,  117  to reduce direct transmission of neutrons from the source  115  to the detectors  116 ,  117 . Thus, detected radiation may be characterized at each of a plurality of distances from the source  115 , and thus have different lateral response (depth of investigation) into the formations surrounding the wellbore  32 . In some example embodiments, two or more different types of well logging instruments, each having a different type of source and different types of corresponding detectors may be included in the same instrument assembly of “tool string.” 
         [0019]    The instrument housing  111  maybe coupled to an armored electrical cable  33  that may be extended into and retracted from the wellbore  32 . The wellbore  32  may or may not include metal pipe or casing  16  therein. The cable  33  conducts electrical power to operate the instrument  30  from a surface  31  deployed recording system  70 , and signals from the detectors  116 ,  117  may be processed by suitable circuitry  118  for transmission along the cable  33  to the recording system  70 . The recording system  70  may include a processor, computer or computer system as will be explained below with reference to  FIG. 1C  for analysis of the detected signals as well as devices for recording the signals communicated along the cable  33  from the instrument  30  with respect to depth and/or time. 
         [0020]    The well logging tool described above can also be used, for example, in logging-while-drilling (“LWD”) equipment. As shown, for example, in  FIG. 1B , a platform and derrick  210  are positioned over a wellbore  212  that may be formed in the Earth by rotary drilling. A drill string  214  may be suspended within the borehole and may include a drill bit  216  attached thereto and rotated by a rotary table  218  (energized by means not shown) which engages a kelly  220  at the upper end of the drill string  214 . The drill string  214  is typically suspended from a hook  222  attached to a traveling block (not shown). The kelly  220  may be connected to the hook  222  through a rotary swivel  224  which permits rotation of the drill string  214  relative to the hook  222 . Alternatively, the drill string  214  and drill bit  216  may be rotated from the surface by a “top drive” type of drilling rig. 
         [0021]    Drilling fluid or mud  226  is contained in a mud pit  228  adjacent to the derrick  210 . A pump  230  pumps the drilling fluid  226  into the drill string  214  via a port in the swivel  224  to flow downward (as indicated by the flow arrow  232 ) through the center of the drill string  214 . The drilling fluid exits the drill string via ports in the drill bit  216  and then circulates upward in the annular space between the outside of the drill string  214  and the wall of the wellbore  212 , as indicated by the flow arrows  234 . The drilling fluid  226  thereby lubricates the bit and carries formation cuttings to the surface of the earth. At the surface, the drilling fluid is returned to the mud pit  228  for recirculation. If desired, a directional drilling assembly (not shown) could also be employed. 
         [0022]    A bottom hole assembly (“BHA”)  236  may be mounted within the drill string  214 , preferably near the drill bit  216 . The BHA  236  may include subassemblies for making measurements, processing and storing information and for communicating with the Earth&#39;s surface. Such measurements may correspond to those made using the instrument string explained above with reference to  FIG. 1A . The bottom hole assembly is typically located within several drill collar lengths of the drill bit  216 . In the illustrated BHA  236 , a stabilizer collar section  238  is shown disposed immediately above the drill bit  216 , followed in the upward direction by a drill collar section  240 , another stabilizer collar section  242  and another drill collar section  244 . This arrangement of drill collar sections and stabilizer collar sections is illustrative only, and other arrangements of components in any implementation of the BHA  236  may be used. The need for or desirability of the stabilizer collars will depend on drilling conditions as well as on the demands of the measurement. 
         [0023]    In the arrangement shown in  FIG. 1B , the components of the well logging instrument may be located in the drill collar section  240  above the stabilizer collar  238 . Such components could, if desired, be located closer to or farther from the drill bit  216 , such as, for example, in either stabilizer collar section  238  or  242  or the drill collar section  244 . 
         [0024]    The BHA  236  may also include a telemetry subassembly (not shown) for data and control communication with the Earth&#39;s surface. Such telemetry subassembly may be of any suitable type, e.g., a mud pulse (pressure or acoustic) telemetry system, wired drill pipe, etc., which receives output signals from LWD measuring instruments in the BHA  236  (including the one or more radiation detectors) and transmits encoded signals representative of such outputs to the surface where the signals are detected, decoded in a receiver subsystem  246 , and applied to a processor  248  and/or a recorder  250 . The processor  248  may comprise, for example, a suitably programmed general or special purpose processor. A surface transmitter subsystem  252  may also be provided for establishing downward communication with the bottom hole assembly. 
         [0025]    The BHA  236  may also include conventional acquisition and processing electronics (not shown) comprising a microprocessor system (with associated memory, clock and timing circuitry, and interface circuitry) capable of timing the operation of the source and the data measuring sensors, storing data from the measuring sensors, processing the data and storing the results, and coupling any desired portion of the data to the telemetry components for transmission to the surface. The data may also be stored in the instrument and retrieved at the surface upon removal of the drill string. Power for the LWD instrumentation may be provided by battery or, as known in the art, by a turbine generator disposed in the BHA  236  and powered by the flow of drilling fluid. The LWD instrumentation may also include directional sensors (not shown separately) that make measurements of the geomagnetic orientation or geodetic orientation of the BHA  236  and the gravitational orientation of the BHA  236 , both rotationally and axially. 
         [0026]    The foregoing computations may be performed on a computer system such as one shown in the processor at  248  in  FIG. 1B , or in the surface unit  70  in  FIG. 1A . However, any computer or computers may be used to equal effect. 
         [0027]      FIG. 1C  shows an example computing system  100  in accordance with some embodiments for carrying out example methods such as those to be explained below with reference to  FIGS. 2 through 11 . The computing system  100  can be an individual computer system  101 A or an arrangement of distributed computer systems. The computer system  101 A includes one or more analysis modules  102  that are configured to perform various tasks according to some embodiments, such as the tasks described above. To perform these various tasks, an analysis module  102  executes independently, or in coordination with, one or more processors  104 , which is (or are) connected to one or more storage media  106 . The processor(s)  104  is (or are) also connected to a network interface  108  to allow the computer system  101 A to communicate over a data network  110  with one or more additional computer systems and/or computing systems, such as  101 B,  101 C, and/or  101 D (note that computer systems  101 B,  101 C and/or  101 D may or may not share the same architecture as computer system  101 A, and may be located in different physical locations, e.g. computer systems  101 A and  101 B may be on a ship underway on the ocean, in a well logging unit disposed proximate a wellbore drilling, while in communication with one or more computer systems such as  101 C and/or  101 D that are located in one or more data centers on shore, other ships, and/or located in varying countries on different continents). Any one or more of the computer systems may be disposed in the well logging instrument (whether wireline as in  FIG. 1A  or LWD as in  FIG. 1B ). 
         [0028]    A processor can include a microprocessor, microcontroller, processor module or subsystem, programmable integrated circuit, programmable gate array, digital signal processor (DSP), or another control or computing device. 
         [0029]    The storage media  106  can be implemented as one or more non-transitory computer-readable or machine-readable storage media. Note that while in the embodiment of  FIG. 1C  storage media  106  is depicted as within computer system  101 A, in some embodiments, storage media  106  may be distributed within and/or across multiple internal and/or external enclosures of computing system  101 A and/or additional computing systems. Storage media  106  may include one or more different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; optical media such as compact disks (CDs) or digital video disks (DVDs); or other types of storage devices. Note that the instructions discussed above can be provided on one computer-readable or machine-readable storage medium, or alternatively, can be provided on multiple computer-readable or machine-readable storage media distributed in a large system having possibly plural nodes. Such computer-readable or machine-readable storage medium or media is (are) considered to be part of an article (or article of manufacture). An article or article of manufacture can refer to any manufactured single component or multiple components. The storage medium or media can be located either in the machine running the machine-readable instructions, or located at a remote site from which machine-readable instructions can be downloaded over a network for execution. 
         [0030]    It should be appreciated that computing system  100  is only one example of a computing system, and that computing system  100  may have more or fewer components than shown, may combine additional components not depicted in the embodiment of  FIG. 1C , and/or computing system  100  may have a different configuration or arrangement of the components depicted in  FIG. 1C . The various components shown in  FIG. 1C  may be implemented in hardware, software, or a combination of both hardware and software, including one or more signal processing and/or application specific integrated circuits. 
         [0031]    Further, the steps in the methods described above may be implemented by running one or more functional modules in information processing apparatus such as general purpose processors or application specific chips, such as ASICs, FPGAs, PLDs, SOCs, or other appropriate devices. These modules, combinations of these modules, and/or their combination with general hardware are all included within the scope of protection of the invention. 
         [0032]      FIG. 1D  shows a schematic cross section of an example neutron well logging instrument structure. Three detectors  116 ,  117 ,  118  at various longitudinal spacings from a neutron source  115 , e.g., a pulsed neutron source as explained with reference to  FIG. 1A , are shown. The detectors  116 ,  117 ,  118  may each be a gamma ray detector such as a scintillation crystal optically coupled to a photomultiplier. Shielding  119  between the source  115  and the detectors  116 ,  117 ,  118 , reduces or prevents the direct passage of radiation from the neutron source  115  to the detectors  116 ,  117  and  118 . The source  115  (pulsed neutron generator) may be based on the deuterium-tritium reaction (with source energy of 14.1 MeV, the deuterium-deuterium reaction (with a source energy of about 2.45 MeV) or any other suitable reaction wherein the energy of the emitted neutrons exceeds 1 MeV. Methods according to the present disclosure may be applied to any form of neutron logging tools with the above described detector and/or neutron source options. Although the example instrument shown in  FIG. 1D  has three longitudinally spaced apart detectors  116 ,  117 ,  118 , for purposes of methods according to the present disclosure only two such detectors may be used. 
         [0033]    The neutron source  115 , when it is on and generating neutrons, will emit high energy neutrons (typically 14 MeV) monochromatically; the energy of the neutrons generated will depend on the particular nuclear reaction use in the pulsed neutron generator. The detectors  116 ,  117 ,  118  will detect gamma rays arriving at each detector with respect to time. The gamma rays are generated as a result of interaction of the emitted neutrons with materials in the wellbore and in the surrounding formations. There are two important mechanisms through which a neutron-induced gamma rays can be generated. One important mechanism is neutron inelastic scattering, which can be triggered only by “fast” neutrons (neutrons having energy above approximately 1 MeV; the exact energy threshold depends on the type of nucleus). The other important mechanism for generating neutron induced gamma rays is neutron capture by susceptible nuclei, which can be triggered primarily by thermal neutrons (with energy below around 0.4 eV) or epi-thermal neutrons (with energy from 1 to 100 eV). When the source  115  is on, the gamma rays arriving at the detectors can result from both mechanisms because the source keeps emitting fast neutrons which can inelastically collide with nuclei in the wellbore and surrounding formations as well as slow down to epi-thermal or thermal energy almost instantly (“instantly” in the present context meaning relative to the acquisition system timing). When the source is off, the gamma rays arriving at the detectors can only result from epi-thermal or thermal neutron capture because of the essentially instantaneous reduction of neutron energy by nuclear collision. Thus, the measured gamma ray counting rate at each of the detectors during the source off time is an indirect measurement of the numbers of epi-thermal and thermal neutrons. Such measurements can be used to provide a measurement related to formation hydrogen index (HI). In the present example embodiment, numbers of radiation events detected by the detectors  116 ,  117 ,  118  may be normalized for variations in the neutron output of the source  115  by using measurements of neutrons emitted from the source  115 , e.g., using a neutron detector  115 A disposed inside the instrument  30  proximate the source  115 . 
         [0034]      FIG. 2  just shows one example of a pulsed neutron source operating cycle that may be used in some embodiments. The graph in  FIG. 2  shows gamma ray counting rate at one of the detectors with respect to time. The neutron source ( 115  in  FIG. 1D ) may be operated for a plurality of controlled duration “bursts”, shown at  40 , each followed by a selected duration wait time  46 . In the present embodiment, each burst may have a duration, shown at  42 , of 20 μs, each wait time, shown at  44 , may have a duration of 30 μs, and after a selected number of bursts, the source ( 115  in  FIG. 1D ) may be switched off for a selected duration (“long wait time”  48 ) to enable radiation events caused by the bursts to decay to background level. In the present example embodiment, the long wait time  48  may be 1080 μs. The number of bursts in one cycle in the present embodiment may be 23, although the exact number of bursts in other embodiments may be different. 
         [0035]    A burst count rate ratio (BRAT) may be calculated using all the numbers of gamma rays detected during all the source “on” times (all 23 bursts in the present embodiment) at one of the detectors (e.g.,  117  in  FIG. 1D ) with respect to the numbers of gamma rays detected during the source on times at a closer spaced detector (e.g.,  116  in  FIG. 1D ); such detection time intervals may be referred to as “burst gates”. A capture ratio (TRAT) may be calculated from measurements of gamma rays made by the same two detectors using one or more “capture” gamma ray detection timing gates. Below are two examples of capture ratio calculated using gamma rays detected during two different timing gates. 
         [0036]    Referring to  FIG. 3 , TRAT may be calculated using gamma rays detected during substantially all the capture gamma ray detection times, including all the short wait time intervals ( 46  in  FIG. 2 ) and some or all of the long wait time interval ( 48  in  FIG. 2 ). A TRAT computed using the foregoing detected gamma rays has substantially no wellbore fluid salinity effect. Other wellbore effects (casing, cement and the type of wellbore fluid, e.g., liquid or gas) can be compensated by a simple equation: 
         [0000]        x =log(TRAT)− a ·log(BRAT)  (1)
 
         [0037]    Where x is the compensated logarithm of the detector count rate ratio, TRAT is the capture count rate ratio and BRAT is the burst count rate ratio. Coefficient “a” is a constant which can be determined experimentally. In the present example, a has been determined to be unity. 
         [0038]      FIG. 3  shows a graph of the compensated ratio x with respect to thermal neutron elastic scattering cross section, for a number of different formation conditions and in the same wellbore conditions. For quartz, calcite and dolomite formations, the fluids filling the pore spaces of each such formation are fresh water, salty water (32.5 ppk, 130 ppk, 260 ppk NaCl concentation; ppk is parts per thousand concentration), methane gas of different densities (0.1 g/cm 3 , 0.2 g/cm 3 , and 0.3 g/cm′ density) and oil (of densities 0.4 g/cm 3 , 0.5 g/cm 3 , 0.6 g/cm 3  and 0.7 g/cm 3 ). Shaly-sand for purposes of evaluation is equivalent to 50% quartz mixed with different types of clay minerals. 
         [0039]    A compensated ratio x calculated as explained above displays some correlation with thermal neutron capture cross section of the formation, however there are still other properties which can affect the compensated ratio x. For example, quartz and carbonate have identifiably different ratio responses; formation pore water salinity has an effect on the ratio response, 0 pu (1 pu=1% porosity) anhydrite is an outlier, and shaly-sand and quartz (clean sand) behave differently. 
         [0040]      FIG. 4  shows a graph of log(TRAT)−log(BRAT) using only gamma rays detected during the short wait times ( 46  in  FIG. 2 ) and excludes gamma rays detected during the long wait time ( 48  in  FIG. 2 ). The TRAT will still have essentially no wellbore fluid salinity effect. Other wellbore effects (casing, cement and borehole fluid/gas) may be compensated using BRAT from the same detection times and using the same equation as explained above.  FIG. 4  shows the compensated ratio using only gamma rays detected during the short wait times ( 46  in  FIG. 2 ) may be accurately described substantially only by the thermal neutron capture cross section. 
         [0041]    This example demonstrates the possibility that one can measure the formation thermal neutron elastic scattering cross section quite accurately in cased hole, free of wellbore effect, for many different formation conditions. To obtain such results, the burst timing, the timing gate, and the compensation may be optimized using the burst count rate ratio (BRAT). By adjusting the burst timing ( 42  and  44  in  FIG. 2 ) and timing gates ( 46  in  FIG. 2 ) for the capture ratio (TRAT), one can eliminate wellbore salinity effect and formation salinity dependency from the determined capture ratio (TRAT). By compensating TRAT using the burst ratio (BRAT), one can compensate not only for casing/cement effects and the wellbore fluid/gas effect, but also some secondary formation responses that are related to fast neutron transport. 
         [0042]    Once a good thermal neutron elastic scattering cross section measurement is obtained using TRAT measured only during the short wait times, compensated by BRAT, such measurement may be converted to hydrogen index and/or porosity φ by assuming a fluid composition in the formation pore space and a mineral composition of the formations. See Eqs. (2) and (3) below, wherein Σtotal represents the determined thermal neutron elastic scattering cross section of the formation, Σ matrix  represents the thermal neutron elastic scattering cross section of the formation minerals and Σ fluid  represents the thermal neutron elastic scattering cross-section of the formation pore fluid 
         [0000]    
       
         
           
             
               
                 
                   
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         [0043]    For neutron porosity made using other known types of neutron porosity instruments, e.g., one having an  241 AmBe chemical isotope source using two, spaced apart  3 He proportional neutron detectors, it is usually assumed that the formation fluid is fresh water, and the rock mineral composition may be sandstone (quartz), limestone (calcium carbonate) or dolomite (calcium magnesium carbonate) or combinations thereof. The neutron porosity computed using compensated TRAT determined as explained above matches the neutron-neutron porosity measured using other known neutron porosity instruments well, especially in shaly formation conditions. 
         [0044]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.