Abstract:
A method includes emitting a burst of neutrons having a first duration into earth formations. Neutrons are detected at a first position spaced apart from the emitting in two time intervals following the burst. After a selected delay time, a second duration neutron burst is emitted into the formations. Gamma rays are detected in selected time intervals following the second burst. The detected neutrons in the two time intervals are used to calculate a thermal neutron capture cross section. Gamma rays detected at the first position in following the second duration burst are used to determine an apparent formation thermal neutron capture cross section and to adjust a time interval for each of the first duration, the second duration and the starting time thereof for detecting gamma rays. The estimated wellbore thermal neutron capture cross section is used to determine an apparent formation thermal neutron capture cross section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    This disclosure is related to the field of pulsed neutron well logging instruments. More specifically, the disclosure relates to pulsed neutron well logging instruments having neutron burst and measurement timing controlled by measurements made by a detector in the instrument. 
         [0004]    Pulsed neutron well logging instruments known in the art include instruments that have gamma ray detectors operated to detect gamma rays emitted as a result of thermal neutron capture (“capture gamma rays”) by selected elemental nuclei in subsurface formations having high neutron capture cross section. The most common of such chemical elements is chlorine, and measurements from such instruments are commonly used as a proxy for brine content in the formations. Brine content may be related to the fractional volume of pore space (porosity) in the formation and the fractional volume of the pore space that is occupied by brine (water saturation). 
         [0005]    Measurements made by such instruments may be first used to calculate a parameter referred to as the thermal neutron decay time (tau). The value of tau calculated may then be converted to a value of the thermal neutron capture cross section (sigma) of the formation by the expression: 
         [0000]      Σ=4550/τ
 
         [0006]    SPE paper no. 2252, Sep. 19, 1968, revised manuscript received Oct. 8, 1970 published by SPE International, Richardson, Tex. explains in detail the advantages of the “Complete Scale-Factor Method”, and explains many of the technical aspects of thermal neutron decay time well log measurements. 
         [0007]    An “automatic tau loop” data acquisition technique, where the neutron burst duration and the detector acquisition timing gates (both starting time and duration) are all adjusted according to the thermal decay time of the formation, is fully described in U.S. Pat. No. 3,662,179 issued to Frentrop et al., and is defined as “the Complete Scale Factor Method.” 
         [0008]    The “Dual Burst” data acquisition technique is fully described in U.S. Pat. No. 4,721,853 issued to Wraight. The dual burst data acquisition technique, as described in the foregoing patent, is used in a fixed timing instrument where the short burst duration is always about 20 microseconds and the long burst is about 150 microseconds. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows an example pulsed neutron well logging instrument. 
           [0010]      FIG. 2  shows a schematic diagram of example electronics and how pulsed neutrons are generated and the decay rate measured. 
           [0011]      FIG. 3  shows a schematic diagram of an example electronic circuit part of an instrument cartridge. 
           [0012]      FIG. 4  shows an example of the overall system with a surface deployed recording and control unit. 
           [0013]      FIG. 5  shows an example of a measurement made by the instrument of  FIG. 1 . 
           [0014]      FIG. 6  shows an example of neutron burst timing and gamma ray detection timing gates for a near spaced gamma ray detector. 
           [0015]      FIG. 7  shows an example of neutron burst timing and gamma ray detection timing gates for a far spaced gamma ray detector. 
           [0016]      FIG. 8  shows a graph of a porosity indicator ratio with respect to porosity in limestone. 
           [0017]      FIG. 9  shows a graph of the square of the ratio shown in  FIG. 8 . 
           [0018]      FIG. 10  shows a graph of the porosity indicator with an endpoint calibrated to a dense shale (80 p.u.). 
           [0019]      FIG. 11  shows a graph of the porosity indicator with respect to porosity in three different lithologies. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows an example embodiment of a pulsed neutron well logging instrument according to the present disclosure. The example instrument  10  may be made in three parts. The bottom part  16  may be a simple mechanical cross-over which is may be used to connect the instrument  10  to other well logging instruments disposed below the example pulsed neutron instrument  10 . The two main parts are a cartridge  12  and a sonde  14 . The cartridge  12  be disposed in a pressure resistant housing  112  configured to traverse a wellbore and may include therein circuitry  12 B for communication with the surface, a power supply  12 A, e.g., batteries for powering the sonde  14 , circuits for controlling the safety of the sonde  14 , especially control over operation of a pulsed neutron generator  14 F and circuits (on board  12 B) for processing data and counts from gamma ray detectors  14 C,  14 D in the sonde  14 . The batteries  12 A powering the system may be disposed inside the cartridge  12  as shown in  FIG. 1 . The cartridge  12  also may contain a pressure-operated switch  12 C between the battery  12 A and the electronic circuit board  12 C. The switch  12 C may be closed once a selected pressure (&gt;100 psi) is applied to the instrument  10 . Thus, at surface, the battery  12 A is disconnected and it is not possible to generate neutrons. 
         [0021]    The sonde  14  may be disposed similarly in a pressure resistant housing  114  configured to traverse a wellbore and to couple to the cartridge housing  112 . The sonde housing  114  may contain therein the pulsed neutron generator  14 F (PNG), the high voltage power supply  14 E for the PNG  14 F, the gamma-ray detectors  14 C,  14 D and some electronic circuits  14 G configured for driving the PNG  14 F. The sonde  14  will be described in more detail with respect to  FIG. 3 . The PNG  14 F may be any type known in the art, and in the present example may be a deuterium-tritium reaction accelerator type PNG that emits neutrons having initial energy of about 14 million electron volts (MeV). The gamma ray detectors  14 C,  14 D may be scintillation counters having a scintillation crystal of any known composition and a photomultiplier tube of any known configuration for use in wellbore instruments, although the type of gamma ray detector is not a limitation on the scope of the present disclosure. 
         [0022]    The cartridge  12  may include any form of electrical/mechanical connector  8 A at its upper end for coupling the cartridge  12  to a cable head or another instrument above the instrument  10  between the cable head (not shown) and the instrument. The sonde  14  may include an electrical/mechanical connector  8 B at its lower end for coupling to another well logging instrument, or such connector  8 B may be a termination or “bull plug” if no instruments are to be connected below the pulsed neutron well logging instrument  10 . 
         [0023]      FIG. 2  shows a schematic diagram of example electronics and how pulsed neutrons are generated and the decay rate measured. An operating loop of the system may include: (i) the cartridge electronics ( 12 B in  FIG. 1 ) such as a master controller (also shown as  52  an  54  in  FIG. 3 ) generates a burst signal that drives the PNG ( 14 F in  FIG. 1 ) through a driver board (e.g.,  56  in  FIG. 3 ) to generate neutrons; (ii) neutrons are generated and imparted into a formation surrounding a wellbore in which the instrument is disposed; (iii) neutrons react with the formation and gamma-rays (GR) are emitted; GR detectors ( 14 C,  14 D in  FIG. 1 ) in the sonde measure those GR; (iv) data from GR detectors are received by the circuitry inside the cartridge (e.g., the master controller); and (v) depending on the numbers of gamma rays detected at various times during a cycle, a controller in the circuitry will shorten or enlarge the duration of the burst signal. A cycle may be repeated between every 500 microseconds and 4.55 milliseconds. The signals may be processed and some portion of the signals may be communicated by telemetry ( 62  in  FIG. 3 ) to the surface. Commands and other data may be received from the surface, demodulated by the telemetry and communicated to the master controller. The present example is configured so that more than one well logging instrument may be included in a “string” of well logging instruments, and commands and other data may be communicated from the surface and data may be communicated to the surface through a common telemetry channel, but this is not a limitation on the scope of the present disclosure. 
         [0024]      FIG. 3  shows a schematic diagram of the electronic circuit part of the cartridge. A safety microcontroller  52 , which may be any form of microprocessor may be configured for communicating with the surface through a telemetry transceiver  62  and detecting and decoding command signals communicated from the surface. The safety microcontroller  52  may communicate with a field programmable gate array (FPGA)  54  to obtain gamma ray measurements (through pulse shaper  60 ) and other data from the sonde and to command the FPGA  54 . The safety microcontroller  52  also controls a switching-on of power supplies  50  (that convert power from the batteries  12 A) that is a safety aspect of the present example embodiment. 
         [0025]    The FPGA  54  controls the above described measurement loop for the sonde (obtaining detector measurement data and setting the duration of the pulsed neutron burst). The FPGA  54  may receive specific commands from the surface for safety reasons. Thus both the FPGA  54  and the safety microcontroller  52  may be configured to detect a specific sequence to start operation of the PNG ( 14 F in  FIG. 1 ). Operation of the PNG is effected by sending commands to a grid controller  56  to apply a voltage pulse to an ionizer grid (not shown separately) in the PNG ( 14 F in  FIG. 1 ). Such operation control of a PNG is well known in the art. The power supply  50  may generate 3 DC voltages 5V, 10V and 40V using power supplied by the batteries ( 12 A in  FIG. 1 ). Data received by the safety controller  52  may be stored in a memory  58  such as a solid state memory or similar mass storage device. 
         [0026]      FIG. 4  shows an example of the overall system with a surface deployed recording and control unit  21 , which may include a general purpose, programmable computer, electric slickline or armored electrical cable  20  to convey the instrument  10  and the instrument connected to the end of the slickline or cable  20 . A telemetry or other type of interface system, shown generally at  23 , may be included in some embodiments if there are further instruments connected to the cable  20  below the instrument  10 . A non-limiting example of an electric slickline is described in U.S. Pat. No. 5,495,755 issued to Moore. It should be understood that the particular conveyance used in any example is not a limitation on the scope of the present disclosure. 
         [0027]      FIG. 5  shows a measurement made by the instrument  10 , i.e., the detection of a boundary between 1 hydrocarbon bearing section  30  of a formation and a water baring section  32  of the formation. Since a pulsed neutron well logging instrument according to the present disclosure uses adaptive timing, where the neutron burst duration tracks the apparent thermal neutron decay time of the formation, the apparent formation sigma measurement may be largely insensitive to the borehole environment. The instrument may therefore be used with only a small amount of response characterization and still provide a clear measurement of the oil water contact. 
         [0028]    In  FIG. 5 , the instrument  10  is moved along the interior of a casing  22  cemented in place in the wellbore. The casing  22  may have a smaller diameter conduit called a “tubing” or “velocity string”  24  disposed therein and sealingly engaged to the casing  22  using a packer  26  or similar annular sealing device. In the present example, the instrument  10  maybe conveyed by electric slickline  20 . The casing  22  may include perforations  28  adjacent the formation from which hydrocarbons are to be extracted through the wellbore. Curve  34  provides an example of a calculated formation sigma measurement that shows an abrupt change in value at the boundary between the hydrocarbon bearing section  30  and the water bearing section  32  of the formation. 
         [0029]      FIG. 6  shows an example of neutron burst timing and gamma ray detection timing gates for the near gamma ray detector (shown as  14 D in  FIG. 1 ). Collectively, the neutron burst times and gamma ray detection times comprise a measurement cycle. The duration of the entire cycle in the present example may correspond to ten Apparent Formation Tau Downhole loop (AFTDL) times. A long neutron burst LB (i.e., an operation of the PNG) may be 1 AFTDL long and a short burst SB may be 0.1 AFTDL long. Near detector counts may be acquired in nine time windows (N 0  through N 8 ) as shown. The first time window N 0  is for gamma ray detections occurring during the short burst SB. Since the capture background is at a minimum at this time, the N 0  count rate has a high proportion of inelastic gamma rays. The N 0  count rate may be combined with the far detector count rate occurring during the same time window (F 0  in  FIG. 7 ) to enable calculation of an inelastic count rate ratio, which can be used as an Apparent Gas Indicator or “AGI.” 
         [0030]    Counts detected in gates N 1  and N 2  are acquired starting after a delay of 0.1 AFTDL delay following the end of the short burst SB and are used to determine an Apparent Borehole Tau (ABT). A long duration neutron burst LB may begin at a time of 1 AFTDL after the beginning of the short bursts. The duration of the long burst LB may be equal to 1 AFTDL. Counting gate N 3  may start after a time delay of 1 AFTDL following the end of the long neutron burst LB. Counting gate N 3  may have a duration equal to the duration of the long burst LB and may be followed by contiguous detection timing gates N 4 , N 5 , N 6  each having a duration equal to the duration of the long burst LB. There may then be time delay of 1 AFTDL, after which contiguous counting gates N 7  and N 8  may occur. At the time at which gate N 7  begins, the thermal neutron capture count rate may have decreased to essentially zero and during gates N 7  and N 8  a long term activation count rate that builds in the near detector may be measured. The gamma ray detection measurements made in gates N 7  and N 8  may be referred to as the “background” radiation measurement. 
         [0031]      FIG. 7  shows corresponding neutron burst (SB, LB) and the acquisition timing gates (F 0 , F 3  through F 8 ) for the far gamma ray detector (shown as  16 C in  FIG. 1 ). F 0  counts gamma rays detected during the short burst (SB) and has a high proportion of inelastic counts, as explained with reference to N 0  in  FIG. 6 . Since the far detector measurement may not be used in the calculation of borehole tau, the timing gates F 1  and F 2  may be omitted from the measurement sequence Counting gate F 3  starts after a delay time of 1 AFTDL following the end of the long neutron burst LB. F 3  may be followed by contiguous counting gates F 4 , F 5 , F 6 , each having a duration of 1 AFTDL. There may then be a delay of 1 AFTDL before contiguous counting gates F 7  and F 8  occur. As is the case for gates N 7  and N 8  explained with reference to  FIG. 6 , by the time N 7  starts, the thermal neutron capture count rate has decreased to essentially zero and the F 7  and F 8  counting gates are a measurement of the long term activation count rate that builds in the far detector. The duration of the time gates F 3 , F 4 , F 5 , F 6 , F 7 , F 8  may each be 1 AFTDL as explained above. 
         [0032]    The downhole tau loop, which regulates the overall neutron burst timing and detection counting gate timing may be controlled by counting rate data from the near detector, in a manner very similar to that described in U.S. Pat. No. 3,662,179. However, because the background is only collected for 2 AFTDL times rather than 3 AFTDL times, as described in the foregoing patent, the count rate equation that needs to be balanced becomes: 
         [0000]      4*( N 5+ N 6)−2* N 4−3*( N 7+ N 8)=0
 
         [0033]    The controller adjusts the duration of the neutron burst timing until the above equation condition is met. The overall measurement cycle lasts 10 AFTDL times, the long neutron burst LB may be 1 AFTDL and the measurement counting gates are either 0.1 AFTDL or 1 AFTDL in duration, as explained above. 
         [0034]    The downhole tau loop, which in the present disclosure may be called “Adaptive Timing” provides an Apparent Formation Tau (AFTDL) which is quite accurate and precise, but an improved result may be obtained by calculating an “Apparent Formation Tau Calculated” (AFTC) from the near detector timing gates (N 3 +N 4 ), (N 5 +N 6 ) and (N 7 +N 8 ). By using the counts from gate N 3  the statistical precision of the measurement may be improved and the rate at which AFTC may change is not limited by the downhole tau loop regulation time. Even if the downhole loop (or Adaptive Timing) is not exactly locked in to the changing Apparent Formation Tau, the AFTC will be correct. 
         [0035]    The following operations may be performed on the counts in specific counting gates in order to determine AFTC. 
         [0036]    The Adaptive Timing loop operation may be programmed into the controller and may balance the equation 
         [0000]      4*( N 5+ N 6)−2* N 4−3*( N 7+ N 8)=0
 
         [0037]    In one example counts in the foregoing gates transmitted to the surface may be averaged over a 1 second sample period. So the following count rates may be transmitted to the surface: 
         [0038]    N 0 , N 1 , N 2 , N 3 , N 4 , (N 5 +N 6 ), (N 7 +N 8 ) 
         [0039]    F 0 , F 3 , F 4 , (F 5 +F 6 ), (F 7 +F 8 ) 
         [0040]    (N 5 +N 6 ), (N 7 +N 8 ), (F 5 +F 6 ) and (F 7 +F 8 ) may be transmitted to the surface using the telemetry as sums because the individual count rates in the foregoing individual gates are not needed and by combining them saves bandwidth in the telemetry. 
         [0041]    First, all the count rates may be expressed as instantaneous count rates before the dead time correction, i.e., 
         [0000]        N 0′=100* N 0
 
         [0000]        N 1′=100* N 1
 
         [0000]        N 2′=100* N 2
 
         [0000]        N 3′=10* N 3
 
         [0000]        N 4′=10* N 4
 
         [0000]      ( N 5+ N 6)′=5*( N 5+ N 6)
 
         [0000]      ( N 7+ N 8)′=5*( N 7+ N 8)
 
         [0000]        F 0′≦100* F 0
 
         [0000]        F 3′=10* F 3
 
         [0000]        F 4′=10* F 4
 
         [0000]      ( F 5+ F 6)′=5*( F 5+ F 6)
 
         [0000]      ( F 7+ F 8)′=5*( F 7+ F 8)
 
         [0042]    Next, the counts in each of the time windows may be corrected for detector dead time: 
         [0000]        N 0″= N 0′(1− N 0′* K )
 
         [0000]    where in the present example, K is the dead time per pulse and in the present example K=0.000001. Other methods for correcting detector counts for dead time are known in the art. 
         [0000]        N 1″= N 1′/(1− N 1′* K )
 
         [0000]        N 2″= N 2′/(1− N 2′* K )
 
         [0000]        N 3″= N 3′/(1− N 3′* K )
 
         [0000]        N 4″= N 4′/(1− N 4′* K )
 
         [0000]      ( N 5+ N 6)″=( N 5+ N 6)′/(1−( N 5+ N 6)′* K )
 
         [0000]      ( N 7+ N 8)″=( N 7+ N 8)′/(1−( N 7+ N 8)′* K )
 
         [0000]        F 0″= F 0′/(1− F 0′* K )
 
         [0000]        F 3″= F 3′/(1− F 3′* K )
 
         [0000]        F 4″= F 4′/(1− F 4′* K )
 
         [0000]      ( F 5+ F 6)″=( F 5+ F 6)′/(1−( F 5+ F 6)′* K )
 
         [0000]      ( F 7+ F 8)″=( F 7+ F 8)′/(1−( F 7+ F 8)′* K )
 
         [0043]    The result is a set of instantaneous, dead time corrected count rates. 
         [0044]    One may then perform a background subtraction on all the count rates in gates other than N 7 , N 8 , F 7 , F 8 . The background may be averaged over a selected time interval, in the present example at least 21 seconds to smooth the background count rate before subtraction. First the average background may be calculated from the counting rates in gates N 7  and N 8 , and F 7  and F 8  to subtract from all the windows: 
         [0000]        BKG   —   N =(Σ i−n   i+n ( N 7+ N 8)″)/(2 n+ 1)
 
         [0000]        BKG   —   F =(Σ i−n   i+n ( F 7+ F 8)″)/(2 n+ 1)
 
         [0000]    wherein n represents the number of acquisition cycles. If n=10 then the background will be averaged over 10 acquisition intervals of 1 second before the i-th level and 10 intervals after, i.e., it is a balanced 21 second filter. Now the background subtraction may be performed for all the counting gates other than the background gates (N 7 , N 8 , F 7 , F 8 ). 
         [0000]        N 0′″( i )= N 0″( i )−(( BKG   —   N )/2)
 
         [0000]        N 1′″( i )= N 1″( i )−(( BKG   —   N )/2)
 
         [0000]        N 2′″( i )= N 2″( i )−(( BKG   —   N )/2)
 
         [0000]        N 3′″( i )= N 3″( i )−(( BKG   —   N )/2)
 
         [0000]        N 4′″( i )= N 4″( i )−(( BKG   —   N )/2)
 
         [0045]    The reason why the background count rate is divided by 2 for the foregoing measurement gates is because the BKG_N is calculated over two gate times (N 7 +N 8 ). The background counts during one gate time interval is thus half of that. 
         [0000]      ( N 5+ N 6)′″( i )=( N 5+ N 6)″( i )− BKG   —   N  
 
         [0046]    No division by two is needed for the foregoing gate measurement because there are two timing gates in the represented value. Similarly for the fare detector ( 14 C in  FIG. 1 ): 
         [0000]        F 0′″( i )= F 0″( i )−(( BKG   —   F )/2)
 
         [0000]        F 3′″( i )= F 3″( i )−(( BKG   —   F )/2)
 
         [0000]        F 4′″( i )= F 4″( i )−(( BKG   —   F )/2)
 
         [0000]      ( F 5+ F 6)′″( i )=( F 5+ F 6)″( i )− BKG   —   F  
 
         [0047]    It is then possible to calculate the outputs at each i-th level. First one may generate an output corresponding to the Apparent Formation Sigma derived from the Downhole tau Loop (AFSDL), and this is 4550/AFTDL. For example, if the tau time of the i th measurement is 180 microseconds then AFSDL=4550/180 which equals 25.28 capture units. This output may be used as a quality control indicator. 
         [0048]    Next, determine the Apparent Formation Tau Calculated from the transmitted, dead time corrected count rates: 
         [0000]    
       
         
           
             
               AFTC 
                
               
                 ( 
                 i 
                 ) 
               
             
             = 
             
               
                 2 
                 * 
                 
                   AFTDL 
                    
                   
                     ( 
                     i 
                     ) 
                   
                 
               
               
                 
                   ln 
                    
                   
                     ( 
                     
                       
                         N 
                          
                         
                             
                         
                          
                         
                           3 
                           ′′′ 
                         
                          
                         
                           ( 
                           i 
                           ) 
                         
                       
                       + 
                       
                         N 
                          
                         
                             
                         
                          
                         
                           4 
                           ′′′ 
                         
                          
                         
                           ( 
                           i 
                           ) 
                         
                       
                     
                     ) 
                   
                 
                 - 
                 
                   ln 
                    
                   
                     ( 
                     
                       
                         
                           ( 
                           
                             
                               N 
                                
                               
                                   
                               
                                
                               5 
                             
                             + 
                             
                               N 
                                
                               
                                   
                               
                                
                               6 
                             
                           
                           ) 
                         
                         ′′′ 
                       
                        
                       
                         ( 
                         i 
                         ) 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0049]    Next determine an Apparent Formation Sigma Calculated (AFSC)(i), 4550/AFTC(i). The output of AFSC may be averaged over 5 seconds for presentation on a well log: 
         [0000]      AFSC(log output)=(Σ i−n   i+n AFSC( i ))/(2 n+ 1), where  n= 2
 
         [0050]    At this time one may also average over 21 levels the count rates N 1 ′″ and N 2 ′″. These averaged count rates may be used to calculate an Apparent Borehole Tau Calculated (ABTC) and they need to be averaged before taking logarithms. Also average AFTC over the same 21 levels. 
         [0000]        N 1′″(averaged)=(Σ i−n   i+n   N 1′″)/(2 n+ 1), where  n= 10
 
         [0000]        N 2′″(averaged)=(Σ i−n   i+n   N 2′″)/(2 n+ 1, where  n= 10
 
         [0000]      AFTC(averaged)=(Σ i−n   i+n AFTC)/(2 n+ 1), where  n= 10
 
         [0051]    Next calculate the averaged ABTC from the following equation: 
         [0000]    
       
         
           
             
               ABTC 
                
               
                 ( 
                 average 
                 ) 
               
             
             = 
             
               
                 ( 
                 
                   
                     AFTC 
                      
                     
                       ( 
                       average 
                       ) 
                     
                   
                   / 
                   10 
                 
                 ) 
               
               
                 
                   ln 
                    
                   
                       
                   
                    
                   N 
                    
                   
                       
                   
                    
                   
                     1 
                     ′′′ 
                   
                    
                   
                     ( 
                     average 
                     ) 
                   
                 
                 - 
                 
                   ln 
                    
                   
                       
                   
                    
                   N 
                    
                   
                       
                   
                    
                   
                     2 
                     ′′′ 
                   
                    
                   
                     ( 
                     averaged 
                     ) 
                   
                 
               
             
           
         
       
     
         [0052]    Next calculate an apparent borehole sigma value: 
         [0000]    
       
         
           
             
               Apparent 
                
               
                   
               
                
               Borehole 
                
               
                   
               
                
               Sigma 
                
               
                   
               
                
               Calculated 
                
               
                   
               
                
               
                 ( 
                 ABSC 
                 ) 
               
                
               
                 ( 
                 averaged 
                 ) 
               
             
             = 
             
               4550 
               / 
               
                 ABTC 
                  
                 
                   ( 
                   averaged 
                   ) 
                 
               
             
           
         
       
     
         [0053]    The output to be displayed on a well log for the foregoing parameter will be ABSC(averaged). Next one may calculate a porosity indicator ratio: 
         [0000]      RatPor( i )=( N 3′″( i )+ N 4′″( i )+( N 5+ N 6)′″)( i )/( F 3′″( i )+ F 4′″( i )+( F 5+ F 6)′″( i ))
 
         [0000]      Log Output of RatPor=(Σ i−n   i+n RatPor( i ))/(2 n+ 1) where  n= 2
 
         [0054]    It may be observed how RatPor varies with limestone porosity by examining the graph in  FIG. 8 . The output in the present example embodiment may be an Apparent Porosity Indicator; it may not output a true formation porosity corrected for the borehole environment and the different lithologies of the formation. The “End Point” of the porosity transform is not the 100% water point but rather a 100% dense shale point. In an actual wellbore logging environment the highest neutron attenuating condition will be in high density shales. Large, water filled caverns are not encountered under normal logging conditions. By making the “end point” a high density shale, the Apparent Porosity Indicator transform will then be monotonic and stable over the entire range from low porosities, found in tight, clean, formations, to very high apparent porosities found in “gumbo” shales. The apparent porosity of the dense shale end point may be defined by what a standard “open hole” neutron porosity well logging instrument measures in such particular formation, which in this case is 80 PU. 
         [0055]    As can be observed in  FIG. 8 , RatPor does not vary linearly with porosity. However, the square of RatPor does vary substantially linearly with porosity, as may be observed in  FIG. 9 . To have an Apparent Porosity Indicator which not only varies approximately linearly with porosity but is also of approximately the correct magnitude one may use the output RatPor̂2/2.8 as shown in  FIG. 10 , therefore: 
         [0000]      Log Output of Apparent Porosity Indicator=Log Output of RatPor̂2/2.8
 
         [0056]    The foregoing output may be an adequate indicator of varying porosity that can be calibrated in situ with a known open hole porosity value (e.g., from a well log) if such data are available. A very significant amount of response characterization and Monte Carlo modeling would be needed to have a characterized porosity response. To show the response of RatPor in limestone, sandstone and dolomite, in and 8 inch diameter wellbore having therein a 20 pound weight per foot length, 7 inch external diameter casing, one may observe such results in  FIG. 11 . 
         [0057]    Next one may use the inelastic count rate ratio (IRAT) as an Apparent Gas Indicator. The inelastic ratio may be calculated as: 
         [0000]      IRAT= N 0′″/ F 0′″
 
         [0058]    IRAT may be displayed as a raw on a scale of 0 to 20 entitled, “Apparent Gas Indicator.” There may be an indicator on the log (e.g., a darkened or other coded scale line) at a value of 10 and an indicator that shows an Apparent gas Indicator value of less than 10 there is a high probability of gas being present either in the borehole or the surrounding formation. As IRAT moves higher than 10 there is a decreasing probability that there is gas present. 
         [0059]    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.