Neutron Porosity Based On One Or More Gamma Ray Detectors And A Pulsed Neutron Source

Abstract

A method for pulsed neutron well logging of a subsurface formation, includes irradiating the formation with a plurality of bursts of neutrons of a group of selected durations; detecting gamma rays resulting from interaction of the neutrons during a group of selected time gates which contains at least some early and late gamma ray counts. The gamma rays are detected at at least two axially spaced apart locations from a position of the irradiating. In a computer, a ratio TRat is determined between the sum of detected gamma rays at a first axial spacing to the sum at a second axial spacing. A borehole correction is performed according to a function related to the ratio TRat before converting the ratio TRat to a hydrogen index or porosity of the subsurface formation.

What is claimed is: 
     
         1 . A method for pulsed neutron well logging of a subsurface formation, comprising:
 irradiating the formation with a plurality of bursts of neutrons of a group of selected durations;   detecting gamma rays resulting from interaction of the neutrons during a group of selected time gates, the gamma rays detected at at least two axially spaced apart locations from a position of the irradiating;   in a computer, determining a ratio TRat of the sum of detected gamma rays at a first axial spacing to the sum at a second axial spacing; and   in the computer, performing a borehole correction according to a function related to the ratio TRat before converting the ratio TRat to a hydrogen index or porosity of the subsurface formation.   
     
     
         2 . The method of  claim 1 , further comprising:
 in the computer, using the function to convert the ratio to a pre-defined formation property TRat 0 ; and   in the computer, determining a hydrogen index or porosity of the subsurface formation using the TRat 0 .   
     
     
         3 . The method of  claim 1 , further comprising:
 repeating irradiating the formation and detecting gamma rays for a selected number of times.   
     
     
         4 . The method of  claim 1 , wherein the function comprises a linear function. 
     
     
         5 . The method of  claim 1 , wherein the function is:
     T Rat= f   1 ( BH )· T Rat 0   +f   2 ( BH )  (1)
   
       wherein, f 1 (BH) and f 2 (BH) are dependent on borehole conditions, and independent of formation conditions. 
     
     
         6 . The method of  claim 1 , in which the group of selected time gates does not contain any time within a neutron burst. 
     
     
         7 . The method of  claim 1 , in which the group of the selected time gates contains at least some early and late gamma ray counts. 
     
     
         8 . The method of  claim 1 , wherein the group of time gates, burst length, and number of bursts are optimized to eliminate/reduce the borehole effect on the ratio TRat. 
     
     
         9 . The method of  claim 8 , wherein the borehole effect is borehole salinity effect. 
     
     
         10 . A method for pulsed neutron well logging of a subsurface formation, comprising:
 irradiating the formation with a plurality of bursts of neutrons of a group of selected durations;   detecting gamma rays resulting from interaction of the neutrons during a group of selected time gates, the gamma rays detected at at least two axially spaced apart locations from a position of the irradiating;   in a computer, determining a ratio of the sum of detected gamma rays at a first axial spacing to the sum at a second axial spacing;   in the computer, performing a borehole correction according to a function to convert the ratio to a value independent from borehole conditions; and   in the computer, using the converted value to determine a hydrogen index or porosity of the subsurface formation.   
     
     
         11 . The method of  claim 10 , further comprising:
 repeating irradiating the formation and detecting gamma rays for a selected number of times.   
     
     
         12 . The method of  claim 10 , wherein the function comprises a linear function. 
     
     
         13 . The method of  claim 10 , wherein the function is:
     T Rat= f   1 ( BH )· T Rat 0   +f   2 ( BH )  (1)
   
       wherein, TRat represents the ratio of the sum of detected gamma rays at the first axial spacing to the sum at the second axial spacing, TRat 0  represents the converted value, f 1 (BH) and f 2 (BH) are dependent on wellbore conditions, and independent of formation conditions. 
     
     
         14 . The method of  claim 10 , wherein the converted value represents a pre-defined formation property. 
     
     
         15 . The method of  claim 10 , in which the group of selected time gates does not contain any time within a neutron burst. 
     
     
         16 . The method of  claim 10 , in which the group of the selected time gates contains at least some early and late gamma ray counts. 
     
     
         17 . The method of  claim 10 , wherein the group of time gates, burst length, and number of bursts are optimized to eliminate/reduce the borehole effect on the ratio. 
     
     
         18 . The method of  claim 17 , wherein the borehole effect is borehole salinity effect. 
     
     
         19 . A method for pulsed neutron well logging of a subsurface formation, comprising:
 (a) irradiating the formation with a plurality of bursts of neutrons of a first selected duration;   (b) detecting gamma rays resulting from interaction of the neutrons during time intervals of a second selected duration following each burst until the beginning of the last burst in the plurality thereof, the gamma rays detected by a detector positioned at at least one axially spaced apart location from a position of the irradiating;   (c) detecting gamma rays following the last burst for a third selected time interval, the third selected time interval beginning substantially at the end of the last burst in the plurality thereof and ending when gamma ray detection has decayed substantially to background level;   (d) in a computer, summing numbers of gamma rays detected in each of the time intervals;   (e) in the computer, determining coefficients, based on wellbore conditions, of a linear function related to the summed numbers of gamma rays detected by the at least one detector; and   (f) in the computer, using the coefficients and the summed numbers of gamma rays detected by the at least one detector to determine a hydrogen index of the subsurface formation.   
     
     
         20 . The method of  claim 19  further comprising:
 repeating (a), (b) and (c) for a selected number of times; 
 in the computer, summing numbers of gamma rays detected in each of the time intervals; 
 in the computer, determining coefficients, based on wellbore conditions of a linear function related to the summed numbers of gamma rays detected by the at least one detector; and 
 in the computer, using the coefficients and the summed numbers of gamma rays detected by each detector to determine a hydrogen index of the subsurface formation.