Method of calculating formation characteristics

Abstract

A method of calculating a formation characteristic includes measuring with at least two detectors spaced apart from each other an intensity of gamma rays, and calculating the formation characteristic by calculating a ratio of the intensity of the gamma rays detected by the two detectors.

1 . A method of calculating a formation characteristic, comprising:
 measuring with at least two detectors, spaced apart from each other, an intensity of gamma rays; and   calculating the formation characteristic by calculating a ratio of the intensity of the gamma rays detected by the two detectors.   
     
     
         2 . The method of  claim 1 , wherein measuring the intensity of the gamma rays includes measuring gamma rays generated by neutrons reacting with a boron isotope B10, cadmium or samarium. 
     
     
         3 . The method of  claim 2 , wherein measuring the intensity of gamma rays includes measuring a spectrum peak intensity of gamma rays generated by neutrons reacting with the boron isotope B10, cadmium or samarium. 
     
     
         4 . The method of  claim 2 , wherein each of the at least two detectors is covered by a separate layer of B10 isotope, cadmium or samarium. 
     
     
         5 . The method of  claim 1 , wherein each of the at least two detectors includes a scintillation crystal and a light detector, and
 the intensity of the gamma rays is measured by measuring light emitted by the scintillation crystal when gamma rays react with the scintillation crystal material.   
     
     
         6 . The method of  claim 1 , wherein the formation characteristic is porosity. 
     
     
         7 . The method of  claim 6 , wherein the porosity is calculated according to the formula: 
       
         
           
             
               
                 
                   
                     GI 
                     SS 
                   
                   
                     GI 
                     LS 
                   
                 
                 = 
                 
                   f 
                    
                   
                     ( 
                     ρ 
                     ) 
                   
                 
               
               , 
             
           
         
         where GI SS  and GI IS  each represent a gamma ray intensity created in a neutron reaction with a material covering the at least two detectors and detected by a respective one of the at least two detectors, and f(ρ) is a function of the porosity of the formation. 
       
     
     
         8 . The method of  claim 7 , wherein the material is a B10 isotope, cadmium or samarium. 
     
     
         9 . A gamma ray measurement system, comprising:
 a neutron source;   a first detector;   a second detector; and   a computing device configured to receive from the first and second detectors detection signals corresponding to a detected gamma ray intensity of each of the first and second detectors, and configured to calculate a formation characteristic based on a ratio of a gamma ray intensity detected by the first detector to a gamma ray intensity detected by the second detector.   
     
     
         10 . The gamma ray detection system of  claim 9 , wherein at least one of the first and second detectors is coated in a layer of boron isotope B10, cadmium or samarium. 
     
     
         11 . The gamma ray detection system of  claim 10 , wherein each of the first detector and the second detector includes a scintillation crystal and a photodetector to detect gamma radiation, and
 the B10 isotope, cadmium or samarium coating surrounds an outer circumference of the scintillation crystal and an end of the crystal facing the neutron source.   
     
     
         12 . The gamma ray detection system of  claim 11 , wherein the scintillation crystal is one of an NaI, LnBr 3 :Ce, GYSO, YAP or BGO crystal. 
     
     
         13 . The gamma ray detection system of  claim 9 , wherein the neutron source and the first and second detectors are co-linear. 
     
     
         14 . The gamma ray detection system of  claim 9 , wherein the neutron source and the first and second detectors are laterally adjacent to each other. 
     
     
         15 . A method of measuring a characteristic of an earth formation, the method comprising:
 covering at least two detectors with a layer of a boron isotope B10, cadmium or samarium;   inserting the at least two detectors and a neutron source into a borehole;   emitting neutrons from the neutron source;   detecting gamma rays generated by a reaction of the B10 isotope, cadmium or samarium and neutrons; and   calculating the characteristic of the formation by detecting a ratio of intensities of gamma rays detected by the at least two detectors.   
     
     
         16 . The method of  claim 15 , wherein the characteristic is porosity. 
     
     
         17 . The method of  claim 15 , wherein the at least two detectors and the neutron source are arranged co-linearly. 
     
     
         18 . The method of  claim 17 , wherein one of the at least two detectors is located between another of the at least two detectors and the neutron source. 
     
     
         19 . The method of  claim 15 , wherein the at least two detectors and the neutron source are arranged laterally adjacent to each other. 
     
     
         20 . The method of  claim 15 , wherein detecting the ratio of intensities of gamma rays includes detecting a ratio of spectrum peak intensities of gamma rays detected by the at least two detectors.