Apparatus and method for determining density, porosity and fluid saturation of formations penetrated by a borehole

Abstract

A borehole logging system for determining bulk density, porosity and formation gas/liquid fluid saturation of formation penetrated by a borehole. Measures of fast neutron radiation and inelastic scatter gamma radiation, induced by a pulsed neutron source, are combined with an iterative numerical solution of a two-group diffusion model to obtain the formation parameters of interest. Double-valued ambiguities in prior art measurements are removed by using the iterative solution of the inverted two-group diffusion model. The system requires two gamma ray detectors at different axial spacings from the source, and a single neutron detector axially spaced between the two gamma ray detectors. The system can be embodied as a wireline system or as a logging-while-drilling system.

What is claimed is:  
     
         1 . A method for determining at least one parameter of material, comprising the steps of: 
 (a) irradiating said material with a source that emits neutrons;    (b) measuring neutron radiation at a first axial spacing from said radiation source;    (c) measuring gamma radiation at a second axial spacing and at a third axial spacing from said radiation source; and    (d) combining said measures of said neutron radiation and said gamma radiation in an iterative solution of at least a two-group radiation diffusion model to obtain a single-valued measure of said at least one parameter.    
     
     
         2 . The method of  claim 1  wherein: 
 (a) said material is earth formation penetrated by a borehole; and  
 (b) said source produces fast neutrons; and  
 (c) a plurality of said parameters are determined.  
 
     
     
         3 . The method of  claim 1  wherein said neutron radiation is fast neutron radiation and said gamma radiation is inelastic scatter gamma radiation.  
     
     
         4 . The method of  claim 1  wherein said third axial spacing is greater than said first axial spacing, and said first axial spacing is greater than said second axial spacing.  
     
     
         5 . The method of  claim 2  wherein said plurality of parameters comprise formation bulk density, formation porosity, and formation fluid saturation.  
     
     
         6 . The method of  claim 1  wherein: 
 (a) said neutron radiation is measured with a first type of radiation detector;  
 (b) said gamma radiation is measured with a second type of radiation detector; and  
 (c) said first and said second types of radiation detectors and said neutron source are disposed within a logging tool.  
 
     
     
         7 . The method of  claim 6  further comprising the step of conveying said logging tool within said borehole with a wireline.  
     
     
         8 . The method of  claim 6  further comprising the step of conveying said logging tool within said borehole with a drill string.  
     
     
         9 . A method for determining a plurality of parameters of earth formation penetrated by a borehole, comprising the steps of: 
 (a) irradiating said formation with fast neutrons produced by a fast neutron source that is sequentially pulsed;    (b) during a sequential pulse, measuring fast neutron radiation at a first axial spacing from said source;    (c) during said sequential pulse, measuring inelastic scatter gamma radiation at a second axial spacing and at a third axial spacing from said source;    (d) iterating a numerical solution of a two-group radiation diffusion model to obtain a linearized relationship between a fast neutron diffusion coefficient and said measure of fast neutron radiation, wherein said linearized relationship is independent of fluid saturation, fluid type and pore space volume of said formation; and    (e) combining said fast neutron radiation measurement and said inelastic scatter gamma ray measurements with said linearized relationship to obtain a measure of bulk density of said formation.    
     
     
         10 . The method of  claim 9  comprising the additional step of combining said measure of bulk density with said fast neutron radiation measurement to obtain a measure of liquid saturation of said formation.  
     
     
         11 . The method of  claim 10  comprising the additional step of combining said measure of bulk density with said measure of liquid saturation to obtain a measure of porosity of said formation.  
     
     
         12 . The method of  claim 9  wherein said third axial spacing is greater than said first axial spacing, and said first axial spacing is greater than said second axial spacing.  
     
     
         13 . The method of  claim 9  wherein: 
 (a) said fast neutron radiation is measured with a first type of radiation detector;  
 (b) said inelastic scatter gamma radiation is measured with a second type of radiation detector; and  
 (c) said first and said second types of radiation detectors and said neutron source are disposed within a logging tool.  
 
     
     
         14 . The method of  claim 13  further comprising the step of conveying said logging tool within said borehole with a wireline.  
     
     
         15 . The method of  claim 13  further comprising the step of conveying said logging tool within said borehole with a drill string.  
     
     
         16 . A system for determining a plurality of parameters of earth formation penetrated by a borehole, comprising: 
 (a) a logging tool comprising a pressure housing in which are disposed 
 (i) a source of neutrons,  
 (ii) a neutron detector disposed at a first axial spacing from said source,  
 (iii) a first gamma ray detector disposed at a second axial spacing from said source, and  
 (iv) a second gamma ray detector disposed at a third axial spacing from said source; and  
   (b) a processor which combines gamma radiation measured by said first and said second gamma ray detectors and neutron radiation measured by said neutron detector using an iterative numerical solution of a two-group radiation diffusion model to obtain said plurality of parameters.    
     
     
         17 . The system of  claim 16  further comprising: 
 (a) a conveyance unit located at the surface of the earth; and  
 (b) a conveyance member with a first end operationally connected to said logging tool and a second end operationally connected to said conveyance unit.  
 
     
     
         18 . The system of  claim 16  wherein: 
 (a) said neutron source is operated to emit sequential pulses of fast neutrons,  
 (b) said neutron detector measures fast neutron radiation during a pulse of fast neutrons; and  
 (c) said gamma ray detectors measure inelastic scatter gamma radiation during a pulse of fast neutrons.  
 
     
     
         19 . The system of  claim 16  wherein: 
 (a) said iterative solution linearizes a relationship between said measure of fast neutron radiation and a fast neutron diffusion coefficient; and  
 (b) said fast neutron radiation measurement and said inelastic scatter gamma ray measurements are combined with said linearized relationship to obtain a measure of bulk density of said formation.  
 
     
     
         20 . The system of  claim 16  wherein said third axial spacing is greater than said first axial spacing, and said first axial spacing is greater than said second axial spacing.  
     
     
         21 . The system of  claim 16  wherein said conveyance member comprises a wireline.  
     
     
         22 . The system of  claim 16  wherein said conveyance member comprises a drill string.  
     
     
         23 . The system of  claim 16  wherein said neutron detector measures fast neutron radiation during a pulse of fast neutrons and said gamma ray detectors measure inelastic scatter gamma radiation during the same pulse of fast neutrons.  
     
     
         24 . A method for determining a parameter of material from a measure of neutron radiation at a first axial spacing from a neutron source and two measures of gamma radiation at a second axial spacing and at a third axial spacing from the source, said method comprising combining said measures of said neutron radiation and said gamma radiation in an iterative solution of a two-group radiation diffusion model to obtain single-valued measure of said parameter.  
     
     
         25  A method for determining a plurality of parameters of earth formation from a measure of fast neutron radiation at a first axial spacing from a neutron source and two measures of inelastic scatter gamma radiation at a second axial spacing and at a third axial spacing from the source, said method comprising: 
 (a) iterating a numerical solution of a two-group radiation diffusion model to obtain a linearized relationship between a fast neutron diffusion coefficient and said measure of fast neutron radiation, wherein said linearized relationship is independent of fluid saturation, fluid type and pore space volume of said formation; and  
 (b) combining said fast neutron radiation measurement and said inelastic scatter gamma ray measurements with said linearized relationship to obtain a measure of bulk density of said formation.  
 
     
     
         26 . A system for determining a plurality of parameters of earth formation with a logging tool which induces and measures neutron radiation and gamma radiation, said system comprising a processor which combines two axially spaced measures of said gamma radiation and a measure of said neutron radiation using an iterative numerical solution of a two-group radiation diffusion model to obtain said plurality of parameters.