System and Method for Correction of Borehole Effects in a Neutron Porosity Measurement

Abstract

Systems, methods, and devices for determining a porosity of a subterranean formation corrected for borehole effects are provided. One such device may be a downhole tool capable of being lowered into a borehole of a subterranean formation that may include a neutron source, two or more neutron detectors, and data processing circuitry. The neutron source may emit neutrons into the subterranean formation. The two or more neutron detectors may be respectively disposed at two or more azimuthal orientations within the downhole tool, and may detect neutrons scattered by the subterranean formation or borehole fluid in the borehole, or both. Based on the neutrons detected by the neutron detectors, the data processing circuitry may determine a porosity of the subterranean formation corrected for borehole effects.

What is claimed is: 
     
         1 . A downhole tool comprising:
 a neutron source configured to emit neutrons into a subterranean formation;   at least one formation-facing neutron detector disposed nearer to a formation-facing side of the downhole tool than a borehole-facing side of the downhole tool and configured to detect a first count of neutrons that have been scattered by the subterranean formation or borehole fluid in the borehole, or a combination thereof;   at least one borehole-facing neutron detector disposed nearer to the borehole-facing side of the downhole tool than the formation-facing side of the downhole tool and configured to detect a second count of neutrons that have been scattered by the subterranean formation or borehole fluid in the borehole, or a combination thereof; and   data processing circuitry configured to determine a porosity of the subterranean formation corrected for borehole effects based at least in part on the first neutron count and the second count by determining a first apparent porosity based at least in part on the first count of neutrons and a second apparent porosity based at least in part on the second count of neutrons and at least in part on a correction function relating the first apparent porosity and the second apparent porosity to a true porosity associated with the subterranean formation for various borehole geometries and borehole fluids.   
     
     
         2 . The downhole tool of  claim 1 , comprising shielding configured to enhance a sensitivity of one of the formation-facing neutron detector and the borehole-facing neutron detector to scattered neutrons arriving via a side of the downhole tool toward which that neutron detector is oriented. 
     
     
         3 . The downhole tool of  claim 1 , wherein the formation-facing neutron detector and the borehole-facing neutron detector are configured to detect epithermal neutrons. 
     
     
         4 . The downhole tool of  claim 1 , wherein the formation-facing neutron detector and the borehole-facing neutron detector are configured to detect thermal neutrons. 
     
     
         5 . The downhole tool of  claim 1 , wherein one of the formation-facing neutron detector and the borehole-facing neutron detector comprises a thermal neutron detector and one the formation-facing neutron detector and the borehole-facing neutron detector comprises an epithermal neutron detector. 
     
     
         6 . The downhole tool of  claim 1 , wherein the data processing circuitry is configured to determine the porosity of the subterranean formation corrected for borehole effects based at least in part on the correction function relating the first apparent porosity and the second apparent porosity to the true porosity associated with the subterranean formation for various borehole geometries and borehole fluids, wherein the various borehole fluids comprise fresh water, barite mud, hematite mud, an NaCl brine, a KCl brine, or an NaBr brine, or any combination thereof. 
     
     
         7 . A method comprising:
 emitting neutrons into a subterranean formation using a neutron source of a downhole tool, wherein the downhole tool is located in a borehole of the subterranean formation and wherein the borehole has a geometry and is filled with a borehole fluid;   detecting a first count of neutrons scattered by the subterranean formation or by the borehole fluid in the borehole, or by a combination thereof, using a formation-facing epithermal neutron detector of the downhole tool disposed nearer to a formation-facing side of the downhole tool than a borehole-facing side of the downhole tool;   detecting a second count of neutrons scattered by the subterranean formation or by the borehole fluid in the borehole, or by a combination thereof, using a borehole-facing epithermal neutron detector of the downhole tool disposed nearer to the borehole-facing side of the downhole tool than the formation-facing side of the downhole tool; and   determining, using data processing circuitry, a porosity of the subterranean formation that corrects for borehole effects due to the geometry and the borehole fluid of the borehole based at least in part on the first count of neutrons and the second count of neutrons by determining a first apparent porosity based at least in part on the first count of neutrons and a second apparent porosity based at least in part on the second count of neutrons and at least in part on a correction function relating the first apparent porosity and the second apparent porosity to a true porosity associated with the subterranean formation for various borehole geometries and borehole fluids.   
     
     
         8 . The method of  claim 7 , wherein the porosity is determined based at least in part on a polynomial correction function relating the first apparent porosity and the second apparent porosity to a true porosity associated with the subterranean formation for various borehole geometries and borehole fluids, wherein coefficients of the polynomial correction function have been chosen to minimize a difference between the determined porosity and the true porosity. 
     
     
         9 . The method of  claim 8 , wherein the porosity is determined based at least in part on the following relationship: 
       
         
           
             
               
                 
                   ϕ 
                   corr 
                 
                 = 
                 
                   
                     ∑ 
                     ij 
                     
                       
                         i 
                         + 
                         j 
                       
                       ≤ 
                       n 
                     
                   
                    
                   
                     
                       a 
                       ij 
                     
                      
                     
                       ϕ 
                       near 
                       i 
                     
                      
                     
                       ϕ 
                       back 
                       j 
                     
                   
                 
               
               , 
             
           
         
         wherein φ corr  represents the determined porosity, φ near  represents the first apparent porosity, φ back  represents the second apparent porosity, n represents the degree of the polynomial, and a ij  represents coefficients chosen to minimize a difference between the determined porosity and a true porosity of the subterranean formation. 
       
     
     
         10 . The method of  claim 7 , wherein the porosity is determined directly from the first count of neutrons and the second count of neutrons using a transform derived from modeled or experimental data, or a combination thereof, that relates the first count of neutrons and the second count of neutrons to a true porosity associated with the subterranean formation for various borehole geometries and borehole fluids. 
     
     
         11 . The method of  claim 7 , wherein the porosity is determined based at least in part on operator-provided external parameters associated with the borehole effects due to the geometry and the borehole fluid of the borehole. 
     
     
         12 . A system comprising:
 a downhole tool configured to be lowered into a borehole of a subterranean formation, to emit neutrons into the subterranean formation using a neutron source, and to detect neutrons that have been scattered by the subterranean formation or borehole fluid in the borehole, or a combination thereof,   using two or more neutron detectors respectively configured to detect neutrons arriving via different azimuthal faces of the downhole tool; and   data processing circuitry configured to determine a porosity of the subterranean formation corrected for borehole effects based at least in part on the neutrons detected by the two or more neutron detectors and based at least in part on an inversion of a forward model giving expected count rates of the neutrons detected by the two or more neutron detectors or giving apparent porosities based on the expected count rates, or giving a combination thereof, as a function of the porosity and the borehole effects.   
     
     
         13 . The system of  claim 12 , wherein the neutron source of the downhole tool comprises an electronic neutron generator configured to emit pulses of neutrons and wherein the data processing circuitry is configured to determine at least one epithermal slowing down time associated with neutrons detected by one of the two or more neutron detectors, to determine at least one apparent porosity based at least in part on the at least one epithermal slowing down time, and to determine the porosity based at least in part on the at least one apparent porosity. 
     
     
         14 . The system of  claim 12 , wherein the two or more neutron detectors comprise at least one thermal neutron detector and at least one epithermal neutron detector, wherein the data processing circuitry is configured to determine at least one thermal apparent porosity based at least in part on thermal neutrons detected by the at least one thermal neutron detector and to determine at least one epithermal apparent porosity based at least in part on epithermal neutrons detected by the at least one epithermal neutron detector, and wherein the data processing circuitry is configured to determine the porosity based at least in part on a correction function that is a function of at least one thermal apparent porosity and the at least one epithermal apparent porosity. 
     
     
         15 . The system of  claim 14 , wherein the data processing circuitry is configured to determine an epithermal slowing down time based at least in part on the epithermal neutrons detected by the epithermal neutron detector and to determine the at least one epithermal apparent porosity based at least in part on an epithermal slowing down time. 
     
     
         16 . The system of  claim 14 , wherein the data processing circuitry is configured to determine the porosity based at least in part on the correction function, wherein the correction function is configured not to consider a thermal decay time. 
     
     
         17 . The system of  claim 14 , wherein the neutron source of the downhole tool comprises a pulsed electronic neutron generator. 
     
     
         18 . The system of  claim 14 , wherein the neutron source of the downhole tool comprises a radioisotopic source.