Downhole Neutron Activation Measurement

Abstract

Systems and methods for measuring neutron-induced activation gamma-rays in a subterranean formation are provided. In one example, a downhole tool for measuring neutron-induced activation gamma-rays may include a neutron source and a gamma-ray detector. The neutron source may emit neutrons according to a pulsing scheme that includes a delay between two pulses. The delay may be sufficient to allow substantially all neutron capture events due to the emitted neutrons to cease. The gamma-ray detector may be configured to detect activation gamma-rays produced when elements activated by the emitted neutrons decay to a non-radioactive state.

What is claimed is: 
     
         1 . A downhole tool comprising:
 a neutron source configured to emit neutrons according to a pulsing scheme, wherein the pulsing scheme includes a delay between two pulses, wherein the delay is sufficient to allow substantially all neutron capture events due to the emitted neutrons to cease; and   a gamma-ray detector configured to detect activation gamma-rays produced when elements activated by the emitted neutrons decay to a non-radioactive state.   
     
     
         2 . The downhole tool of  claim 1 , wherein the delay is greater than or equal to approximately 2 ms. 
     
     
         3 . The downhole tool of  claim 1 , wherein the delay is greater than or equal to approximately 1 s. 
     
     
         4 . The downhole tool of  claim 1 , wherein the pulsing scheme is configured to vary depending on a logging speed of the downhole tool. 
     
     
         5 . The downhole tool of  claim 1 , wherein the pulsing scheme is configured to vary depending on whether the downhole tool is moving or is approximately stationary. 
     
     
         6 . The downhole tool of  claim 1 , wherein the pulsing scheme is configured to be independent of a logging speed of the downhole tool and configured to comprise a plurality of predetermined burst patterns for a plurality of logging speeds. 
     
     
         7 . The downhole tool of  claim 1 , wherein the pulsing scheme is configured such that one of the pulses of the pulsing scheme is subdivided into a plurality of microbursts. 
     
     
         8 . The downhole tool of  claim 7 , wherein the plurality of microbursts comprises a carbon/oxygen (C/O) microbursting scheme, a gamma-ray spectroscopy microbursting scheme, a dual-burst microbursting scheme, or a Sigma measurement microbursting scheme, or any combination thereof. 
     
     
         9 . The downhole tool of  claim 7 , wherein the plurality of microbursts comprises less than or equal to approximately 50% of the one of the pulses of the pulsing scheme and wherein a plurality of delays between the plurality of microbursts comprises greater than or equal to approximately 50% of the one of the pulses of the pulsing scheme. 
     
     
         10 . A downhole tool for use in a subterranean formation, comprising:
 a neutron source configured to emit neutrons into the subterranean formation in pulses separated by a delay of at least approximately 2 ms; and   a gamma-ray detector configured to detect activation gamma-rays and either or both inelastic gamma-rays or neutron capture gamma-rays resulting from interactions between the emitted neutrons and the subterranean formation.   
     
     
         11 . The downhole tool of  claim 10 , wherein the neutron source comprises a d-D neutron generator or a d-T neutron generator, or a combination thereof. 
     
     
         12 . The downhole tool of  claim 10 , wherein the gamma-ray detector is configured for detecting counts of the activation gamma-rays or detecting spectra of the activation gamma-rays, or any combination thereof. 
     
     
         13 . The downhole tool of  claim 10 , comprising a second gamma-ray detector, wherein the neutron source is disposed in the downhole tool between the gamma-ray detector and the second gamma-ray detector. 
     
     
         14 . The downhole tool of  claim 10 , comprising a second gamma-ray detector, wherein the second gamma-ray detector is disposed in the downhole tool between the gamma-ray detector and the neutron source. 
     
     
         15 . A method comprising:
 injecting fracture fluid containing an inert tracer material into a subterranean formation;   emitting neutrons into the subterranean formation to activate the tracer material using a neutron generator configured to emit neutrons according to a pulsing scheme that includes a delay between pulses of at least approximately 2 ms; and   detecting activation gamma-rays from the activated tracer material using a gamma-ray detector.   
     
     
         16 . The method of  claim 15 , wherein injecting the fracture fluid comprises injecting fracture fluid containing the inert tracer material, wherein the inert tracer material is configured to be activated through thermal neutron capture and wherein the emitted neutrons have energies sufficient to cause neutron capture events but not to cause substantially any inelastic scattering events. 
     
     
         17 . The method of  claim 15 , wherein the activation gamma-rays are detected at least approximately 2 ms after an emitted neutron pulse has ended. 
     
     
         18 . The method of  claim 15 , comprising detecting either or both inelastic gamma-rays or neutron capture gamma-rays resulting from interactions between the emitted neutrons and the subterranean formation or the tracer material.