Determination of near wellbore properties using natural gamma rays

1. A method comprising:

detecting gamma rays in a detector in a tool, the tool disposed downhole in a well;

determining, via a processor, deviation of energy spectra of the detected gamma rays in each selected energy window of a set of selected energy windows with respect to a reference energy spectra of the respective selected energy window; and

determining, via the processor, one or more properties of one or more regions around the tool from the deviation of the energy spectra, the regions being between the tool and a source of the detected gamma rays in a formation around the well,

wherein determining one or more properties includes determining at least one of a borehole fluid density and an annular space density.

2. The method of claim 1 , wherein determining the deviation of the energy spectra includes determining differences between energy spectra of the detected gamma rays in two different energy windows.

3. The method of claim 1 , wherein determining the one or more properties includes using a relationship between the energy spectra in different energy windows.

4. The method of claim 3 , wherein the energy spectra in the different energy windows are normalized by the reference energy spectra channel by channel.

5. The method of claim 1 , wherein the reference energy spectra is energy spectra measured in fresh water-filled borehole.

6. The method of claim 1 , wherein the borehole fluid density is determined using an index correlated to difference of normalized counting rates in different energy windows.

7. The method of claim 1 , wherein the annular space density is determined using an index correlated to a ratio of normalized counting rates in different energy windows.

8. A system comprising:

a tool having a detector to detect gamma rays, the tool operable downhole in a well; and

a processor arranged to

determine deviation of energy spectra of gamma rays detected in downhole operation of the tool in each selected energy window of a set of selected energy windows with respect to a reference energy spectra of the respective selected energy window, and

determine one or more properties of one or more regions around the tool, when the tool is disposed downhole in the well, from the deviation of the energy spectra, the regions being between the tool and a source of the detected gamma rays in a formation around the well,

wherein determination of one or more properties includes a determination of at least one of a borehole fluid density and an annular space density.

9. The system of claim 8 , wherein the detector is a gamma ray scintillator coupled with a photomultiplier tube or a photodiode.

10. The system of claim 8 , wherein the detector is a semiconductor detector that converts photons incident to the semiconductor detector to an electrical signal.

11. The system of claim 8 , wherein the processor is arranged to determine the one or more properties by use of a relationship between the energy spectra in different energy windows.

12. The system of claim 11 , wherein the energy spectra in the different energy windows are normalized by the reference energy spectra channel by channel.

13. The system of claim 8 , wherein the reference energy spectra is energy spectra measured in fresh water-filled borehole.

14. The system of claim 8 ,

wherein the borehole fluid density is determined using a first index correlated to a difference of normalized counting rates in different energy windows, and

wherein the annular space density is determined using a second index correlated to a ratio of the normalized counting rates in the different energy windows.

15. A machine-readable storage device having instructions stored thereon, which, when performed by a machine, cause the machine to perform operations, the operations comprising:

determining, via a processor, deviation of energy spectra of gamma rays in each selected energy window of a set of selected energy windows with respect to a reference energy spectra of the respective selected energy window, wherein the gamma rays are detected in a detector of a tool disposed downhole in a well; and

determining, via the processor, one or more properties of one or more regions around the tool from the deviation of the energy spectra, the regions being between the tool and a source of the detected gamma rays in a formation around the well,

wherein determining one or more properties includes determining at least one of a borehole fluid density and an annular space density.

16. The machine-readable storage device of claim 15 , wherein determining the deviation of the energy spectra includes determining differences between the energy spectra of the detected gamma rays in two different energy windows.

17. The machine-readable storage device of claim 15 , wherein determining the one or more properties includes using a relationship between the energy spectra in different energy windows.

18. The machine-readable storage device of claim 17 , wherein the energy spectra in the different energy windows are normalized by the reference energy spectra channel by channel.

19. The machine-readable storage device of claim 15 , wherein the reference energy spectra is energy spectra measured in fresh water-filled borehole.

20. The machine-readable storage device of claim 15 ,

wherein the borehole fluid density is determined using a first index correlated to difference of normalized counting rates in different energy windows, and

wherein the annular space density is determined using a second index correlated to a ratio of the normalized counting rates in the different energy windows.