API unit calibration of gamma ray logging tools using simulated data

1. A method for calibrating a gamma ray logging tool, the method comprising:

simulating gamma ray emissions of a formation by:

simulating a low activity zone of the formation and calculating a first counting rate for the low activity zone; and

simulating a high activity zone of the formation and calculating a second counting rate for the high activity zone;

calculating an American Petroleum Institute (“API”) unit sensitivity factor for a logging tool based upon the simulated gamma ray emissions, wherein the API unit sensitivity factor is calculated based upon the first and second counting rates;

deploying the logging tool along a wellbore;

obtaining a counting rate response of the logging tool; and

converting the counting rate response to API units using the API unit sensitivity factor.

2. A calibration method as defined in claim 1 , wherein the gamma ray emissions are simulated using Monte Carlo modeling.

3. A calibration method as defined in claim 1 , wherein simulating the gamma ray emissions comprises:

simulating a single activity zone of the formation; and

calculating a counting rate for the single activity zone,

wherein the API unit sensitivity factor is calculated based upon the counting rate.

4. A calibration method as defined in claim 1 , wherein simulating the gamma ray emissions comprises:

calculating photon activities of the gamma ray emissions for one or more elemental sources;

defining an elemental probability distribution for sampling the gamma ray emissions based upon the photon activities;

defining a mixed source probability distribution for sampling the gamma ray emissions based upon the elemental probability distributions; and

using the mixed source probability distribution, calculating a tally multiplier to express a tally of scoring photons as a counting rate,

wherein the counting rate will be utilized to calculate the API unit sensitivity factor.

5. A calibration method as defined in claim 1 , wherein simulating the gamma ray emissions comprises:

simulating one or more activity zones of the formation and calculating a first counting rate for each zone;

calculating a first API unit sensitivity factor using the first counting rate;

developing a first proxy formation model whose borehole size matches a reference logging condition of a first logging tool simulated to calculate the first counting rate;

using the first proxy formation model, calculating a second counting rate of the first logging tool;

converting the second counting rate to API units using the first API unit sensitivity factor;

developing a second proxy formation model whose borehole size matches a reference logging condition of a second logging tool; and

using the second proxy formation model, calculating a third counting rate of the second logging tool,

wherein the API unit sensitivity factor is a second API unit sensitivity factor calculated using the third counting rate.

6. A gamma ray logging tool, comprising:

one or more gamma ray emission sensors communicably coupled to processing circuitry that is operable to perform a method comprising:

obtaining a counting rate response of the logging tool; and

converting the counting rate response to API units using an API unit sensitivity factor obtained using any of methods 1-6.

7. A calibration method as defined in claim 1 , wherein the logging tool is deployed via a wireline, slickline or tubular assembly.

8. A method for generating a gamma ray calibration model, the method comprising:

simulating gamma ray emissions of a formation by:

simulating a low activity zone of the formation and calculating a first counting rate for the low activity zone; and

simulating a high activity zone of the formation and calculating a second counting rate for the high activity zone; and

calculating an American Petroleum Institute (“API”) unit sensitivity factor for a logging tool based upon the simulated gamma ray emissions, wherein the API unit sensitivity factor is calculated based upon the first and second counting rates.

9. A method as defined in claim 8 , wherein the gamma ray emissions are simulated using Monte Carlo modeling.

10. A method as defined in claim 9 , wherein simulating the gamma ray emissions comprises:

simulating a single activity zone of the formation; and

calculating a counting rate for the single activity zone,

wherein the API unit sensitivity factor is calculated based upon the counting rate.

11. A method as defined in claim 9 , wherein simulating the gamma ray emissions comprises:

calculating photon activities of the gamma ray emissions for one or more elemental sources;

defining an elemental probability distribution for sampling the gamma ray emissions based upon the photon activities;

defining a mixed source probability distribution for sampling the gamma ray emissions based upon the elemental probability distributions; and

using the mixed source probability distribution, calculating a tally multiplier to express a tally of scoring photons as a counting rate,

wherein the counting rate will be utilized to calculate the API unit sensitivity factor.

12. A method as defined in claim 9 , wherein simulating the gamma ray emissions comprises:

simulating one or more activity zones of the formation and calculating a first counting rate for each zone;

calculating a first API unit sensitivity factor using the first counting rate;

developing a first proxy formation model whose borehole size matches a reference logging condition of a first logging tool simulated to calculate the first counting rate;

using the first proxy formation model, calculating a second counting rate of the first logging tool;

converting the second counting rate to API units using the first API unit sensitivity factor;

developing a second proxy formation model whose borehole size matches a reference logging condition of a second logging tool; and

using the second proxy formation model, calculating a third counting rate of the second logging tool,

wherein the API unit sensitivity factor is a second API unit sensitivity factor calculated using the third counting rate.

13. A non-transitory computer program product comprising instructions which, when executed by at least one processor, causes the processor to perform any of the methods in claims 1 or 9 .

14. A system for calibrating a gamma ray logging tool, the system comprising:

a logging tool having one or more gamma ray sensors therein; and

processing circuitry communicably coupled to the logging tool and operable to perform operations comprising applying simulated gamma ray emission data to convert logging tool counting rate responses to American Petroleum Institute (“API”) units, wherein the conversion is achieved by:

simulating a low activity zone of a formation and calculating a first counting rate for the low activity zone;

simulating a high activity zone of the formation and calculating a second counting rate for the high activity zone;

calculating an API unit sensitivity factor for a logging tool based upon the first and second counting rates;

deploying the logging tool along a wellbore;

obtaining a counting rate response of the logging tool; and

converting the counting rate response to API units using the API unit sensitivity factor.

15. A system for generating a gamma ray calibration model, the system comprising processing circuitry operable to perform operations comprising:

simulating gamma ray emissions of a formation by:

simulating a low activity zone of the formation and calculating a first counting rate for the low activity zone; and

simulating a high activity zone of the formation and calculating a second counting rate for the high activity zone; and

calculating an American Petroleum Institute (“API”) unit sensitivity factor for a logging tool based upon the simulated gamma ray emissions, wherein the API unit sensitivity factor is calculated based upon the first and second counting rates.

16. A system as defined in claim 15 , wherein the gamma ray emissions are simulated using Monte Carlo modeling.

17. A system as defined in claim 15 , wherein simulating the gamma ray emissions comprises:

simulating a single activity zone of the formation; and

calculating a counting rate for the single activity zone,

wherein the API unit sensitivity factor is calculated based upon the counting rate.

18. A system as defined in claim 15 , wherein simulating the gamma ray emissions comprises:

calculating photon activities of the gamma ray emissions for one or more elemental sources;

defining an elemental probability distribution for sampling the gamma ray emissions based upon the photon activities;

defining a mixed source probability distribution for sampling the gamma ray emissions based upon the elemental probability distributions; and

using the mixed source probability distribution, calculating a tally multiplier to express a tally of scoring photons as a counting rate,

wherein the counting rate will be utilized to calculate the API unit sensitivity factor.

19. A system as defined in claim 15 , wherein simulating the gamma ray emissions comprises:

simulating one or more activity zones of the formation and calculating a first counting rate for each zone;

calculating a first API unit sensitivity factor using the first counting rate;

developing a first proxy formation model whose borehole size matches a reference logging condition of a first logging tool simulated to calculate the first counting rate;

using the first proxy formation model, calculating a second counting rate of the first logging tool;

converting the second counting rate to API units using the first API unit sensitivity factor;

developing a second proxy formation model whose borehole size matches a reference logging condition of a second logging tool; and

using the second proxy formation model, calculating a third counting rate of the second logging tool,

wherein the API unit sensitivity factor is a second API unit sensitivity factor calculated using the third counting rate.