Multiphysics NMR logging techniques for the determination of in situ total gas in gas reservoirs

1. A method of measuring total gas in place for a subterranean formation traversed by a borehole, comprising:

emplacing a nuclear magnetic resonance (NMR) tool into the borehole;

logging the borehole with the NMR tool to obtain an NMR log;

determining a NMR signal corresponding to gas using the NMR log;

determining a value corresponding to a number of hydrogen nuclei within the formation using the NMR signal corresponding to gas; and

calculating a total gas in place (TGiP) log of the formation using the value corresponding to the number of hydrogen nuclei.

2. The method of claim 1 , wherein the NMR tool comprises at least one of a wireline NMR tool, a logging-while-drilling NMR tool, a modular formation dynamics tester, and a laboratory NMR instrument.

3. The method of claim 1 , further comprising:

emplacing a second tool into the formation and determining a second log, wherein determining the NMR signal corresponding to gas further comprises determining a NMR signal corresponding to water using the second log and separating the NMR signal corresponding to water from the NMR signal corresponding to gas.

4. The method of claim 3 , wherein the second log comprises a NMR log, a dielectric log, a resistivity log, cuttings analysis as a function of depth, or a combination thereof.

5. The method of claim 1 ,

wherein the value corresponding to the number of hydrogen nuclei is a value corresponding to a number of methane hydrogen nuclei, and

wherein the value corresponding to the number of methane hydrogen nuclei is determined using a gas composition.

6. The method of claim 5 , further comprising:

determining the gas composition using downhole fluid analysis, mud logging, production data from the borehole, production data from a nearby well, or a combination thereof.

7. The method of claim 6 , wherein determining the gas composition comprises determining partial pressures of constituents of the gas composition.

8. The method of claim 1 , wherein the value corresponding to the number of hydrogen nuclei within the formation is a number of moles of hydrogen nuclei within the formation.

9. The method of claim 8 , wherein the following relationship is used to determine the number of moles of hydrogen nuclei within the formation:

v

=

ϕ

MR

⁡

(

gas

)

·

n

w

⁢

ρ

w

M

W

·

1

n

_

g

wherein ν is the number of moles of hydrogen nuclei within the formation, ρ w is the mass density of water, n w is the number of hydrogen nuclei in each molecule of water, M w is the molecular weight of water, φ MR (gas) is the NMR signal corresponding to gas, and n g is the mean number of hydrogen nuclei per gas molecule in a gas mixture.

10. The method of claim 8 , wherein the number of moles of hydrogen nuclei within the formation is a number of moles of hydrogen nuclei per unit volume of gas.

11. The method of claim 1 , further comprising:

separating a NMR signal corresponding to water from the NMR log, wherein the separating is performed using NMR well log relaxation analysis, NMR well log dimensional relaxation analysis, NMR well log diffusion-relaxation analysis, resistivity well log analysis, dielectric well log analysis, cuttings analysis, or a combination thereof; and

after separating the NMR signal corresponding to water from the NMR log, determining the NMR signal corresponding to gas using the NMR log.

12. The method of claim 11 , further comprising using one or more of D, T 1 , T 2 , and any ratio thereof to separate the NMR signal corresponding to water from the NMR log.

13. The method of claim 1 , wherein the method is performed without using a hydrogen index.

14. The method of claim 1 , wherein the method is performed without using a density for the gas.

15. A method of measuring total gas in place for a subterranean formation, comprising:

emplacing a nuclear magnetic resonance (NMR) tool into the borehole;

logging the borehole with the NMR tool to obtain an NMR log;

determining a mean number of hydrogen nuclei per gas molecule in the formation;

determining a NMR signal corresponding to gas within the formation using the NMR log; and

calculating a total volume of gas using the mean number of hydrogen nuclei per gas molecule and the NMR signal corresponding to gas within the formation.

16. The method of claim 15 , wherein calculating the total volume of gas comprises calculating a volume of gas per unit volume and calculating the total volume by integration of the NMR log for an entire depth of the gas reservoir.

17. The method of claim 15 , wherein determining the mean number of hydrogen nuclei comprises determining partial pressures of constituents of the gas within the formation.

18. The method of claim 15 , wherein determining the NMR signal corresponding to gas further comprises determining the NMR signal corresponding to water and separating the NMR signal corresponding to water from the NMR signal corresponding to gas.

19. The method of claim 18 , wherein the separating is performed using NMR well log relaxation analysis, NMR well log two dimensional relaxation analysis, NMR well log diffusion-relaxation analysis, resistivity well log analysis, dielectric well log analysis, cuttings analysis, or a combination thereof.

20. The method of claim 15 , wherein the determining the mean number of hydrogen nuclei per gas molecule for the gas comprises:

determining gas composition information for the gas in the formation; and

calculating the mean number of hydrogen nuclei per gas molecule for the gas.

21. The method of claim 15 , wherein the determining the mean number of hydrogen nuclei per gas molecule for the gas comprises estimating the mean number.

22. The method of claim 15 , wherein the method is performed without using a hydrogen index.

23. The method of claim 15 , wherein the method is performed without using a density for the gas.