Method for determining the composition of natural gas liquids, mean pore-size and tortuosity in a subsurface formation using NMR

1. A method of measuring at least one of:

(i) respective downhole concentrations in a subsurface formation of one or more members of the C1-C5 alkane group consisting of methane, ethane, propane, butane and pentane;

(ii) a ratio between respective downhole concentrations of multiple members of the C1-C5 alkane group, and

(iii) a linear combination of ratios between respective downhole concentrations of multiple members of the C1-C5 alkane group:

the method comprising:

a. obtaining sample(s) of core from a target depth in a subsurface formation;

b. obtaining downhole NMR log data of the subsurface formation at the target depth;

c. when a given one of the core sample(s) is saturated with a first pressurized saturation fluid comprising a first member of the C1-C5 alkane group, obtaining first laboratory NMR data of the saturated core-sample(s); and

d. when the same given one or a different one of the core sample(s) is saturated with a second pressurized saturation fluid that is different from the first pressurized saturation fluid, the second saturation fluid comprising a second member of the C1-C5 alkane group that is different from the first member, obtaining second laboratory NMR data of the saturated core-sample(s), wherein the first and second laboratory NMR data collectively form a laboratory NMR data set, and wherein the method further comprises:

e. computing from the lab-NMR data set and the downhole NMR log data at least one downhole parameter selected from the group consisting of:

(i) respective downhole concentrations of one or more members of the C1-C5 alkane group consisting of methane, ethane, propane, butane and pentane; and

(ii) a ratio between respective downhole concentrations of multiple members of the C1-C5 alkane group.

2. The method of claim 1 wherein:

i. the method further comprises: when the same given one or a different one of the core sample(s) is saturated with a third pressurized saturation fluid comprising a third member of the C1-C5 alkane group that is different from the first and second members, obtaining third laboratory NMR data of the saturated core-sample(s); and

ii. the third pressurized saturation fluid is different from both of the first and second pressurized saturation fluids; and

iii. the first, second and third laboratory NMR data collectively form the laboratory NMR data set.

3. The method of claim 2 wherein:

i. the method further comprises: when the same given one or a different one of the core sample(s) is saturated with a fourth pressurized saturation fluid comprising a fourth member of the C1-C5 alkane group that is different from the first, second and third members, obtaining fourth laboratory NMR data of the saturated core-sample(s); and

ii. the 4th pressurized saturation fluid is different from all of the first, second, and 3rd pressurized saturation fluids; and

iii. the first, second, third and fourth laboratory NMR data collectively form the laboratory NMR data set.

4. The method of claim 3 wherein:

i. the method further comprises: when the same given one or a different one of the core sample(s) is saturated with a fifth pressurized saturation fluid comprising a fifth member of the C1-C5 alkane group that is different from the first, second, third and fourth members, obtaining fifth laboratory NMR data of the saturated core-sample(s); and

ii. the fifth pressurized saturation fluid is different from all of the first, second, 3rd and 4th pressurized saturation fluids; and

iii. the first, second, third, fourth and fifth laboratory NMR data collectively form the laboratory NMR data set.

5. The method of claim 1 wherein the downhole pressure and/or temperature are estimated for the subsurface formation at the target depth, and wherein the 1 st and/or 2 nd and/or 3 rd and/or 4 th and/or 5 th laboratory NMR data is obtained when the core sample(s) are saturated with the first and/or second and/or third and/or fourth and/or fifth pressurized saturation fluid at the estimated pressure and temperature.

6. The method of claim 1 wherein the laboratory NMR data and/or downhole NMR log data comprises one or more of (i.e. any combination of): T2 distribution data, T2apparent distribution data, T1 distribution data, T1/T2 data, T1/T2apparent, D (diffusion) vs. T2 data, D (diffusion) vs T2apparent data.

7. The method of claim 1 wherein the computing of the at least one downhole parameter comprises optimizing a fit of:

i. a mathematical combination of the 1St and/or 2 nd and/or 3 rd and/or 4th and/or 5th laboratory NMR data; to

ii. downhole NMR data.

8. The method of claim 7 wherein the computed downhole concentration and/or computed molar fraction of the 1 st and/or 2 nd and/or 3 rd and/or 4 th and/or 5 th member of the C1-C5 alkane group corresponds to a weighting coefficient for the 1 st and/or 2 nd and/or 3 rd and/or 4 th and/or 5 th laboratory NMR data.

9. The method of claim 1 performed to compute at least one of the following:

(i) a ratio between a downhole methane concentration and a downhole ethane concentration;

(ii) a ratio between a downhole methane concentration and a downhole propane concentration;

(iii) a ratio between a downhole methane concentration and a downhole butane concentration;

(iv) a ratio between a downhole methane concentration and a downhole pentane concentration; and/or

(v) any ratio involving any of C1-C5 alkane group.

10. The method claim 1 , performed to compute at least one of the following:

(i) a multi-alkane sum of downhole concentrations of ethane and/or propane and/or butane and/or pentane; and

(ii) a ratio between the multi-alkane sum and a downhole methane concentration.

11. The method of claim 1 performed for a plurality of target depths to characterize the subsurface reservoir at multiple target depths.

12. The method of claim 1 wherein the computing comprises converting T2 data into T2apparent data or vice versa using at least one of:

(i) an estimated restricted diffusion coefficient for one or more members of the C1-C5 alkane group; and/or

(ii) an estimated mean pore-size of the core sample(s).

13. The method of claim 12 wherein the restricted diffusion coefficient is computed by interpolating the following method:

a method using NMR to determine the mean pore-size and tortuosity of the light hydrocarbon-filled porosity in a reservoir formation by:

a) Obtaining core in selected zones from the reservoir formation,

b) Measuring NMR on the core in the laboratory including normalized diffusion coefficient D/Do as a function of diffusion time using one or more hydrogen-bearing fluids selected from the list (H2, HD, CH4, C2H6, C3H8, C4H10, C5H12) saturating the core,

c) Varying the pressure and/or temperature of the one or more hydrogen-bearing fluids to vary the diffusion length in the NMR measurement, and

d) Computing the mean pore-size and tortuosity of the light hydrocarbon-filled porosity of the reservoir formation from D/Do versus diffusion length of the hydrogen-bearing fluid using a numerical model for restricted diffusion in a porous medium.

14. The method of claim 1 wherein the NMR log is from a gradient-based NMR tool.

15. The method of claim 1 wherein

(i) a molar fraction of the first member of the C1-C5 alkane group within the first saturation fluids is at least 0.9, and

(ii) a molar fraction of the second member of the C1-C5 alkane group within the second saturation fluids is at least 0.9.

16. The method of claim 2 wherein

(i) a molar fraction of the first member of the C1-C5 alkane group within the first saturation fluids is at least 0.9,

(ii) a molar fraction of the second member of the C1-C5 alkane group within the second saturation fluids is at least 0.9; and

(iii) a molar fraction of the third member of the C1-C5 alkane group within the third saturation fluids is at least 0.9.

17. The method of claim 3 wherein

(i) a molar fraction of the first member of the C1-C5 alkane group within the first saturation fluids is at least 0.9,

(ii) a molar fraction of the second member of the C1-C5 alkane group within the second saturation fluids is at least 0.9;

(iii) a molar fraction of the third member of the C1-C5 alkane group within the third saturation fluids is at least 0.9; and

(iv) a molar fraction of the fourth member of the C1-C5 alkane group within the fourth saturation fluids is at least 0.9.

18. The method of claim 4 wherein

(i) a molar fraction of the first member of the C1-C5 alkane group within the first saturation fluids is at least 0.9,

(ii) a molar fraction of the second member of the C1-C5 alkane group within the second saturation fluids is at least 0.9;

(iii) a molar fraction of the third member of the C1-C5 alkane group within the third saturation fluids is at least 0.9;

(iv) a molar fraction of the fourth member of the C1-C5 alkane group within the fourth saturation fluids is at least 0.9; and

(v) a molar fraction of the fifth member of the C1-C5 alkane group within the fifth saturation fluids is at least 0.9.

19. A method of measuring at least one of:

(i) respective downhole concentrations in a subsurface formation of one or more members of the C1-C5 alkane group consisting of methane, ethane, propane, butane and pentane; and

(ii) a ratio between respective downhole concentrations of multiple members of the C1-C5 alkane group, the method comprising:

a. obtaining sample(s) of core from a target depth in a subsurface formation;

b. obtaining downhole NMR log data of the subsurface formation at the target depth;

c. when a given one of the core sample(s) is saturated with a first pressurized saturation fluid comprising a first member of the C1-C5 alkane group, obtaining first laboratory NMR data of the saturated core-sample(s); and

d. when the same given one or a different one of the core sample(s) is saturated with a second pressurized saturation fluid that is different from the first pressurized saturation fluid, the second saturation fluid comprising a second member of the C1-C5 alkane group that is different from the first member, obtaining second laboratory NMR data of the saturated core-sample(s), wherein the first and second laboratory NMR data collectively form a laboratory NMR data set, and wherein the method further comprises:

e. computing from the lab-NMR data set and the downhole NMR log data at least one downhole parameter selected from the group consisting of:

(i) respective downhole concentrations of one or more members of the C1-C5 alkane group consisting of methane, ethane, propane, butane and pentane; and

(ii) a ratio between respective downhole concentrations of multiple members of the C1-C5 alkane group.

20. A method of measuring at least one of:

(i) respective downhole concentrations in a subsurface formation of one or more members of the C1-C5 alkane group consisting of methane, ethane, propane, butane and pentane; and

(ii) a mathematical function of respective downhole concentrations of multiple members of the C1-C5 alkane group, the method comprising:

a. obtaining sample(s) of core from a target depth in a subsurface formation;

b. obtaining downhole NMR log data of the subsurface formation at the target depth;

c. when a given one of the core sample(s) is saturated with a first pressurized saturation fluid comprising a first member of the C1-C5 alkane group, obtaining first laboratory NMR data of the saturated core-sample(s); and

d. when the same given one or a different one of the core sample(s) is saturated with a second pressurized saturation fluid that is different from the first pressurized saturation fluid, the second saturation fluid comprising a second member of the C1-C5 alkane group that is different from the first member, obtaining second laboratory NMR data of the saturated core-sample(s), wherein the first and second laboratory NMR data collectively form a laboratory NMR data set, and wherein the method further comprises:

e. computing from the lab-NMR data set and the downhole NMR log data at least one downhole parameter selected from the group consisting of:

(i) respective downhole concentrations of one or more members of the C1-C5 alkane group consisting of methane, ethane, propane, butane and pentane; and

(ii) a mathematical function between respective downhole concentrations of multiple members of the C1-C5 alkane group.