Determining rock sample properties

1. A method for determining at least one rock property of a core sample, comprising:

measuring a first nuclear magnetic resonance (NMR) spectrum signal of a test fluid enclosed at a particular pressure in a cylinder of an NMR pressure cell;

measuring a second NMR spectrum signal of a core sample immersed in the test fluid that is enclosed at the particular pressure in the cylinder of the NMR pressure cell;

removing a background NMR spectrum signal from the first and second NMR spectrum signals to determine a bulk test fluid NMR spectrum signal and a combined test fluid and core sample NMR spectrum signal;

determining a porosity of the core sample based on the bulk test fluid NMR spectrum signal, the combined test fluid and core sample NMR spectrum signal, at least one dimension of the core sample, and at least one dimension of the cylinder; and

determining a fluid intake capacity of the core sample based at least in part on the porosity and the at least one dimension of the core sample.

2. The method of claim 1 , wherein removing the background NMR spectrum signal from the first and second NMR spectrum signals comprises:

removing a first background NMR spectrum signal from the first NMR spectrum signal; and

removing a second background NMR spectrum signal from the second NMR spectrum signal.

3. The method of claim 2 , further comprising:

measuring the first background NMR spectrum signal of the cylinder at a vacuum and independent of the test fluid and the rock sample; and

measuring the second background NMR spectrum signal of the cylinder with the rock sample at a vacuum and independent of the test fluid.

4. The method of claim 2 , wherein removing the first background NMR spectrum signal from the first NMR spectrum signal comprises subtracting an amplitude of the first background NMR spectrum signal from an amplitude of the first NMR spectrum signal across a chemical shift; and

removing the second background NMR spectrum signal from the second NMR spectrum signal comprises subtracting an amplitude of the second background NMR spectrum signal from an amplitude of the second NMR spectrum signal across the chemical shift.

5. The method of claim 1 , wherein the at least one dimension of the core sample comprises a cross-sectional radius or diameter of the core sample, and the at least one dimension of the cylinder comprises a cross-sectional radius or diameter of an inner volume of the cylinder.

6. The method of claim 5 , wherein determining the fluid intake capacity of the core sample based at least in part on the porosity and the at least one dimension of the core sample comprises:

determining the fluid intake capacity of the core sample based on the porosity, the cross-sectional radius or diameter of the core sample, and a length of the core sample.

7. The method of claim 6 , wherein the core sample is cylindrical.

8. The method of claim 1 , wherein the test fluid comprises methane, and the NMR spectrum signal comprises a 1 H or a 13 C signal.

9. The method of claim 1 , wherein the test fluid comprises carbon dioxide, and the NMR spectrum signal comprises a 13 C signal.

10. A system for determining at least one rock property of a core sample, comprising:

a nuclear magnetic resonance (NMR) pressure cell, comprising:

a test cylinder configured to hold a test fluid and a core sample; and

an NMR coil positioned around the test cylinder and configured to measure NMR spectrum signals of the test cylinder;

a control system communicably coupled to the NMR coil and configured to perform operations comprising:

measuring a first nuclear magnetic resonance (NMR) spectrum signal of the test fluid enclosed at a particular pressure in the test cylinder;

measuring a second NMR spectrum signal of the core sample immersed in the test fluid that is enclosed at the particular pressure in the test cylinder;

removing a background NMR spectrum signal from the first and second NMR spectrum signals to determine a bulk test fluid NMR spectrum signal and a combined test fluid and core sample NMR spectrum signal;

determining a porosity of the core sample based on the bulk test fluid NMR spectrum signal, the combined test fluid and core sample NMR spectrum signal, at least one dimension of the core sample, and at least one dimension of the test cylinder; and

determining a fluid intake capacity of the core sample based at least in part on the porosity and the at least one dimension of the core sample.

11. The system of claim 10 , wherein the operation of removing the background NMR spectrum signal from the first and second NMR spectrum signals comprises:

removing a first background NMR spectrum signal from the first NMR spectrum signal; and

removing a second background NMR spectrum signal from the second NMR spectrum signal.

12. The system of claim 11 , wherein the control system is configured to perform operations comprising:

measuring the first background NMR spectrum signal of the cylinder at a vacuum and independent of the test fluid and the rock sample; and

measuring the second background NMR spectrum signal of the cylinder with the rock sample at a vacuum and independent of the test fluid.

13. The system of claim 11 , wherein the operation of removing the first background NMR spectrum signal from the first NMR spectrum signal comprises subtracting an amplitude of the first background NMR spectrum signal from an amplitude of the first NMR spectrum signal across a chemical shift; and

the operation of removing the second background NMR spectrum signal from the second NMR spectrum signal comprises subtracting an amplitude of the second background NMR spectrum signal from an amplitude of the second NMR spectrum signal across the chemical shift.

14. The system of claim 10 , wherein the at least one dimension of the core sample comprises a cross-sectional radius or diameter of the core sample, and the at least one dimension of the cylinder comprises a cross-sectional radius or diameter of an inner volume of the cylinder.

15. The system of claim 14 , wherein the operation of determining the fluid intake capacity of the core sample based at least in part on the porosity and the at least one dimension of the core sample comprises:

determining the fluid intake capacity of the core sample based on the porosity, the cross-sectional radius or diameter of the core sample, and a length of the core sample.

16. The system of claim 15 , wherein the core sample is cylindrical.

17. The system of claim 10 , wherein the test fluid comprises methane, and the NMR spectrum signal comprises a 1 H or a 13 C signal.

18. The system of claim 10 , wherein the test fluid comprises carbon dioxide, and the NMR spectrum signal comprises a 13 C signal.

19. A computer-implemented method for determining at least one rock property of a core sample, comprising:

identifying, with at least one hardware processor, measurements of a first nuclear magnetic resonance (NMR) spectrum signal of a test fluid enclosed at a particular pressure in a cylinder of an NMR pressure cell;

identifying, with the at least one hardware processor, measurements of a second NMR spectrum signal of a core sample immersed in the test fluid that is enclosed at the particular pressure in the cylinder of the NMR pressure cell;

removing, with the at least one hardware processor, a background NMR spectrum signal from the first and second NMR spectrum signals to determine a bulk test fluid NMR spectrum signal and a combined test fluid and core sample NMR spectrum signal;

determining, with the at least one hardware processor, a porosity of the core sample based on the bulk test fluid NMR spectrum signal, the combined test fluid and core sample NMR spectrum signal, at least one dimension of the core sample, and at least one dimension of the cylinder; and

determining, with the at least one hardware processor, a fluid intake capacity of the core sample based at least in part on the porosity and the at least one dimension of the core sample.

20. The computer-implemented method of claim 19 , wherein removing the background NMR spectrum signal from the first and second NMR spectrum signals comprises:

removing, with the at least one hardware processor, a first background NMR spectrum signal from the first NMR spectrum signal; and

removing, with the at least one hardware processor, a second background NMR spectrum signal from the second NMR spectrum signal.

21. The computer-implemented method of claim 20 , further comprising:

identifying, with the at least one hardware processor, measurements of the first background NMR spectrum signal of the cylinder at a vacuum and independent of the test fluid and the rock sample; and

identifying, with the at least one hardware processor, measurements of the second background NMR spectrum signal of the cylinder with the rock sample at a vacuum and independent of the test fluid.

22. The computer-implemented method of claim 20 , wherein removing the first background NMR spectrum signal from the first NMR spectrum signal comprises subtracting, with the at least one hardware processor, an amplitude of the first background NMR spectrum signal from an amplitude of the first NMR spectrum signal across a chemical shift; and

removing the second background NMR spectrum signal from the second NMR spectrum signal comprises subtracting, with the at least one hardware processor, an amplitude of the second background NMR spectrum signal from an amplitude of the second NMR spectrum signal across the chemical shift.

23. The computer-implemented method of claim 19 , wherein the at least one dimension of the core sample comprises a cross-sectional radius or diameter of the core sample, and the at least one dimension of the cylinder comprises a cross-sectional radius or diameter of an inner volume of the cylinder.

24. The computer-implemented method of claim 23 , wherein determining the fluid intake capacity of the core sample based at least in part on the porosity and the at least one dimension of the core sample comprises:

determining, with the at least one hardware processor, the fluid intake capacity of the core sample based on the porosity, the cross-sectional radius or diameter of the core sample, and a length of the core sample.

25. The computer-implemented method of claim 19 , wherein the test fluid comprises methane, and the NMR spectrum signal comprises a 1 H or a 13 C signal.

26. The computer-implemented method of claim 19 , wherein the test fluid comprises carbon dioxide, and the NMR spectrum signal comprises a 13 C signal.