NMR contrast logging

1. An apparatus for estimating a property of an earth formation penetrated by a borehole, the apparatus comprising:

a carrier configured to be conveyed through the borehole;

a nuclear magnetic resonance (NMR) instrument disposed at the carrier and configured to perform an NMR measurement on a volume sensitive to the NMR measurement; and

a contrast agent disposed in the volume and comprising particles that form a suspension in a liquid, the suspension of particles being configured increase a magnetic field gradient of at least one earth formation material in the volume to change an NMR relaxation time constant of the at least one earth formation material;

wherein the NMR measurement on the volume containing the at least one earth formation material and the contrast agent is used to estimate the property.

2. The apparatus of claim 1 , wherein the volume in located in the earth formation.

3. The apparatus of claim 2 , further comprising a drilling mud delivery system configured to deliver drilling mud and the contrast agent to the volume in the earth formation.

4. The apparatus of claim 3 , further comprising a contrast agent metering system configured to meter the contrast agent to the drilling mud at a selected ratio of contrast agent to drilling mud.

5. The apparatus of claim 4 , wherein the contrast agent metering system is configured to select from at least a first contrast agent and a second contrast agent to be metered to the drilling mud.

6. The apparatus of claim 4 , wherein the contrast agent metering system comprises at least one of a flow sensor, a metering pump, a control valve, and a controller, each configured to meter the contrast agent at the selected ratio.

7. The apparatus of claim 1 , further comprising a fluid analyzer disposed at the carrier in the borehole and configured to receive a sample of fluid from the earth formation in a sample chamber that includes the volume sensitive to the NMR measurement.

8. The apparatus of claim 7 , wherein the fluid analyzer comprises a contrast agent metering system configured to meter the contrast agent to the sample at a selected ratio of contrast agent to sample.

9. The apparatus of claim 1 , wherein the relaxation time constant change comprises a reduction in the relaxation time constant.

10. The apparatus of claim 1 , wherein the particles comprise a chemical having at least one unpaired electron in an orbital shell.

11. The apparatus of claim 10 , wherein the chemical comprises at least one of Cr3+, Mn2+, Fe3+, Cu2+, Eu3+, Gd3+, and Dy3+.

12. The apparatus of claim 10 , wherein the chemical comprises at least one of superparamagnetic iron oxide-based colloids (SPIO) or ultrasmall paramagnetic iron oxides (USPIO).

13. The apparatus of claim 1 , wherein the particles comprise nanoparticles.

14. The apparatus of claim 1 , wherein the property is at least one of porosity, permeability, invasion profile, fluid composition, and a location of a boundary between layers of the formation.

15. A method for estimating a property of an earth formation penetrated by a borehole, the method comprising:

conveying a carrier through the borehole, a nuclear magnetic resonance (NMR) instrument being disposed at the carrier and configured to perform an NMR measurement on a volume sensitive to the NMR measurement;

providing a contrast agent into the volume, the contrast agent comprising particles that form a suspension in a liquid, the suspension of particles being configured increase a magnetic field gradient of at least one earth formation material in the volume to change an NMR relaxation time constant of the at least one earth formation material;

performing an NMR measurement on at least one earth formation material in the volume containing the contrast agent; and

estimating the property from the performed NMR measurement.

16. The method of claim 15 , further comprising performing another NMR measurement on the at least one earth formation material in the volume not having the contrast agent and estimating the property from the NMR measurement and the another NMR measurement.

17. The method of claim 15 , wherein the volume sensitive to the NMR measurement is in a sample chamber disposed in a fluid analyzer comprising the NMR instrument.

18. A method for estimating vug-porosity of an earth formation penetrated by a borehole, the method comprising:

conveying a carrier through the borehole a first time, a nuclear magnetic resonance (NMR) instrument being disposed at the carrier and configured to perform an NMR measurement on a volume sensitive to the NMR measurement;

performing a first NMR measurement on at least one earth formation material in the volume;

conveying a carrier through the borehole a second time, a nuclear magnetic resonance (NMR) instrument being disposed at the carrier and configured to perform an NMR measurement on a volume sensitive to the NMR measurement;

providing a contrast agent into the volume, the contrast agent being configured to change an NMR relaxation time constant of at least one material in the volume;

performing a second NMR measurement on the at least one earth formation material in the volume containing the contrast agent; and

estimating the vug-porosity using the first NMR measurement and the second NMR measurement.

19. The method of claim 18 , wherein the contrast agent comprises at least one of an aqueous solution comprising a paramagnetic agent and particles that form a suspension in a liquid, the suspension of particles being configured increase a magnetic field gradient of the at least one earth formation material in the volume.

20. The method of claim 19 , wherein the paramagnetic agent comprises at least one of Gd-EDTA, iron sulphate and MnCl 2 .

21. An apparatus for estimating a property of a fluid in an earth formation penetrated by a borehole, the apparatus comprising:

a carrier configured to be conveyed through the borehole;

a fluid analyzer disposed at the carrier and configured to receive a sample of a fluid from the earth formation, the fluid analyzer comprising a nuclear magnetic resonance (NMR) instrument configured to perform an NMR measurement on the sample; and

a contrast agent configured to be mixed with the sample to change the relaxation time of at least one material in the sample;

wherein the NMR measurement is used to estimate the property.

22. The apparatus of claim 21 , wherein the contrast agent comprises at least one of an aqueous solution comprising a paramagnetic agent and particles that form a suspension in a liquid, the suspension of particles being configured increase a magnetic field gradient of the at least one material in the sample.