Method for estimating insitu fluid viscosity from NMR measurements

1. A method for estimating a viscosity of a fluid in a rock formation, the method comprising:

performing a first nuclear magnetic resonance (NMR) measurement with zero magnetic field gradient on at least a portion of a sample of the rock formation containing the fluid in order to obtain a first distribution of transverse relaxation time constants;

estimating a first diffusive couple factor from the first distribution of transverse relaxation time constants, the first diffusive couple factor relating the first distribution of transverse time constants to the viscosity of the fluid and a saturation of the fluid;

replacing the fluid on which the first NMR measurement was performed with a different fluid;

performing a second NMR measurement with zero magnetic field gradient on the at least a portion of the sample containing the different fluid in order to obtain a second distribution of transverse relaxation time constants;

estimating a second diffusive couple factor from the second distribution of transverse relaxation time constants, the second diffusive couple factor relating the second distribution to a viscosity of the different fluid and a saturation of the different fluid; and

estimating the viscosity of the fluid in the rock formation by using the first diffusive couple factor and the second diffusive couple factor.

2. The method of claim 1 , further comprising establishing a non-diffusive porosity distribution reference with at least another portion of the sample of the rock formation containing the fluid.

3. The method of claim 2 , wherein establishing comprises using a mercury injection technique.

4. The method of claim 2 , wherein estimating a first diffusive couple factor comprises normalizing the first transverse relaxation time constant distribution with respect to the non-diffusive porosity distribution reference.

5. The method of claim 2 , wherein estimating a second diffusive couple factor comprises normalizing the second transverse relaxation time constant distribution with respect to the non-diffusive porosity distribution reference.

6. The method of claim 2 , wherein one sample of the rock formation is used for establishing the non-diffusive reference and another sample of the rock formation is used for the first and second NMR measurements.

7. The method of claim 2 , further comprising determining a pore size distribution for the at least a portion of the sample by applying the first diffusive couple factor to normalize the first transverse relaxation time constant distribution to the non-diffusive porosity distribution.

8. The method of claim 7 , further comprising using a correction factor to account for a reduction in effective pore diameters due to additional interfacial tension forces in interfaces between fluids with different wetting characteristics.

9. The method of claim 1 , wherein the first and second NMR measurements are performed with an inter-echo time T e of about 0.6 milliseconds.

10. The method of claim 1 , wherein replacing comprises:

cooling the fluid in the at least a portion of the sample;

leaching the fluid from the at least a portion of the sample;

drying the at least a portion of the sample; and

vacuuming the at least a portion of the sample with the different fluid.

11. The method of claim 1 , wherein the different fluid is brine.

12. The method of claim 1 , wherein the second NMR measurement is performed with the at least a portion of the sample at substantially the same pressure and temperature as when the first NMR measurement was performed.

13. The method of claim 1 , wherein the first diffusive couple factor comprises an exponent to which an inverse of a transverse relaxation time constant measured in the first NMR measurement is raised.

14. The method of claim 1 , wherein the second diffusive couple factor comprises an exponent to which an inverse of a transverse relaxation time constant measured in the second NMR measurement is raised.

15. An apparatus configured for estimating a viscosity of a fluid in a rock formation, the apparatus comprising:

an electronics unit configured for:

estimating a first diffusive couple factor from a first distribution of transverse relaxation time constants obtained from at least one sample of the rock formation containing the fluid, the first diffusive couple factor relating the first distribution of transverse relaxation time constants to the viscosity of the fluid and a saturation of the fluid;

estimating a second diffusive couple factor from a second distribution of transverse relaxation time constants obtained from the at least one sample with the fluid on which the first distribution of transverse relaxation time constants was obtained replaced by a different fluid, the second diffusive couple factor relating the second distribution of transverse relaxation time constants to a viscosity of the different fluid and a saturation of the different fluid; and

estimating the viscosity of the fluid in the rock formation using the first diffusive couple factor and the second diffusive couple factor;

wherein the first distribution and the second distribution are obtained using a zero magnetic field gradient.

16. The apparatus of claim 15 , wherein the electronics unit is further configured for receiving a non-diffusive porosity distribution reference.

17. The apparatus of claim 15 , further comprising a downhole tool configured for taking the at least one sample and an instrument configured for performing nuclear magnetic resonance (NMR) measurements (NMR) in order to provide the first distribution of transverse relaxation time constants and the second distribution of transverse relaxation time constants.

18. The apparatus of claim 15 , wherein the downhole tool comprises the instrument and the electronics unit.

19. A non-transitory computer readable storage medium comprising machine executable instructions estimating a viscosity of a fluid in a rock formation, by implementing a method comprising:

estimating a first diffusive couple factor from a first distribution of transverse relaxation time constants obtained from at least one sample of the rock formation, the first distribution obtained using a zero magnetic field gradient, the first diffusive couple factor relating the first distribution of transverse relaxation time constants to the viscosity of the fluid and a saturation of the fluid;

determining a second diffusive couple factor from a second distribution of transverse relaxation time constants obtained from the at least one sample with the fluid on which the first distribution of transverse relaxation time constants was obtained replaced by a different fluid, the second distribution obtained using a zero magnetic field gradient, the second diffusive couple factor relating the second distribution of transverse relaxation time constants to a viscosity of the different fluid and a saturation of the different fluid;

estimating the viscosity of the fluid in the rock formation using the first diffusive couple factor and the second diffusive couple factor; and

at least one of recording the viscosity and displaying the viscosity to a user.