Oscillating path length spectrometer

1. A method of evaluating a fluid, the method comprising:

transmitting a light beam through the fluid to a detector while oscillating a path length traveled through the fluid by the light beam at a first frequency of oscillation;

measuring a time-dependent intensity of incident light at the detector responsive to an interaction of the light beam with the fluid to produce a time-dependent intensity signal;

filtering the time dependent intensity signal to recover a path-dependent signal oscillating at the first frequency and indicative of an absorbance property of the fluid; and

estimating a parameter of interest of the fluid using the path-dependent signal.

2. The method of claim 1 , wherein the time-dependent intensity is indicative of a decrease in intensity between the light beam and the incident light.

3. The method of claim 2 , wherein:

the time-dependent intensity is indicative of a difference in intensity caused by absorbance and a difference in intensity caused by non-absorbance related optical effects; and

the difference in intensity caused by absorbance is represented by the path-dependent signal.

4. The method of claim 1 , comprising filtering the time-dependent intensity signal using at least one of: i) a band-pass filter centered on a second frequency substantially the same as the first frequency; and ii) a phase-sensitive lock-in amplifier.

5. The method of claim 1 , wherein the parameter of interest comprises at least one of: i) an oil fraction of the fluid; ii) a water fraction of the fluid; iii) a gas fraction of the fluid; and iv) a composition of the fluid.

6. The method of claim 1 , wherein the path length is oscillated between a minimum path length and a maximum path length, the minimum path length corresponds to a maximum in the path-dependent signal, and the maximum path length corresponds to a minimum in the path-dependent signal.

7. The method of claim 6 , estimating the parameter of interest by using a ratio of a first intensity of the minimum in the path-dependent signal to a second intensity of the maximum in the path-dependent signal to estimate an absorbance of the fluid.

8. The method of claim 6 , wherein the difference between the minimum path length and the maximum path length is a non-zero value less than one millimeter.

9. The method of claim 1 , wherein at least one of i) transmitting the light beam through the fluid, and ii) measuring the time-dependent intensity of incident light at the detector, is carried out using an optical fiber immersed in the fluid.

10. The method of claim 1 , wherein oscillating the path length comprises moving at least one of a source of the light beam and the detector along a light beam axis with respect to the other of the source and the detector.

11. The method of claim 1 , comprising estimating the parameter of interest using spectral information from the incident light.

12. The method of claim 1 , wherein the downhole fluid is flowing.

13. The method of claim 1 , comprising estimating an absorbance from the path-dependent signal, and using the absorbance to estimate the parameter of interest.

14. The method of claim 1 , wherein the fluid is a highly scattering mixture.

15. An apparatus for evaluating a fluid, the apparatus comprising:

a spectral detector;

a light source configured to transmit a light beam through the fluid to the detector;

an actuator configured to oscillate a path length traveled through the fluid by the light beam at a first frequency of oscillation; and

at least one processor configured to:

measure a time-dependent intensity of incident light at the detector responsive to an interaction of the light beam with the fluid to produce a time-dependent intensity signal;

filter the time dependent intensity signal to recover a path-dependent signal oscillating at the first frequency and indicative of an absorbance property of the fluid; and

estimate a parameter of interest of the fluid using the path-dependent signal.