Interferometry-based downhole analysis tool
Various systems and methods for performing optical analysis downhole with an interferogram (a light beam having frequency components with a time variation that identifies those frequency components. The interferogram is produced by introducing an interferometer into the light path, with the two arms of the interferometer having a propagation time difference that varies as a function of time. Before or after the interferometer, the light encounters a material to be analyzed, such as a fluid sample from the formation, a borehole fluid sample, a core sample, or a portion of the borehole wall. The spectral characteristics of the material are imprinted on the light beam and can be readily analyzed by processing electronics that perform a Fourier Transform to obtain the spectrum or that enable a comparison with one or more templates. An interferometer designed to perform well in the hostile environments downhole is expected to enable laboratory-quality measurements.
1. A downhole tool that comprises:
a downhole light detector;
a downhole dual-beam interferometer on an optical path between two light sources and the light detector, wherein the interferometer produces, from light received along the optical path, a spectralized beam having a spectral composition with multiple frequency components measurable from time variations of the spectralized beam's intensity;
a window on the optical path that enables light received along the optical path to illuminate a material to be analyzed, wherein the material is a borehole fluid or part of a borehole wall; and
processing electronics coupled to the light detector to detect an electrical signal representing time variations of the spectralized beam's intensity and to determine therefrom the multiple frequency components and a parameter of the material.
2. The tool of claim 1 , wherein the material is a fluid drawn from a port seated against a borehole wall.
3. The tool of claim 2 , wherein the window is on a sample cell that stores the fluid for transport to the surface, and the optical path passes through the fluid.
4. The tool of claim 2 , wherein the parameter is a contamination level for the fluid.
5. The tool of claim 2 , wherein the parameter includes a quantity of at least one fluid type.
6. The tool of claim 2 , wherein the parameter includes a concentration of at least one substance.
7. The tool of claim 2 , wherein the parameter is a size distribution of particles in the fluid.
8. The tool of claim 1 , wherein the parameter is a fluid density or a quantity of at least one fluid type.
9. The tool of claim 1 , wherein the light detector senses light reflected from the material.
10. The tool of claim 1 , wherein the light detector senses light transmitted through the material.
11. The tool of claim 1 , wherein the interferometer includes an integrated light path component comprised of a solid block of a transparent material.
12. The tool of claim 1 , wherein one of the two light sources is a reference light source providing light that traverses at least that portion of the light path that includes the interferometer and the light detector, wherein the processing electronics determine a motion compensation based on measurements of light from the reference light source.
13. The tool of claim 1 , wherein the tool is adapted to be part of at least one of: a wireline logging assembly, a drilling assembly, and a tubing-conveyed logging assembly.
14. The tool of claim 1 , wherein the processing electronics correlate the signal with one or more templates to determine a relative concentration of at least one substance.
15. The downhole tool of claim 1 , wherein the interferometer comprises a fixed mirror and a movable mirror, wherein the movable mirror is movable as a function of time to spectralize the light received along the optical path to produce the spectralized beam as a function of time.
16. The downhole tool of claim 1 , wherein the interferometer produces the spectralized beam using a reciprocating or rotating mirror, and wherein the processing electronics determine the multiple frequency components using mirror position as a function of time.
17. A downhole tool that comprises:
a downhole light detector;
a downhole interferometer on an optical path between a broadband light source and the light detector, wherein the interferometer produces, from light received along the optical path, a spectralized beam having a spectral composition with multiple frequency components measurable from time variations of the spectralized beam's intensity;
a window on the optical path that enables light received along the optical path to illuminate a material to be analyzed, wherein the material is a borehole fluid or part of a borehole wall; and
processing electronics coupled to the light detector to detect an electrical signal representing time variations of the spectralized beam's intensity and to determine therefrom the multiple frequency components and a parameter of the material,
wherein the interferometer includes a spinning retroreflector having a position encoder coupled to the processing electronics.
18. A downhole analysis method that comprises:
directing light from two downhole light sources along an optical path that includes a downhole interferometer and a window for downhole sample illumination, wherein the downhole sample is a borehole fluid or part of a borehole wall;
modulating one arm length of the interferometer as a function of time to produce a spectralized beam before the light reaches a downhole detector, the spectralized beam having a spectral composition with multiple frequency components measurable from time variations of the spectralized beam's intensity;
measuring time variations of the spectralized beam's intensity with the downhole detector; and
using the measured time variations of the spectralized beam's intensity to determine the multiple frequency components and a property of the downhole sample.
19. The method of claim 18 , wherein the downhole sample is a fluid drawn from a port seated against a borehole wall.
20. The method of claim 18 , wherein the property is at least one of; a contamination level, a quantity of at least one fluid type, a concentration of at least one substance, and a size distribution of particles.
21. The method of claim 18 , wherein modulating one arm length of the interferometer as a function of time comprises reciprocating or rotating a mirror, and wherein mirror position as a function of time is used to determine the multiple frequency components.
22. The method of claim 18 , wherein one of the two light sources is a reference light source providing light that traverses at least the interferometer and a light detector, wherein using the measured time variations of the spectralized beam's intensity to determine the multiple frequency components and a property of the downhole sample is based on measurements of light from the reference light source.