Optics: measuring and testing – By shade or color – With color transmitting filter
Reexamination Certificate
2002-06-04
2004-09-28
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By shade or color
With color transmitting filter
C356S419000, C250S339090
Reexamination Certificate
active
06798518
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of downhole sampling and in particular to derivative spectrometry in a downhole environment.
2. Summary of the Related Art
Oil companies take samples from potential hydrocarbon-bearing formations to determine a formation's propensity to produce hydrocarbons. Oil companies desire the most accurate measure of sample contamination percentage in real time as they are pumping fluid from a formation so that they can decide when to divert a sample being pumped to a sample collection tank. As formation fluid is pumped from the formation, the percentage of filtrate contained in the formation fluid sample diminishes in the pumped fluid. Thus, an oil company typically pumps until a pure sample, relatively free of filtrate can be obtained in order to accurately appraise the hydrocarbon producing potential of the surrounding formation. The oil company does not, however, want to pump unnecessarily long and waste very expensive rig time. Conversely, they do not want to pump too little and collect a useless sample, which is full of contaminants and does not reflect the properties of the formation fluid. If the contamination of filtrate contained in the sample is more than about 10%, the sample may be useless for its intended purpose. Moreover, it may not be discovered that the sample is useless until the sample is retrieved at the surface, making a return trip downhole necessary to collect another sample. In such cases, the PVT properties indicative of formation and formation fluid properties that are measured in the lab cannot be corrected back to true reservoir conditions because of excessive contamination. It is therefore desirable to perform sample contamination measurements downhole. One method of investigation is to use a spectrometer to perform optical measurements on the fluid samples collected in a downhole environment.
Numerous factors can affect downhole spectrometer measurements. In the downhole environment, photodetectors operate at high ambient temperatures and thus are very noisy and produce a substantially diminished signal. Also, contaminated samples consisting of flowing streams of crude oil containing scatterers such as sand particles or bubbles tend to add noise to the system. These scatterers cause the optical spectrum to momentarily “jump” up (get darker) as they pass through the sample cell. At high concentrations, these scatterers cause the measured spectrum to move or jump constantly. To first order, the effect of the scatters is just a momentary baseline offset. An operator can greatly improve the signal-to-noise ratio of a downhole spectrometer by modulating the wavelength of light and using a lock-in amplifier. Thus, there is a need for a spectrometer that operates in a downhole environment and diminishes the effects of the scatterers and the associated offsets.
Spectrometers typically disperse white light into constituent colors. The resulting rainbow of colors can be projected through a sample to be analyzed and onto a fixed array of photodetectors which sense light projected though the sample. Alternatively, by rotating a dispersive element (i.e. grating, prism), the rainbow can be mechanically scanned past a single photodetector one color at a time. In either case, an operator can obtain a sample's darkness versus wavelength, in other words, the sample's spectrum.
Photodetectors and their amplifiers always have some thermal noise and drift, which limit the accuracy of a spectral reading. As temperature increases, noise and drift increase dramatically higher at the same time that photodetector signal becomes significantly weaker. If an operator oscillates the wavelength (color) of light about some center wavelength, then the operator can reject most photodetector and amplifier noise and drift by using an electronic bandpass filter that passes only that electrical frequency at which the wavelength of light is being oscillated. The operator can further reject noise by using a phase-sensitive (“lock-in”) amplifier that not only rejects signals that have the wrong frequency but also rejects signals that have the correct frequency but do not have a fixed phase relationship (indicative of noise) relative to the wavelength oscillation. A lock-in amplifier can improve signal to noise by as much as 100 db, which is a factor of 10
100 db/10
or 10 billion.
The output of the lock-in amplifier used in this procedure is proportional to the root-mean-square (RMS) amplitude of that portion of the total signal, which is at the same frequency and has a fixed phase relationship relative to the optical frequency being observed. The more that the darkness of the sample changes with color, the larger this RMS value will be. Thus, the output of lock-in amplifier for a system with an oscillating-wavelength input is proportional to the derivative of the spectrum (with respect to wavelength) at the center wavelength of the oscillation.
A spectrometer based on an oscillating-wavelength and a lock-in amplifier can be used to obtain high accuracy spectral measurements as described in the related art below. U.S. Pat. No. 4,070,111 entitled Rapid Scan Spectrophotometer, by Harrick, Jan. 24, 1978 discloses a spectrophotometer capable of rapid spectral scanning by mounting a low inertia reflective grating directly on the output shaft of a galvanometer-type optical scanner, and sweeping the beam dispersed from the grating across a spherical mirror and after reflection there from across a beam exit slit. The invention also describes rapid wavelength switching for a laser spectrometer.
U.S. Pat. No. 4,225,233 Rapid Scan Spectrophotometer, by Ogan Sep. 30, 1980 discloses a spectrometer capable of providing a predetermined wavelength of output light in accordance with a control voltage signal applied to a scanning element. The scanning element located at the grating image of the spectrometer is a small mirror attached to the rotor of a galvanometer. The angular position of the galvanometer is accurately controlled by a closed-loop electronic control. The spectrum reflected from the mirror is passed through a slit to provide the output light of a predetermined wavelength. Selection of the waveform of the control signal allows the spectrometer to be operated as a dual wavelength spectrometer, to use a linear wavelength scan, or other wavelength scan patterns for absorbance analyses of a sample.
U.S. Pat. No. 4,264,205 Rapid Scan Spectral Analysis System Utilizing Higher Order Spectral Reflections Of Holographic Diffraction Gratings, Landa Apr. 28, 1981 And U.S. Pat. No. 4,285,596 Holographic Diffraction Grating System For Rapid Scan Spectral Analysis, Landa, Aug. 25, 1981 discloses an improved optical system for rapid, accurate spectral analysis of the reflectivity or transmissivity of samples. A concave holographic diffraction grating oscillating at high speed provides a rapid scanning of monochromatic light through a spectrum of wavelengths. The rapid scan by the grating enables the reduction of noise error by averaging over a large number of cycles. It also reduces the measurement time and thus prevents sample heating by excessive exposure to light energy. A filter wheel is rotated in the optical path and is synchronous with the grating.
U.S. Pat. No. 4,968,122 Galvanometer Gimbal Mount, Hlousek et. al., Nov. 6, 1990 discloses an improved mounting in which a rotating diffraction grating assembly directly connects the grating to the galvanometer that rotates the grating. The galvanometer is gimbal-mounted on a plate so that its position, and that of the grating, can be adjusted so that the plane of dispersion of the grating passes through a desired point when the grating is rotated.
U.S. Pat. No. 4,969,739 Spectrometer With Direct Drive High Speed Oscillating Grating, McGee, Nov. 13, 1990 discloses an optical grating oscillating at a high rate to scan a narrow wavelength band of light through the spectrum dispersed by a grating. The grating is connected integrally with the roto
Bergren Paul
DiFoggio Rocco
Walkow Arnold
Baker Hughes Incorporated
Evans F. L.
Madan Mossman & Sriram P.C.
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