Method and apparatus for a high resolution downhole spectrometer

Abstract

The present invention provides a simple, robust, and versatile high-resolution spectrometer that is suitable for downhole use. The present invention provides a method and apparatus incorporating a spinning, oscillating or stepping optical interference filter to change the angle at which light passes through the filters after passing through a sample under analysis downhole. As each filter is tilted, the color or wavelength of light passed by the filter changes. Black plates are placed between the filters to isolate each filter's photodiode. The spectrometer of the present invention is suitable for use with a wire line formation tester, such as the Baker Atlas Reservation Characterization Instrument to provide supplemental analysis and monitoring of sample clean up. The present invention is also suitable for deployment in a monitoring while drilling environment. The present invention provides a high resolution spectometer which enables quantification of a crude oil's percentage of aromatics, olefins, and saturates to estimate a sample's gas oil ratio (GOR). Gases such as CO2 are also detectable. The percentage of oil-based mud filtrate contamination in a crude oil sample can be estimated with the present invention by using a suitable training set and chemometrics, a neural network, or other type of correlation method.

Description

[0001] This application is related to U.S. patent application No. not presently assigned, entitled "A Method and Apparatus for a Downhole Flourescence Spectrometer" by Rocco DiFoggio, Paul Bergen and Arnold Walkow, filed on Jun. 4, 2002 which is hereby incorporated herein by reference in its entirety. This application is related to U.S. patent application No. not presently assigned, entitled "A Method and Apparatus for a Derivative Spectrometer" by Rocco DiFoggio, Paul Bergen and Arnold Walkow, filed on Jun. 4, 2002 which is hereby incorporated herein by reference in its entirety. This application is related to the U.S. patent application Ser. No. 10 / 119,492 filed on Apr. 10, 2002 by Rocco DiFoggio et al., entitled "A Method and Apparatus for Downhole Refractometer And Attenuated Reflectance Spectrometer" which is hereby incorporated herein by reference in its entirety.[0002] 1. Field of the Invention[0003] The invention pertains in general to a high-resolution downhole scanning spe...

AI Technical Summary

Benefits of technology

[0009] Photodetectors and their amplifiers almost always have some thermal noise and drift, which limit the accuracy of a spectral reading. As operating temperature increases, noise and drift increase dramatically 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, centered at the oscillation frequency. The operator can further reject noise by using a phase-sensitive ("lock-in") amplifier that not only rejects signals having the wrong frequency but also rejects signals having the correct frequency but having no 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.sup.100 db / 10 or 10 billion.

Problems solved by technology

Contaminated sample yield invalid results when trying to determine the properties of a hydrocarbon bearing formation.

They do not want to pump unnecessarily long and waste very expensive rig time.

If the contamination is more than about 10%, the sample may be useless.

In such cases, the PVT properties measured in the lab cannot be corrected back to true reservoir conditions because of this excessive contamination.

In the down-hole environment, photodetectors are utilized and must operate at high ambient temperatures, thus, they are very noisy and generate very little signal.

Moreover, dirty or contaminated samples of flowing streams of crude oil containing scatterers such as sand particles or gas bubbles tend to add noise to the system.

Photodetectors and their amplifiers almost always have some thermal noise and drift, which limit the accuracy of a spectral reading.

As operating temperature increases, noise and drift increase dramatically at the same time that photodetector signal becomes significantly weaker.

Button et al. does not enable the possibility of a full rotation or control of the angular deviation.

The down-hole environment is difficult for operating sensors.

Reasons include limited space within a tool's pressure housing, elevated temperatures, and the need to withstand shock and vibration.

These epoxy replicas soften and creep at high temperature, which causes a distortion in the spectrum and a loss in light intensity.

Moreover, the price of an all-glass master grating ($50-100K) is prohibitively expensive.

Known laboratory spectrometers typically utilize Fourier Transform (FT) spectroscopy which are unsuitable for down-hole applications.

FT spectrometers are large (2-3 feet long, 1-2 feet wide), heavy (200-300 lbs), mechanically and electronically complicated, and must maintain perfect alignment of all their optical components to work properly, which is why they are typically built on a very rigid framework.

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