Optics: measuring and testing – Refraction testing
Reexamination Certificate
2002-04-10
2004-01-27
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Refraction testing
C356S133000, C250S577000
Reexamination Certificate
active
06683681
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to refractometry and spectrometry in a wellbore environment. Specifically, it pertains to a robust apparatus and method for measuring refractive index of fluids along a continuum (rather than in steps), of measuring attenuated reflectance spectra, and to the interpretation of measurements made with this apparatus to determine a variety of formation fluid parameters. The refractometer and attenuated reflectance spectrometer disclosed here uses a simplified design, which is very appropriate for a downhole environment.
BACKGROUND OF THE INVENTION
Oil and gas companies spend large sums of money in their attempts to find hydrocarbon deposits. They drill exploration wells in their most promising prospects and use these exploration wells not only to determine whether hydrocarbons are present but also to determine the properties of those hydrocarbons, which are present.
For deep offshore fields, before any hydrocarbons can be produced, it is first necessary to spend several years building very expensive platforms with proper oil and gas handling facilities. The design specifications and cost of materials used in these facilities are strongly dependent on the properties of the hydrocarbons, such as gas to oil ratio, viscosity, bubble point pressure, asphaltene precipitation pressure, and so on. The exploration well itself is generally plugged and abandoned not long after it is drilled. However, the information that it provides is often used throughout the life of the oil or gas field.
To determine hydrocarbon properties, oil and gas companies often withdraw some hydrocarbons from the exploration well. Wireline formation testers, such as the Baker Atlas Reservoir Characterization Instrument (RCI) can be lowered into the well for this purpose.
Initially, fluids that are withdrawn may be highly contaminated by filtrates of the fluids (“muds”) that were used during drilling. To obtain samples that are sufficiently clean (usually <10% contamination) so that the sample will provide meaningful lab data concerning the formation, formation fluids are generally pumped from the wellbore for 30-90 minutes, while clean up is being monitored in real time. Then, these withdrawn fluids can be collected downhole in tanks for subsequent analysis in a laboratory at the surface.
Alternatively, for some properties, samples can be analyzed downhole in real time. The present invention relates both to monitoring sample clean up and to performing downhole analysis of samples at reservoir conditions of temperature and pressure.
A downhole environment is a difficult one in which to operate a sensor. Measuring instruments in the downhole environment must operate under extreme conditions and limited space within a tool's pressure housing, including elevated temperatures, vibration, and shock.
U.S. Pat. No. 5,167,149 by Mullins et al. and U.S. Pat. No. 5,201,220 by Mullins et al., are both entitled Apparatus and Method for Detecting the Presence of Gas in a Borehole Flow Stream. The Mullins apparatus of that invention comprises a downhole 8-channel critical angle (and Brewster angle) refractometer to distinguish oil from gas and to estimate the percentage of gas in a fluid.
The traditional method of measuring the index of refraction of a dark fluid (such as a crude oil) is the critical angle refractometer. A diverging beam of light travels through a transparent solid (e.g., glass) and strikes the interface between this transparent solid and some fluid to be measured, which is in contact with the transparent solid. The reflected diverging beam is dimmer at those angles, which are close to a normal to the interface. At such angles, some of the light is transmitted (refracted) into the fluid.
The reflected diverging beam is much brighter at glancing angles. Starting at the Brewster angle, any incident p-polarized light suffers no reflection loss. Starting at the critical angle, all light, regardless of polarization, suffers no reflection loss but is 100% reflected from the interface so that no light is transmitted into the fluid.
The critical angle can be calculated from Snell's Law, n
0
sin &thgr;
0
=n
1
sin &thgr;
1
, for light refracted as it travels from medium n
0
to medium n
1
. The maximum possible refracted angle (as measured from the normal to the interface) is 90° so by substituting &thgr;
1
=90° into Snells's Law we can calculate the critical angle, &thgr;
c
=arcsin (n
1
0
).
At the critical angle, we see a large change in reflected intensity (a bright/dark demarcation), which can be located using a single moveable detector or an array of stationary photodetectors. A single moveable detector would add substantial mechanical complications to a downhole design.
Laboratory instruments often use an array of 1024 or more stationary photodetectors to detect the critical angle. However, mimicking the lab design downhole would be difficult because multiplexers built into photodetector arrays generally do not work above about 95 C. Even with separate high-temperature multiplexers, multiplexing so many very weak signals at the elevated temperatures encountered downhole would be problematic as they would probably have to be stacked. Therefore, downhole, only a few fixed photodetector elements (e.g., 8) are likely to be used for a critical angle refractometer. Of course, with an 8-channel refractometer, as described in U.S. Pat. Nos. 5,167,149 and 5,201,220 mentioned earlier, the refractive index is measured only in 8 steps rather than as a continuum.
Because such a device only measures refractive index in eight coarse steps, it would be difficult for an operator of this device to monitor sample clean up. Sample clean up refers to the transition from filtrate-contaminated fluid to nearly pure formation fluid while pumping fluid from selected depths in the wellbore.
Accurate sample clean up monitoring cannot be provided by processing a course refractive index reading. Thus, there is a need for a method and apparatus, which can measure refractive index along a continuum so that an operator can accurately monitor the refractive index of a formation sample.
SUMMARY OF THE INVENTION
The present invention provides an continuous refractive index measurement. An advantage to monitoring clean up by using a continuous refractive index measurement is the refractive index is much less sensitive to the passage of sand or other particulates, which can cause sudden spurious increases (“jumps”) in absorbance across the entire spectrum of a downhole transmission spectrometer.
The refractometer of the present invention is less sensitive to particulates because it probes the fluid to a depth of only a few wavelengths of light past the window so it does not see all of the particulates that pass through the 2-mm pathlength cell (
304
). Few particles get within a few wavelengths of light of the window, in part, because there is a coating of fluid around the particles and around the window that is at least a few wavelengths of light.
The present invention does not require measuring the critical angle. Furthermore, it can also be used as an attenuated reflectance spectrometer.
The present invention provides a continuous refractive index measurement and comprises an apparatus and method of simplified refractometer design for durable and accurate operation in a downhole environment. In one aspect of the invention, the present invention provides for novel interpretation of measurements made with the refractometer of the present invention. In another aspect of the invention, the present invention provides a method and apparatus to distinguish between gas and liquid based on the much lower index of refraction of gas. In another aspect of the invention, the present invention provides a method for determining the refractive index of a wellbore or formation fluid from the fraction of light, R, reflected off of the interface between a transparent window and the wellbore or formation fluid. In another aspect of the invention, the present invention can be used to
Bergren Paul
DiFoggio Rocco
Walkow Arnold
Baker Hughes Incorporated
Font Frank G.
Madan Mossman & Sriram P.C.
Nguyen Sang H.
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