Dual Path Length Optical Cell for Downhole Fluid Analysis
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Solution Overview
Problem
Existing downhole fluid optical analysis systems face challenges in accurately estimating fluid properties due to issues like reflection differences at the window-fluid interface, variability in optical fiber positioning, and the need for precise empty-cell reference spectra, which can be affected by environmental changes and spectrometer aging.
Innovation Solution
A dual path length optical cell system is introduced, utilizing a moveable optically transmissive member, such as a sapphire plate, between two sapphire windows to create a shorter optical path, allowing for self-referencing and minimizing environmental variations, while maintaining accurate absorbance measurements by using the ratio of light transmission through both paths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single optical path is used for fluid analysis, then the device structure is simple, but measurement precision deteriorates due to reflection errors and path length variations under pressure
Solution Approach 1:
The optical cell is segmented into two distinct optical paths of different lengths (first optical path and second optical path). This segmentation allows independent measurement of light transmission through different path lengths, enabling the system to calculate and compensate for reflection errors and path length variations, thereby improving measurement precision without requiring a completely redesign of the optical cell structure.
Solution Approach 2:
The system changes the optical path length parameter by providing two different path lengths through the fluid sample. By measuring light transmission at two different path lengths and using the ratio of these measurements, the system can eliminate the effects of reflection errors and pressure-induced path length changes, improving absorbance measurement accuracy while maintaining a relatively simple optical cell design.
2Reliability
If empty-cell reference spectra are used for calibration, then measurement precision can be maintained, but reliability deteriorates due to environmental changes and spectrometer aging
Solution Approach 1:
The system performs self-calibration using the two optical paths themselves as references. By measuring light transmission through the fluid at two different path lengths and calculating the ratio, the system inherently compensates for environmental changes and instrument drift without requiring external empty-cell reference spectra. This self-service approach maintains both reliability and measurement precision.
Solution Approach 2:
The system uses feedback from the dual-path length measurements to continuously correct for environmental variations and spectrometer aging. The ratio of light transmission at the two different path lengths provides real-time feedback that compensates for drift in the optical system, maintaining reliable and accurate fluid property estimates without requiring periodic recalibration with empty-cell references.
3Measurement precision
If a longer optical path is used, then absorbance measurement sensitivity is improved, but measurement precision deteriorates due to excessive light absorption and signal loss
Solution Approach 1:
The system changes the optical path length parameter by providing two different path lengths. The shorter path length ensures sufficient light transmission intensity by preventing excessive absorption, while the longer path length provides measurement sensitivity. By using the ratio of measurements at the two path lengths, the system achieves both adequate signal intensity and precise fluid property detection.
Solution Approach 2:
The system uses a shorter optical path length than would be ideal for maximum sensitivity, but this partial action ensures sufficient light transmission intensity. The dual-path approach compensates for the reduced sensitivity by using the ratio of the two path lengths to calculate absorbance, ultimately achieving both adequate signal intensity and precise measurement.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces reflection errors, compensates for path length changes under pressure, and eliminates the need for separate empty-cell reference spectra, providing reproducible and accurate fluid property estimates, including gas-oil ratio and API gravity, directly downhole.
Implementation Method 1
interspersing an optically transmissive member into a first optical path through the fluid, thereby creating a second shorter optical path through the fluid
Implementation Method 2
measuring an intensity of light, I1 transmitted through the first optical path; measuring an intensity of light, I2 transmitted through the second optical path; and estimating an optical property for the second optical path from the ratio, I1/I2
Data Source
AI summary
A method is disclosed for changing an optical path length through a fluid downhole, comprising interspersing an optically transmissive member into a first optical path through the fluid, thereby creating a second shorter optical path through the fluid. In another embodiment, the method further comprises measuring an intensity of light, I1 transmitted through the first optical path; measuring an intensity of light, I2 transmitted through the second optical path; and estimating an optical property for the second optical path from the ratio, I1/I2. A system is disclosed for changing an optical path length through a fluid downhole, comprising a fluid passage between two optically transmissive windows for the fluid downhole, the fluid passage having a first optical path through the fluid; and an optically transmissive member for insertion into the first optical path, thereby creating a second shorter optical path through the fluid.