Various types of photodetectors are known and used in the art. Of these, some materials, such as germanium (Ge), Indium-gallium-arsenide (InGaAs), indium-antimony (InSb), etc., can be used for near-infra red (NIR) photodetectors. An example of an NIR photodetector is the PD-24 photodiode manufactured by IBSG Co. Ltd.
As shown in
Due to these operating parameters, the PD-24 photodiode is typically cooled to maintain its temperature to within recommended operational temperatures. For example, the PD24-TEC package shown in
Various types of sensors are used in downhole tools to test the formation, analyze fluids, and perform other operations. Because the downhole environment can involve high temperatures, harsh chemicals, vibrations, and other extreme conditions, the downhole tool and sensors must be designed to handle problems resulting from such conditions. In some cases, any sensitive electronics must be independently cooled to be able to operate in the high downhole temperatures. Added to all of these difficulties, the downhole environment has limited space into which the downhole tool and sensors must fit.
In some implementations, NIR photodetectors are used in a downhole tool to measure optical absorption spectra or other optical characteristics of downhole fluids. For example, NIR photodetectors can be used in spectrometers to identify downhole fluid (e.g., oil, water, and gas phase), to quantify filtrate contamination, and to determine hydrocarbon composition (e.g., amount of methane, ethane, propane) and the gas-to-oil ratio (GOR).
Using NIR photodetectors downhole can be problematic because the NIR photodetectors can experience elevated dark currents due to the high thermal conditions in the downhole environment. The rated operational temperatures for NIR photodetectors is typically far lower than temperatures of above 125° C. experienced during downhole operation. As is known, cooling photodetectors downhole can be considerably complicated because the downhole tool has a limited power budget that makes any current/power used for cooling problematic. For these reasons, what is needed is a NIR photodetector that can deal with these problems without requiring significant complexity and cooling.
A downhole tool includes a measurement device for in-situ sampling and analysis of fluids in a wellbore. The measurement device includes at least one spectral source of light for interacting with a sample of downhole fluid and includes at least one detector for detecting the spectral source of light after interaction with the fluid.
The photodetector can have a near infra-red photodiode for measuring incident light in the near infra-red region. The NIR photodetector is operated in the high temperature downhole environment without cooling to the standard operating temperature of the photo detector. The photodetector may include a photodiode having a quaternary material of AlGaAsSb, which meets the requirements for downhole implementation based almost solely on the properties of the quaternary material itself.
Analysis of high temperature properties of quaternary photodiodes shows that the responsivity curve for the detector red-shifts, i.e., shifts towards longer wavelengths, with increase in temperature. This advantageously provides detection of spectral sources at and above 2400-nm. At lower temperatures, the detector can be operated in photoconductive mode since the shunt resistance is high enough to maintain low dark current with an applied reverse bias. At elevated temperatures, the detector performance is improved by operating in photovoltaic mode (with no reverse bias) to reduce dark currents. In both photovoltaic and photoconductive mode, the detector is connected to a standard trans-impedance amplifier circuit.
The magnitude of the signal generated by the photodiode reduces with increasing temperature. At the same time, the magnitude of noise voltage at the output of the trans-impedance amplifier increases with the increasing temperature due to the shunt resistance dropping with increasing temperature. The photodiode's signal is independent of the wavelength and modulation frequency of the incident light so long as the gain bandwidth of the transimpediance amplifier circuit is sufficient. At constant temperature, the photodiode's signal is also constant across various samples of the same material. Responsivity generally decreases with increase in temperature. For example, at 200° C. the responsivity is approximately 30% of the responsivity at 100° C.
The photodiode exhibits stable operation when exposed to long periods of high temperature. The measurement device can be adapted to provide cooling for downhole temperatures exceeding a predetermined value. For example, cooling can be activated for temperatures exceeding 125° C.
A. Downhole Tool
A downhole tool 10 in
As shown in
A pump 20 then pumps collected fluid from the probe 12 into the tool 10 via the flow lines 24/25. The fluid, which can contain hydrocarbon components (liquid and/or gas) as well as drilling mud filtrate or other contaminants, flows through the tool 10, and various instruments and sensors in the tool 10 analyze the fluid. For example, a measurement section 14 can have sensors that measure various physical parameters (i.e., pressure, temperature, etc.) of the fluid, and the measurement device 30 in the fluid analysis section 16 can determine physical and chemical properties of oil, water, and gas constituents of the fluid downhole. Eventually, fluid directed via the flow lines 24/25 can either be purged to the annulus or can be directed to the sample carrier 18 where the samples can be retained for additional analysis at the surface.
Additional components 22 of the tool 10 can hydraulically operate valves and other elements within the tool 10, can provide control and power to various electronics, and can communicate data via wireline or fluid telemetry to the surface. Uphole, surface equipment 28 can have a surface telemetry unit (not shown) to communicate with the downhole tool's telemetry components. The surface equipment 28 can also have a surface processor (not shown) that performs additional processing of the data measured by the tool 10.
As noted above, the fluid analysis section 16 uses the measurement device 30 for downhole fluid analysis. Depending on the configuration and types of sources and photodetectors used and their orientation relative to a sample, the measurement device 30 can operate as a photometric analyzer, reflectometer, spectroscope, spectrophotometer, spectrometer, fluorimeter, or the like. For example, the measurement device 30 can operate as a multi-channel photometric analyzer in which discrete wavelengths are interrogated over a given measurement range. In common oil field usage, such a multi-channel photometric analyzer can be referred to as a spectrometer. Thus, the measurement device 30 can use various discrete spectral channels to perform spectroscopic analysis of downhole fluid passing relative to it as the fluid is pumped through the tool 10.
As such, the spectroscopic analysis discussed herein can include, but may not be limited to, analysis of transmission, absorbance, fluorescence, or reflectance spectra, upon which chemometrics, derivative spectroscopy, and other techniques known in the art can be applied.
Although shown used in the formation tester tool 10, the measurement device 30 can be deployed in any suitable tool used for wireline formation testing, production logging, Logging While Drilling/Measurement While Drilling (LWD/MWD), or other operations. Therefore, the downhole tool 10 can be a wireline formation tester, a drilling formation tester, a production logging tool, or other temporary, permanent, or semi-permanent tool to take fluids from the borehole.
B. Measurement Device For Downhole Tool
As schematically shown in
When operated, the source assembly 40 generates spectral signals partitioned into two channels—a reference channel 50 and a measurement channel 60. The reference channel 50 travels directly to the detector assembly 100. The measurement channel 60, however, interacts with a sample fluid via the sample assembly 70 and then travels to the detector assembly 100. In turn, the detector assembly 100 includes a reference detector unit 110 for the reference channel 50, a measurement detector unit 120 for the measurement channel, and control circuitry 130 coupled to these units 110/120.
Each detector unit 110/120 has dual photodetector 112A-B/122A-B for detecting two beams or bands of spectral energy from their respective channels 50/60. For example, first photodetector 112A/122A can include photodiodes capable of sensing in the near infra-red (NIR) spectrum, while second photodetector 112B/122B can be photodiodes capable of sensing in the ultraviolet (UV)/visible (Vis) spectral ranges, although other spectral ranges could be used. In combination, each dual band detector unit 110/120 can detect a wavelength range of about 350 to about 2400-nm, for example.
Inside each unit 110/120, a high pass beam splitter 116/126 splits the incoming channel (50/60) into a first (NIR) band and a second (UV-Vis) band by reflecting all wavelengths shorter than a cutoff wavelength and by passing all longer wavelengths. The cutoff wavelength of the splitter 116/126 can be between 800 and 1200-nm.
Once the channels (50/60) are split into bands, the first (NIR) photodetectors 112A/122A detect the first isolated bands passing through the splitters 116/126. These first (NIR) photodetectors 112A/122A can include quaternary photodiodes used for sensing the NIR wavelength range, for example. For their part, the second (VIS) photodetectors 112B/122B detect the second isolated bands from the splitters 116/126. These second (VIS) photodetectors 112B/122B can be silicon-based photodiodes used for sensing the visible and/or ultraviolet wavelength range, for example.
After detection, the control circuitry 130 coupled to each of the photodetectors 112A-B/122A-B interrogates the photodetectors' responses for processing and analysis. Additional details regarding the detector assembly 100 and its operation can be found in U.S. patent application Ser. No. 12/613,808, filed on Nov. 6, 2009, entitled “Multi-channel Detector Assembly for Downhole Spectroscopy;” which is incorporated herein by reference in its entirety.
As noted above, the optical photodetectors 112A/122A can use NIR photodiodes composed of a quaternary material. In
The header 153 can preferentially be a TO-18 type of header, although any other standard TO package available commercially or a custom designed package could be used. High temperature solder or conductive epoxy can be used to affix the photodiode die 160 to the header 153. The cap 154 attaches to the header 153, and the optically transparent window 155 attaches to the cap 154. In all, the packaging of the photodiode die 160 in the photodetector 150 can be designed for high temperature operation. Likewise, any electronic components (i.e., resistors, op-amps, etc.) associated with the photodetector 150 preferably have very high precision or tolerance values.
Contrary to the standard practices in the art and the standard temperature ratings for such a quaternary photodiode, the photodiode die 160 of the NIR photodetector 150 of
1. Device Without Cooling
In one embodiment of the detector assembly 100 of
2. Device With Cooling
In another embodiment of the detector assembly 100 of
The selection of the predetermined temperature can be based on the magnitude of noise that can be tolerated during downhole operation at elevated temperatures. As discussed below, for example,
Due to the complexities of cooling in a downhole environment associated with packaging, power supply, etc. . . . , the cooling apparatus 140 is preferably designed to cool the photodiode die (160) by a preset amount of temperature. For example, the cooling apparatus 140 may cool the photodiode die (160) by 25° C. below a current operating temperature. Therefore, if the operating temperature detected by the device 100 is currently 150° C., the cooling apparatus 140 is activated to reduce the temperature of the photodiode die (160) by about 25° C. to about 125° C.
The cooling apparatus 140 can be a thermal electric cooler (TEC), although other forms of cooling can be used. For example, the cooling apparatus 140 can use sorptive cooling, thermo-tunneling, evaporators, Dewar, etc. As a TEC, the apparatus 140 can use BiTe-based Peltier elements, which require a high temperature solder be used to assemble the Peltier to avoid reflow at operational temperatures. An appropriate solder would be 80/20 AuSn, which reflows at about 280° C. The ceramics forming the top and bottom plates of the TEC for the apparatus 140 may be typically alumina or aluminum nitride. The electrical connections are preferably made using wirebonding to avoid lower temperature solders, but solders such as Sn-3.5% Ag (reflow 221° C.) may be used with the understanding that operational temperatures may be close to reflow temperature so that such lower temperature solders are less desirable.
3. Operation of Device
In the next step 204, a light source, such as one or more spectral sources 42, can be activated to generate spectral signals across a spectral range. For the reference channel 50, the generated spectral signal images onto the photodiode die (160) of the NIR photodetector 112A. For the measurement channel 60, the generated spectral signal first interfaces with the sample in the interface 70 and then images onto the photodiode die (160) of the NIR photodetector 122A (
Once the amplified output of the photodetector 112A/122A is collected, the process 200 extracts the signal, which represents the actual response to the incident light, from the dark current and thermal noise. As discussed below with respect to
For example,
The FFT analysis of the signal can be compared to the FFT analysis of an ideal square wave (shown by indicators 304) of 50 Hz in order to identify noise. Because there is a close match between the measured FFT analysis and the ideal FFT analysis, the exemplary signals of
Returning to
C. Analysis of Photodiode Operation in Downhole Environment
As noted above, the photodetector 150 of
1. Test Setup
Light generated by the light source 403 is fed to the photodetector 150 via a fiber optic cable 406. To mimic the sensing environment for the photodetector's response, a function generator 404 modulates the light source 403 so that the output of the light source 403 can have various waveform patterns, such as a square wave, a sine wave, etc. having varying duty cycles. For example, the function generator 404 can modulate the output of the light source 403 to have a square wave of modulation frequency range of 50 Hz to 1 kHz with 50% duty cycle. A temperature meter 407 measures the instant temperature of the photodetector 150 by way of a thermocouple.
For testing, output of the photodetector 150 is fed to a trans-impedance amplifier (TIA) 409, which is essentially a current to voltage converter. TIA 409 converts the output current of photodetector 150 into voltage, which is captured by a signal capturing device 408. The signal capturing device 408, which can be an oscilloscope, receives a trigger input signal from the function generator 404.
2. Wavelength Responsivity
The wavelength range of a quaternary material (AlGaAsSb) extends from approximately 800-nm though the NIR to approximately 2400-nm. As noted in the Background Section of the present disclosure, a quaternary photodiode composed of AlGaAsSb, such as a commercially available PD-24, typically has a long wavelength cutoff at approximately 2400-nm at room temperature. In other words, the responsivity of such a quaternary photodiode is negligible above 2400-nm. As evidenced by temperature testing, the responsivity of the disclosed quaternary photodiode die 160 composed of AlGaAsSb shifts toward longer wavelengths at higher temperatures, which is beneficial for downhole use. In fact, any changes in wavelength response at elevated temperature experienced by the quaternary material actually favors the near-infrared wavelengths because the temperature actually produces a red-shifting of the material's responsivity curve towards longer wavelengths.
The test setup 400 of
As temperature is further increased, although the magnitude of responsivity reduces, the disclosed photodiode die 160 is responsive to even longer wavelengths, as evident, for example, from curve 503 corresponding to temperature of 205° C. Therefore, an increase in the operating temperature of the disclosed photodetector 150 under downhole conditions is advantageous at least because the responsivity of the disclosed photodetector 150 shifts towards longer wavelengths, allowing the measurement of downhole fluids at these longer wavelengths.
3. Photodiode Mode of Operation
As is known, photodiodes generate a current in response to incident light. This response can be measured in two different modes of operation—photovoltaic mode and photoconductive mode. In photovoltaic mode, the photodiode is operated in zero-bias condition, i.e., no external voltage source is connected to the diode terminals. In contrast, in photoconductive mode, the photodiode is biased by applying voltage to one of its terminals.
Whether the photodetector 150 is operating in photovoltaic or photoconductive mode (
To do this, the effect of reverse bias voltage Vbias on the photodetector 150 operating at room temperature (27° C.) is first analyzed. Examples of this are shown in
Thus, at room temperature (27° C.), despite increase in reverse bias voltage, signal amplitude can be successfully measured. Therefore, at room temperature, the photodetector 150 can be successfully operated in either photovoltaic mode or photoconductive mode. This is what is expected due to the operating range for the quaternary detector material.
4. Dark Current and Signal Amplitude
Because the quaternary detector with the disclosed photodetector 150 is to be used downhole without any cooling (or at least without significant cooling), further analysis of the photodiode's behavior at higher temperatures is needed. Accordingly, analysis of dark current and signal amplitude for high temperatures is discussed.
Further, input voltage noise and offset on the op-amp 602 (
Therefore, for the disclosed photodetector 150 to be able to operate at high temperatures such as 200° C., it is preferable that the photodetector 150 be operated with minimum, or zero, reverse bias voltage. This means that the photodetector 150 can be operated in photovoltaic mode for high temperature operation (>200° C.). This is in contrast to how the photodetector 150 can operate at lower temperatures in photoconductive mode, as noted previously.
5. Signal Amplitude and Responsivity
Discussion now turns to analyzing the effect of temperature on the signal amplitude and responsivity of the disclosed photodetector 150.
In both cases, the signal amplitude decreases with the increase in temperature. This is evident from the smaller magnitude of ΔS204, which is measured at 204° C., compared to the magnitude ΔS31, which is measured at 31° C. Moreover, signal noise also increases with increase in temperature. In short, the signal-to-noise ratio decreases with the increase in temperature. This means, as the temperature increases, discerning the signal from the noise becomes more difficult to perform.
As discussed previously, responsivity of a photodiode is the current magnitude generated for a given power and wavelength of incident light. The test setup 400 of
Magnitude of the current (represented in volts) generated by the photodetector 150 is measured in terms of the magnitude of the fundamental frequency of the spectrum of the signal spectrum. For example,
As evident from the plots in
To determine the effect of each of the variables: temperature, wavelength, modulation frequency, and the device under test; each of the twelve traces are accumulated into a single two dimensional plot of signal responsivity vs. temperature, as shown in
6. Temperature Stresses
Changes in temperature can cause stresses in the photodetector 150. One way of determining the material stability of photodiode 150 is shown in
The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.
Number | Date | Country | |
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20120298850 A1 | Nov 2012 | US |
Number | Date | Country | |
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Parent | 12613808 | Nov 2009 | US |
Child | 13113359 | US |