Abstract:
A fiber optic sensor to determine a property in an environment with a temperature exceeding 150 degrees Celsius includes a light source to emit broadband light, an etendue of the light source being less than 1000 square micro meter-steradians (μm 2  sr), and an optical fiber to carry incident light based on the broadband light and a reflection resulting from the incident light. A photodetector detects a resultant light based on the reflection and outputs an electrical signal, and a processor processes the electrical signal from the photodetector to determine the property.

Description:
BACKGROUND 
       [0001]    Sensors and measurement devices are used in many environments. The information provided by these devices facilitate decision making. For example, in sub-surface exploration and production efforts, many sensors are used to obtain information about the sub-surface environment and the formation properties. This information may be used to make a variety of decisions including, for example, decisions about drilling direction, speed, and equipment maintenance. Exemplary sensors include acoustic sensors, nuclear magnetic resonance (NMR) sensors, and fiber optic sensors. Fiber optic sensors include sensors in which the optical fiber is the sensing element and senses properties such as, for example, strain, temperature, and pressure. Fiber optic sensors also include sensors in which the optical fiber supplies the light used in sensing such as, for example, in spectroscopy. The sensors used in a given environment must be able to withstand the conditions of the environment to function effectively. 
       SUMMARY 
       [0002]    According to an embodiment, a fiber optic sensor to determine a property in an environment with a temperature exceeding 150 degrees Celsius includes a light source configured to emit broadband light, an etendue of the light source being less than 1000 square micro meter-steradians (μm 2  sr); an optical fiber configured to carry incident light based on the broadband light and a reflection resulting from the incident light; a photodetector configured to detect a resultant light based on the reflection and output an electrical signal; and a processor configured to process the electrical signal from the photodetector to determine the property. 
         [0003]    According to another embodiment, a method of obtaining a property with a fiber optic sensor in an environment with a temperature exceeding 150 degrees Celsius includes disposing a light source in the environment, the light source emitting broadband light and having an etendue less than 1000 square micro meter-steradians (μm 2  sr); disposing an optical fiber to carry incident light based on the broadband light and a reflection resulting from the incident light; disposing a photodetector to detect a resultant light based on the reflection and output an electrical signal; and configuring a processor to process the electrical signal from the photodetector to determine the property. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
           [0005]      FIG. 1  is a block diagram of a fiber optic sensor according to embodiments of the invention in an exemplary sub-surface environment; 
           [0006]      FIG. 2  illustrates features of the light source and optical fiber of the fiber optic sensor according to embodiments; 
           [0007]      FIG. 3  is a block diagram of a fiber optic sensor as a pressure sensor according to an embodiment; 
           [0008]      FIG. 4  is a block diagram of a fiber optic sensor with the optical fiber supplying light for spectroscopy according to an embodiment; and 
           [0009]      FIG. 5  is a block diagram of an exemplary embodiment of the light source of the fiber optic sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    As noted above, sensors such as fiber optic sensors may be used in a variety of environments. Some environments, such as the sub-surface environment, are high temperature environments with temperatures exceeding 150 degrees Celsius. The different types of fiber optic sensors have in common an optical fiber and a light source. As noted above, the light from the light source may be supplied by the optical fiber (as in spectroscopy) or the light from the light source may facilitate using the optical fiber as the sensing element. Different types and configurations of fiber optic sensor may be used to measure strain, temperature, pressure, and other quantities. 
         [0011]    While the light source may be located at the surface in some cases, the fiber optic sensors according to embodiments detailed herein relate to fiber optic sensors that include the light source in a high temperature environment, such as the downhole environment. Light from the downhole light source is introduced into the optical fiber, and this light is modified (in intensity, phase, polarization, wavelength or transit time) based upon the value of the quantity to be measured or the light is provided to the sensing element (e.g., spectrographic analyzer). For measuring physical properties such as pressure, temperature, and strain, a Fiber Bragg Grating (FBG) is often written into the fiber. As the spacing between FBG lines changes due to environmental pressure, temperature, or strain, the specific color of light, out of a band of colors of incident light, which is reflected back from the FBG changes. Correspondingly, there is a reduction in transmitted light intensity at that same color within the band of colors of the light that are transmitted. Using a broad band of colors, rather than a very narrow band of colors (a laser) for the light input allows interrogation of multiple FBGs that are written into the fiber at different locations, each having a sufficiently different grating line spacing so that the environmentally-induced shifts in the respective FBG reflected colors do not overlap between different FBGs, thus allowing distributed sensing and measurements all along the fiber using a single light source. The optical fiber may have a core diameter on the order of microns (micrometers). 
         [0012]    Thus, the light source must not only withstand high temperatures without decreasing light intensity but must also exhibit low etendue. Etendue is a property indicating how spread out the light is in both area and angle. When the etendue of light from a light source is high, only a tiny portion of that light can be launched into the narrow optical fiber core, which is extremely inefficient. Embodiments of the systems and methods herein relate to a fiber optic sensor including a light source that maintains light intensity at high temperatures (over 150 degrees Celsius) and exhibits low etendue (below 1000 square micrometer steradians, μm 2  sr). One exemplary embodiment detailed herein is a fiber optic sensor with a graphene light source. 
         [0013]      FIG. 1  is a block diagram of a fiber optic sensor  110  according to embodiments of the invention in an exemplary sub-surface environment.  FIG. 1  shows a borehole  1  that penetrates the earth  3  which includes a formation  4 . A set of tools  10  may be lowered into the borehole  1  by a string  2 . In embodiments of the invention, the string  2  may be a casing string, production string, an armored wireline, a slickline, coiled tubing, or a work string. In measure-while-drilling (MWD) embodiments, the string  2  may be a drill string, and a drill would be included below the tools  10 . Information from the sensors and measurement devices included in the set of tools  10  (e.g., the fiber optic sensor  110 ) may be sent to the surface for processing by the surface processing system  120  via a fiber link or telemetry. Different embodiments of the fiber optic sensor  110  are detailed further below with reference to  FIGS. 3 and 4 . 
         [0014]      FIG. 2  illustrates features of the light source  210  and optical fiber  220  of the fiber optic sensor  110  according to embodiments.  FIG. 2  shows the etendue (extent of spatial and angular spread) of the light  212  emitted by the light source  210 . Etendue increases in a lossy or scattering optical system, and etendue is conserved in a lossless optical system with only perfect lenses and mirrors. However, etendue cannot be reduced by a lens  215  or other component. Source etendue can be calculated as the integral, over all infinitesimal areas on the light source, of the dot product of the light direction with the normal to that area, over all angles multiplied by the square of the refractive index, n, in which the source is immersed. For a source that is immersed in vacuum or air, n is essentially equal to one. The concept of etendue conservation can be expressed more simply by the common approximation: 
         [0000]      light source area*source solid angle=image area*image solid angle  [EQ. 1]
 
         [0000]    Source solid angle and image sold angle refer to three-dimensional angles. An optical fiber  220  has a cone of acceptance for light impinging upon that fiber. When shown in cross-section, the cone of acceptance appears as a planar half-angle (shown in  FIG. 2  as θ  221 ). When this planar half-angle is rotated about one side, a conical volume of revolution corresponding to the cone of acceptance is generated. 
         [0015]    In the cross-sectional view shown in  FIG. 2 , the source cone planar angle  211  and the image cone planar angle  216  are indicated. The relationship between cross-sectional planar angle  211 ,  216  and solid angle is explained below with reference to the planar angle θ  221  associated with the opening in the optical fiber  220 . Ideally, the image planar angle  216  would match the planar angle θ  221  of the optical fiber  220  such that all light  212  generated at the light source  210  enters the optical fiber  220 . However, the image planar angle  216  associated with most light sources  210  (not shown to relative scale in  FIG. 2 ) is likely to be larger than the planar angle θ  221  of the optical fiber  220  such that most of the light  212  from the light source  210  is not introduced into the optical fiber  220 . This represents a source of inefficiency. Accordingly, embodiments herein describe a low etendue light source  210  (i.e., one resulting in a smaller image planar angle  216  for a given light source area and image area). 
         [0016]    The solid angle Ω  222  shown in  FIG. 2  corresponds with the planar angle θ  221  in the cross-sectional view shown for the optical fiber  220 . The planar angle θ  221  (in the cross-sectional view) is regarded as the acceptance angle or angle at which incoming light is accepted into the optical fiber  220  in air. The optical fiber  220  includes a fiber cladding  225  surrounding a fiber core  227 . The light from the light source  210  is launched into the fiber core  227  at an opening of the optical fiber  220 . The planar angle θ  221  (in the cross-sectional view) is expressed in radians and is given by: 
         [0000]    
       
         
           
             
               
                 
                   θ 
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                     s 
                     R 
                   
                 
               
               
                 
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         [0000]    As shown in  FIG. 2 , s is the subtended arc length, and R is the radius of the optical fiber  220  which corresponds to the area A. That is, the planar angle θ  221  in radians is the ratio of a subtended arc (s) of a circle to the radius (R) of the circle. The solid angle Ω  222  is expressed in steradians (sr) and is given by: 
         [0000]    
       
         
           
             
               
                 
                   Ω 
                   = 
                   
                     A 
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
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                     . 
                     
                         
                     
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                     3 
                   
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         [0000]    That is, the solid angle Ω  222  is the ratio of the subtended area (A) of the surface of a sphere to the square of the radius (R) of the sphere. Thus, written in terms of the planar angle θ  221 , the solid angle Ω  222  is given by: 
         [0000]      Ω=2π(1−cos θ)  [EQ. 4]
 
         [0000]    The diameter of the fiber core  227  is generally on the order of 9 micrometers (μm). This corresponds with an acceptance angle or planar angle θ  221  in the cross-sectional view given by: 
         [0000]    
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                       arcsin 
                        
                       
                         ( 
                         
                           NA 
                           fiber 
                         
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                     = 
                     
                       arcsin 
                       ( 
                       
                         
                           
                             
                               n 
                               core 
                               2 
                             
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                               n 
                               cladding 
                               2 
                             
                           
                         
                         
                           n 
                           0 
                         
                       
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         [0000]    The numerical aperture of the optical fiber  220  is indicated as NA fiber . The index of refraction is n, and n 0  indicates the index of refraction of air, which is what the optical fiber  220  is immersed in. The value of n 0  is 1 in vacuum and is also 1, to within 300 parts per million, in air. The index of refraction of the fiber core  227  (n core ) and the index of refraction of the fiber cladding  225  (n cladding ) are known values for the optical fiber  220 . 
         [0017]    For the exemplary optical fiber  220  with a 9 μm fiber core  227  diameter, the acceptance angle or planar angle θ  221  is 15 degrees based on EQ. 5. According to EQ. 4, the solid angle Ω  222  is then 0.2 sr. This corresponds to an etendue of 14 μm 2  sr. Laser diodes are available with etendue below 14 μm 2  sr and even as low as 1 μm 2  sr. However, laser diodes dim dramatically as temperature increases (most laser diodes stop lasing above 110 degrees Celsius), emit too narrow a wavelength spectrum to be used for distributed sensing of many FBGs along a fiber, and have a reliability lifetime that is 10 to 100 times shorter than most light emitting diodes. Incandescent graphene emits a far broader band of light than even superluminescent diodes (which are known for having broad band light emission) so incandescent graphene enables the use of many thousands of spectrally non-overlapping FBGs for distributed sensing along enormous lengths (miles) of fiber. Further, incandescent graphene enables performing optical spectroscopy over a broad range of wavelengths from visible to near-infrared, infrared, and far-infrared. 
         [0018]    Practically speaking, a wide-band light source with an etendue of only 14 μm 2  sr is not available for high temperature use. For example, lasers are used as low-etendue light sources for fiber optic sensor systems with the light source at the surface, but the use of lasers in a borehole  1 , for example, is impractical due to the environmental temperatures. This is due to a dramatic drop in light intensity with temperature of any semiconductor light source (e.g., light emitting diode (LED), super luminescent light emitting diode (SLED), laser diode). For example, above 125 degrees Celsius, the optical power output of an SLED drops below the minimum needed to make a measurement. On the other hand, the light intensity of a graphene incandescent light source  210  would not drop with temperature because it is not a semiconductor light source. The operational temperature of incandescent graphene is far higher than any borehole  1  temperature, for example. If high environmental temperature (e.g., in the borehole  1 ) affected the incandescent graphene, the effect would be a slight increase in temperature that resulted in a corresponding increase (rather than decrease) in the light output. 
         [0019]    To obtain light output that is brighter but more narrow band (similar in bandpass to an SLED), the incandescent filament may be surrounded by a nanophotonic interference filter  517  ( FIG. 5 ) designed to recycle some light (such as infrared) that is outside of the user-selected wavelength bandpass by reflecting it back to the filament (making it hotter and brighter) while transmitting only the desired bandpass of light. Among sources that can withstand high temperatures, etendue is a factor in their utility for purposes of fiber optic sensors  110 . A one square millimeter light emitting diode (LED), for example, provides an image planar angle  216  around 25 degrees, which corresponds with a solid angle of 0.6 sr and an etendue of 600,000 μm 2  sr. As another example, a one square millimeter tungsten (W) filament provides an image planar angle  216  around 180 degrees (into all of space or 4π sr), which corresponds with an etendue of Ser. No. 12/566,370 μm 2  sr. 
         [0020]    According to embodiments herein, the light source  210  exhibits an etendue less than 1000 μm 2  sr. As noted above, one exemplary embodiment involves a graphene filament  510  ( FIG. 5 ) as the light source  210 . This is discussed further with reference to  FIG. 5 . A 5-micron by 5-micron light-emitting portion of graphene filament  510 , emitting into all of space or 4π sr, exhibits an etendue of about 314 μm 2  sr. By way of comparison, the etendue of an exemplary 5-micron by 5-micron graphene filament  510  is approximately 2000 times lower (600,000/314) than a square millimeter LED etendue and 40,000 times lower (Ser. No. 12/566,370/314) than a square millimeter tungsten filament etendue. In addition, as noted above, graphene can withstand temperatures well above 150 degrees Celsius without a loss in light  212  intensity. The operating temperature of a nanoscale graphene light emitter has been estimated to be 2850 Kelvin (K), which is far hotter than the hottest oil or gas wells of 250 degrees Celsius (523 K). Thus, a high temperature environment, such as the borehole  1  environment, will have a negligible effect on the intensity or on the peak wavelength of emission of an incandescent graphene light source  210 . Further, the thermal conductivity of graphene above about 1800 K is greatly reduced (unlike that of tungsten, for example) so that the center of the graphene filament  510  stays very hot but that heat is not easily transferred to the two end supports, which stay far cooler. This simplifies the support of this free-hanging structure on a micron scale and reduces the power draw needed to maintain a hot center temperature. Further, the heat localization within the graphene filament  510  reduces the area of the light emission (about 25 square microns, for example, for the 5-micron by 5-micron region  515  ( FIG. 5 ) of the graphene filament  510 ), thereby reducing the corresponding etendue. 
         [0021]      FIG. 3  is a block diagram of a fiber optic sensor  110  as a pressure sensor according to an embodiment. This exemplary embodiment is provided for explanatory purposes to indicate the arrangement of the light source  210  with the optical fiber  220  and other components of the fiber optic sensor  110 . However, as noted above, a fiber optic sensor  110  in which the optical fiber  220  acts as the sensing element may sense temperature, strain, acoustics, and other properties and is not limited to being a pressure sensor. The fiber optic sensor  110  is disposed partially inside and partially outside a pressure housing, whose wall  310  is indicated in  FIG. 3 . The wall  310  of the pressure housing may coincide with the wall of the tool  10  housing shown in  FIG. 1 , for example. The part of the fiber optic sensor  110  that is outside the wall  310  (the portion that acts as the sensing element) must be exposed to the pressure to be sensed. That is, the portion of the optical fiber  220  that acts as the sensing element is exposed to the wellbore fluid (outside the tool  10  housing) whose pressure is measured according to the exemplary embodiment. 
         [0022]    The portion of the optical fiber  220  that acts as the sensing element includes fiber Bragg gratings (FBGs)  330 . The wavelength of the reflected signal differs from the wavelength of the incident signal based on the FBGs  330 , and the effect of the FBGs  330  further varies based on pressure in a quantifiable way. A sand shield  335  may be disposed to shield the FBGs  330 . The light source  210  may be a graphene filament  510  ( FIG. 5 ) according to an embodiment. A lens  215  may or may not be used to direct the light from the light source  210  into the optical fiber  220  (fiber core  227 ). The reflected signal affected by the FBGs  330  is directed through the lens  325  to a photodetector  320 . The photodetector  320  may provide the reflections for processing (e.g., via the telemetry link to the processing system  120 ) or a processor may be part of the fiber optic sensor  110  ( 545 ,  FIG. 5 ). 
         [0023]      FIG. 4  is a block diagram of a fiber optic sensor  110  with the optical fiber  220  supplying light for spectroscopy according to an embodiment. The orientation of the fiber optic sensor  110  is not limited and may be, for example, vertical or horizontal. According to an exemplary embodiment, the fiber optic sensor  110  may be shown sideways in  FIG. 4  with respect to the orientation shown in  FIG. 1 . That is, the light source  210  may be oriented toward the surface while the pump flow direction indicated in  FIG. 4  may be oriented deeper into the borehole  1 . A wall  411  creates a pressure housing inside of which is low pressure, as indicated. Wellbore fluid at higher pressure is outside the wall  411  and formation fluid flows through a tube  415  that penetrates the housing (intersects the wall  411 , as shown) and the formation fluid flows past the fiber optic sensor  110 . The housing may correspond with the tool  10  housing, shown in  FIG. 1 , for example. The fiber optic sensor  110  may also be oriented such that the housing of the fiber optic sensor  110  does not correspond with the tool  10  housing. The tube  415  includes a mirror  410 . The pathlength for optical absorbance is the round trip distance through the fluid from the optical window  409  to the tube  415  to the mirror  410  and back to the optical window  409 . As  FIG. 4  indicates, half of the pathlength is associated with light before it hits the mirror  410 , and half of the pathlength is associated with light after it hits the mirror  410 . This arrangement may be referred to as a “transflectance” arrangement. 
         [0024]    Light from the light source  210  may pass through a lens  215  before entering the optical fiber  220 . The light source  210  may be a graphene filament  510  ( FIG. 5 ) or another broadband light source with an etendue below 1000 μm 2  sr. The light encounters the formation fluid in the tube  415 , which has a diameter d and is reflected by the mirror  410 . Thus, the light passes through a length of 2*d (to and from the mirror  410 ) through the formation fluid. The reflected light passes through a lens  440   a  to a bandpass filter  430   a  that passes a particular spectrum and the passed light is received at the photodetector  420   a . The same process happens for a different pass band (different part of the spectrum) when the reflected light passes through lens  440   b  and bandpass filter  430   b  to the photodetector  420   b . While two bandpass filters  430   a ,  430   b  are shown, any number of filters  430  may be used to separate the signal reflected by the mirror  410  into more wavelength bands. The signals from the different photodetectors  420  are processed (within the fiber optic sensor  110 ) ( 545 ,  FIG. 5 ) or at the processing system  120 , for example. 
         [0025]    As noted above,  FIGS. 3 and 4  illustrate two exemplary types of fiber optic sensors  110  that may be used in a high-temperature environment such as the sub-surface environment. Relevant characteristics of the fiber optic sensors  110  according to the exemplary embodiments and other embodiments is the high temperature (e.g., greater than 150 degrees Celsius) and the small diameter (on the order of 9 μm) of the light-receiving component (the optical core  227  of the optical fiber  220  of the fiber optic sensor  110 ). As a result, the light source  210  of the fiber optic sensor  110  must function at temperatures exceeding 150 degrees Celsius without a reduction in light intensity and must exhibit an etendue below 1000 μm 2  sr. 
         [0026]      FIG. 5  is a block diagram of an exemplary embodiment of the light source  210  of the fiber optic sensor  110 . The exemplary light source  210  is a free-standing graphene filament  510  supported at its ends by end supports  520 . The supports  520  may comprise a refractory metal that can withstand the temperatures of the graphene filament  510  without melting or sagging. Exemplary supports  520  may include tungsten, molybdenum, tantalum, or rhenium. The graphene filament  510  may be electrically biased by a voltage source  530  to initiate emission of broadband light. As shown in  FIG. 5 , the graphene filament  510  may be biased through the supports  520  rather than directly, according to an exemplar embodiment. The exemplary embodiment involves a strip of graphene filament  510  that is approximately 6.5 to 14 μm in length. A central region of the graphene filament  510  where the graphene filament  510  gets the hottest is indicated as region  515 . The controller  540  that controls the voltage source  530  to thereby control light emission by the graphene filament  510  may include known one or more processors  545  and memory devices  543  that also process the received reflections at the photodetectors  320 ,  420  of the fiber optic sensor  110 . An exemplary nanophotonic interference filter  517  is shown in  FIG. 5  as a wrap-around cylinder with a slit  518  along the edge of the cylinder. The slit  518  facilitates optionally sliding the exemplary nanophotonic interference filter  517  over the graphene filament  510 . As noted above, the nanophotonic interference filter  517  recycles some light that is outside of the user-selected wavelength bandpass by reflecting it back to the filament, thereby making the graphene filament  510  hotter and brighter while transmitting only the desired wavelengths of light. 
         [0027]    The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” can include a range of ±8% or 5%, or 2% of a given value. 
         [0028]    While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.