Patent Publication Number: US-7907277-B2

Title: Method and apparatus for downhole spectroscopy

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to well bore tools and in particular to apparatus and methods for downhole formation testing. 
     2. Background Information 
     Oil and gas wells have been drilled at depths ranging from a few thousand feet to as more than 5 miles. Wireline and drilling tools often incorporate various sensors, instruments and control devices in order to carry out any number of downhole operations. These operations may include formation testing, fluid analysis, and tool monitoring and control. 
     The environment in these wells present many challenges to maintain the tools used at depth due to vibration, harsh chemicals and temperature. Temperature in downhole tool applications presents a unique problem to these tools. High downhole temperatures may reach as high as 200° C. (390° F.) or more, and sensitive electronic equipment usually requires cooling in order to work in the hazardous environment. An added problem is that space in the carrier assembly is usually limited to a few inches in diameter. 
     SUMMARY 
     The following presents a general summary of several aspects of the disclosure in order to provide a basic understanding of at least some aspects of the disclosure. This summary is not an extensive overview of the disclosure. It is not intended to identify key or critical elements of the disclosure or to delineate the scope of the claims. The following summary merely presents some concepts of the disclosure in a general form as a prelude to the more detailed description that follows. 
     Disclosed is an apparatus for estimating a property of a downhole fluid including a carrier that is conveyed in a borehole, and a semiconductor electromagnetic energy source carried by the carrier, the semiconductor electromagnetic energy source having an active region that includes one or more nitride-based barrier layers that are modulation-doped using a nitride-based doped layer. 
     A method for estimating a property of a downhole fluid includes conveying a carrier in a borehole and carrying a semiconductor electromagnetic energy source in a borehole using the carrier, the semiconductor electromagnetic energy source having an active region that includes one or more nitride-based barrier layers that are modulation-doped using a nitride-based doped layer. The method may further include emitting electromagnetic energy from the emitter toward the downhole fluid in-situ and detecting an interaction between the emitted electromagnetic energy and the downhole fluid using a detector. The downhole fluid property is estimated at least in part using an output signal from the detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the several non-limiting embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
         FIG. 1  is an exemplary wireline system according to several embodiments of the disclosure; 
         FIG. 2  is a cross section view of a non-limiting semiconductor electromagnetic energy source; 
         FIG. 3  illustrates a band diagram of the electromagnetic energy source of  FIG. 2 ; 
         FIGS. 4 and 5  illustrate a non-limiting example of a p-down photodetector according to the disclosure; and 
         FIG. 6  is a non-limiting example of a downhole spectrometer that may be used with systems such as depicted in  FIG. 1 ; 
         FIGS. 7 and 8  illustrate downhole Raman spectrometer examples according to several embodiments of the disclosure. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present disclosure uses terms, the meaning of which terms will aid in providing an understanding of the discussion herein. As used herein, high temperature refers to a range of temperatures typically experienced in oil production well boreholes. For the purposes of the present disclosure, high temperature and downhole temperature include a range of temperatures from about 100° C. to about 200° C. (about 212° F. to about 390° F.). 
       FIG. 1  schematically illustrates a non-limiting example of a wireline apparatus  100  according to several disclosed embodiments. In the example shown, a well borehole  110  traverses several subterranean formations  102 . The well borehole  110  will typically be filled or at least partially filled with a fluid mixture which can include various gases, water, drilling fluid, and formation fluids that are indigenous to the subterranean formations penetrated by the well borehole. Such fluid mixtures are referred herein to as “well borehole fluids”. 
     A formation evaluation tool  120  is conveyed in the well borehole  110  using a wire line  104 . Wire line deployment and retrieval may be performed by a powered winch carried by a service truck  108 , for example. The wireline  104  typically is an armored cable that carries data and power conductors for providing power to the formation evaluation tool  120  and to provide two-way data communication between a tool processor  112  and a controller  114  that may be carried by the service truck  108 . The wireline  104  typically is carried from a spool  116  over a pulley  118  supported by a derrick  122 . The spool  116  may be carried by the truck  108  as shown for on-land operations, by an offshore rig for underwater operations, or by any other suitable mobile or fixed supporting structure. The controller  114  may include a processor  142 , such as within a computer or a microprocessor, data storage devices, such as solid state memory and magnetic tapes, peripherals, such as data input devices and display devices, and other circuitry for controlling and processing data received from the tool  120 . The surface controller  114  may further include one or more computer programs embedded in a computer-readable medium accessible to the processor  142  in the controller  114  for executing instructions contained in the computer programs to perform the various methods and functions associated with the processing of the data from the tool  120 . 
     The exemplary wireline  FIG. 1  operates as a carrier for the formation evaluation tool  120 , but any carrier is considered within the scope of the disclosure. The term “carrier” as used herein means any device, device component, combination of devices, media and/or member that may be used to convey, house, support or otherwise facilitate the use of another device, device component, combination of devices, media and/or member. Exemplary non-limiting carriers include drill strings of the coiled tube type, of the jointed pipe type and any combination or portion thereof. Other carrier examples include casing pipes, wirelines, wireline sondes, slickline sondes, downhole subs, bottom hole assemblies (BHA&#39;s), drill string inserts, modules, internal housings and substrate portions thereof. 
     The lower portion of the formation evaluation tool  120  may include an assembly of several tool segments that are joined end-to-end by threaded sleeves or mutual compression unions  124 . An assembly of tool segments suitable for the present invention may include a hydraulic, electrical, or electro-mechanical power unit  126  and a formation fluid extractor  128 . The formation fluid extractor  128  may include an extensible suction probe  138  that is opposed by bore wall feet  140 . Both, the suction probe  138  and the opposing feet  140  may be hydraulically or electro-mechanically extensible to firmly engage the well borehole wall. Construction and operational details of a suitable fluid extraction tool  128  are thoroughly described by U.S. Pat. No. 5,303,775, the specification of which is incorporated herein by reference. 
     A large displacement volume motor/pump unit  130  may be provided below the extractor  128  for line purging. A similar motor/pump unit  132  having a smaller displacement volume may be included in the tool in a suitable location, such as below the large volume pump, for quantitatively monitoring fluid received by the tool  120 . One or more sample tank magazine sections (two are shown  134 ,  136 ) may be included for retaining fluid samples from the small volume pump  132 . Each magazine section  134 ,  136  may have several fluid sample tanks  106 . 
     In several embodiments to be described in further detail later, the tool  120  includes a downhole spectrometer or other evaluation tool using one or more semiconductor components for generating and/or detecting electromagnetic energy. 
       FIG. 2  is a cross section view of a non-limiting gallium-nitride semiconductor  200  for use in hazardous environments including a downhole environment. The gallium-nitride semiconductor  200  may be constructed for operation as any number of semiconductor device, for example a diode, a transistor, a field effect transistor (FET), a laser diode or any other useful semiconductor device using a high-gain media. The example gallium-nitride semiconductor  200  in  FIG. 2  illustrates a semiconductor electromagnetic energy source that may be carried into a well borehole for use in downhole spectroscopy. Any carrier may be used to carry the electromagnetic energy source into a well borehole. For example, a carrier comprising a wireline as described above and shown in  FIG. 1  may be used. In several embodiments, suitable carriers may include drill strings of the coiled tube type, of the jointed pipe type and any combination or portion thereof, casing pipes, wirelines, wireline sondes, slickline sondes, downhole subs, bottom hole assemblies (BHA&#39;s), drill string inserts, modules, internal housings and substrate portions thereof. 
     The electromagnetic energy source  200  may be configured to emit electromagnetic energy having a wavelength responsive to downhole fluid characteristics. In one or more embodiments, the electromagnetic energy source  200  may comprise a substrate  202 , and an electromagnetic energy emitter  204  that includes an electromagnetic energy generating section  206 . 
     In one or more embodiments, the substrate  202  may include several layers forming a template. In one or more embodiments, the substrate  202  may be a hetero-epitaxial lateral overgrowth or more simply, hetero-ELO template  202 . The example shown in  FIG. 2  illustrates a hetero-ELO template that includes a template substrate  208  with a buffer layer  210  disposed on the template substrate  208 . An underlying layer  212  is disposed on the buffer layer  210  and an interlayer  214  is disposed on the underlying layer  212 . A nitride-based layer  216  is then disposed on the interlayer  214 . 
     The template substrate  208  may be selected from any suitable material for forming a semi-conductor substrate. In one embodiment, a sapphire material may be used for the substrate  208 . In another example, silicon carbide (SiC) or other ceramic may be used. The buffer layer  210  may be a low-temperature buffer layer (LT-Buffer Layer) formed using one or more nitride-based materials such as gallium nitride, aluminum nitride, and gallium aluminum nitride. The underlying layer may utilize a nitride-based material. In one example, the underlying layer includes gallium nitride. The interlayer  214  may be a low-temperature aluminum nitride interlayer (LT-AlN interlayer). The top nitride layer may be selected in part based on the preceding layer materials used. In this example, an aluminum-gallium-nitride (AlGaN) layer is used as the top nitride layer. 
     The electromagnetic energy emitter  204  may be configured to emit electromagnetic energy of any suitable wavelength or band of wavelengths. In one or more embodiments, the electromagnetic energy emitter emits a broad band of electromagnetic energy. In one or more embodiments, the electromagnetic energy source  200  includes a fluorescent material disposed on a surface of the electromagnetic energy emitter that provides an output approximating a white light source. In one or more embodiments, the electromagnetic energy source  200  comprises a structure for emitting ultra-violet (UV) light. In one or more embodiments, the electromagnetic energy source  200  may include a structure for emitting light having a wavelength corresponding to violet light, e.g. about 405 nm. In one or more embodiments, the wavelength may be in a range of about 380 nm to about 450 nm. In one or more embodiments, the electromagnetic energy emitter  204  may operate as a UV laser diode, a violet light laser diode or there may be a combination of UV and violet laser diodes. In one or more embodiments, a laser diode may be grown on a hetero-epitaxial lateral overgrowth or more simply, hetero-ELO template. 
     The UV or violet laser diode  204  in this example includes an electromagnetic energy generator  206 , which in this example is a multiple quantum well structure. Those skilled in the art with the benefit of the present disclosure will appreciate that structures other than MQW may incorporate the concepts described herein. Thus, other electromagnetic energy generators  206  are within the scope of the disclosure. Examples include, but are not limited to, single quantum wells, quantum dots, quantum dash structures, and quantum wire structures. 
     The electromagnetic energy emitter  204  includes a lower cladding layer  218  and a guide layer  220  is disposed on the lower cladding layer  218 . In one non-limiting example, the lower cladding layer  218  is an n-cladding layer formed using AlGaN and the guiding layer  220  is formed using GaN. The MQW  206 , is formed using alternating barrier layers  222  of InGaN material and p-AlGaN blocking layers  224 . The number of barrier layers  222  and blocking layers  224  may be selected depending on the number of quantum wells desired. In this example, three quantum wells are formed using two AlGaN blocking layers  224  for raising the energy band of the wells as shown in the band diagram of  FIG. 3 . An interlayer layer  226  formed using InGaN operates as a guide layer and one or more p-cladding layers  228  are disposed on top of the MQW section  206 . The p-cladding layers may be strained layer superlattice cladding layers formed using AlGaN and/or GaN. 
       FIG. 3  illustrates a band diagram of the electromagnetic energy source of  FIG. 2 . The AlGaN electron blocking layers increase the well depth shown in  FIG. 3  at  308 . The increased well depth provides better performance at high ambient temperatures such as those to be expected in a downhole environment.  FIG. 3  is an illustrative conduction band diagram that may be produced where a semiconductor device is constructed in accordance with the device  200  shown in  FIG. 2 . The Ec band  300  includes a portion  302  associated with the n-AlGaN cladding layer  218 . A second band portion  304  is associated with the GaN guide layer  220 . The non-limiting structure of  FIG. 2  forms 3 MQWs  306  with the extended depth  308  being associated with the p-AlGaN electron blocking layers  224 . The Ec band diagram then has a portion  310  associated with the InGaN guiding interlayer  226 . In one or more embodiments, the Ec level for sections  304  and  310  are substantially the same where the material ratios are the same. A section  312  is associated with the p-AlGaN or GaN superlattice cladding layers  228  and includes one or more additional blocking layers. 
     The exemplary semiconductor device  200  described above also illustrates a method of modulation doping that includes modulating the doping of the barrier layers used in quantum wells. Modulating the doping according to the disclosure may be used to shift the Fermi levels and provides a high barrier that helps in the high temperature environment. Modulation doping also increases absorption due to a high concentration of carriers caused by the doping. This tradeoff is useful in the downhole environment and is counterintuitive with respect to the current direction of surface applications for laser diodes. The doping also decreases the electrical resistance of the device. One may also increase the band offset by changing the alloy composition ratio to increase or decrease the strain. Using a highly doped MQW in a GaN device also allows for the creation of intra-cavity contacts that facilitates the manufacturing process. 
     It should be understood that those skilled in the art with the benefit of the present disclosure will appreciate that other high-gain semiconductor devices may be constructed using the nitride-based semiconductor and/or active region blocking layers described herein. In several non-limiting examples, the semiconductor device may be a diode, a transistor, a field effect transistor (FET), a laser diode or any other useful semiconductor device using a high-gain media. In several embodiments, the high-temperature high-gain media semiconductor device  200  may be modified for use in any number of sensor, communication, switching, amplification and information handling applications. 
       FIGS. 4 and 5  illustrate a non-limiting example of a p-down photodiode according to the disclosure.  FIG. 4  is a cross section of a photodiode, also referred to as a photodetector  400 . The photodetector  400  may be used to detect any number of electromagnetic energy responses. In several embodiments, the photodetector  400  may be incorporated in a downhole tool, such as the tool  120  described above and shown in  FIG. 1 . In one or more embodiments, the photodetector  400  may be used in conjunction with the electromagnetic energy source  200  described above and shown in  FIG. 2 . 
     The non-limiting photodetector  400  is a semiconductor photodetector that includes a substrate  402  and a multi-layer structure disposed on the substrate. The substrate  402  may include any suitable substrate material. In this example, the substrate includes SiC to provide a substrate for a high temperature photodiode, which may also be used as an avalanche photodetector. Avalanche photodetectors are used slightly above their breakdown voltage and are specifically designed to multiply the electrons rather then the holes. This is due to a higher noise caused by hole multiplication. SiC has a high ratio of the multiplication coefficients for electrons/holes and therefore will be a better suited material system for making avalanche devices. 
     An n-type layer  404  is disposed on the substrate. A multiplying layer  406  is disposed on the n-type layer  404 . A blocking  408  layer is disposed on the multiplying layer  406  and an absorption layer  410  is disposed on the blocking layer  408 . A cap layer  412  is positioned on the absorption layer  410  and a mask  414  is deposited over the device with access openings for an upper electrode  416  and a lower electrode  418 . The electrodes  416 ,  418  may be arranged in a p-up or p-down configuration. The configuration shown here is a p-up configuration where the p-electrode is position on an upper mesa of the device. 
     In operation, electromagnetic energy, which may be in the form of light waves  420 , engages the photodiode from above the substrate as shown. The light interacts with the device and an output signal indicative of the light intensity may be detected by connecting one or more leads to the electrodes  416 ,  418 . 
     The performance of the photodetector may be improved for operation in a downhole environment or other harsh environment by use of the p-carrier blocking layer  408  as shown in this example. The structure size, such as the diameter of the detector wafer may be increased to reduce series resistance and to provide better thermal properties. Increasing the diameter of the wafer also provides for an easier optical coupling scheme. 
     Those skilled in the art will appreciate that a high-temperature semiconductor device, such as the device  200  described above and shown in  FIG. 2  may be used in any number of applications. In one or more embodiments, the high-temperature semiconductor device  200  may be used as part of a downhole spectrometer. Several spectrometer examples will be provided below with reference to  FIGS. 6-8 . 
       FIG. 6  schematically illustrates a non-limiting example of a downhole spectrometer  600  according to the disclosure. The downhole spectrometer  600  may be incorporated into any of several wireline tools, including the formation evaluation tool  120  described above and shown in  FIG. 1 . In other embodiments, the downhole spectrometer may be incorporated into a while-drilling tool, such as the tool  120 . 
     The downhole spectrometer  600  in the example shown includes an electromagnetic energy source  602 . In one or more embodiments, the electromagnetic energy source  602  may include a nitride-based semiconductor as described above and shown in  FIG. 2  at  200 . In one or more embodiments, the electromagnetic energy source  602  may include an array of individual sources  622 . The electromagnetic energy source  602  emits energy in the form of light toward a formation fluid cell  604  via an optical path  620 . The optical path may be any path that provides optical transmission. In one embodiment, the optical path  620  may include an air gap. In another embodiment, the optical path  620  includes an optical fiber. In one or more embodiments, the optical path  620  may be a direct interface where the electromagnetic energy source  602  is adjacent the window  606  or is in contact with the fluid  608 . 
     The fluid cell  604  includes at least one window  606  for receiving the emitted light, so that the light may interact with fluid  608  within the cell  604 . Several configurations of sample cells and windows may be used in other embodiments without departing from the scope of the present disclosure. For example, to measure optical transmittance through a cell, one could use a pair of windows. Transflectance measurements may be conducted using a single window with a mirror behind the window and having the fluid sample between the mirror and window. Attenuated reflectance measurements may be conducted using a single window in contact with the fluid sample. Raman scattering and fluorescence measurements may be conducted using a single window and collecting the resulting light on the same side of the window as the source light. In another example, light may be collected through a second window for Raman scattering and fluorescence measurements. Depending on the opacity of the sample, the second window could collect the resulting light at 90 degrees from the direction of the source light. 
     Continuing with the example of  FIG. 6 , a photodetector  610  receives the light after the light interacts with the fluid  606 . The photodetector  610 , which may be a single broadband photodetector, is responsive to light emitted from the array and provides an output signal indicative of the light received at the photodetector  610  after interaction with the fluid  608 . In one or more embodiments, the photodetector  610  may be a SiC photodetector or a GaN detector substantially similar to the photodetector  400  described above and shown in  FIG. 4 . In some cases, the photodetector output signal may be an analog electrical signal. An analog-to-digital converter  612  may be used to convert the photodetector output signal into a digital signal that is received by a processor  624  that is part of a controller  614 ,  616 . The light emitted from the electromagnetic energy source  602  may be modulated by the processor  624  within the same controller  614  that receives the photodetector output or by a separate controller. In the example shown, one modulator/controller  614  is coupled to the photodetector  610  and a second modulator/controller  616  is coupled to the electromagnetic energy source  602 . These controllers may be implemented as a single controller without departing from the scope of the disclosure. In other embodiments, the controller or controllers  614 ,  616  may be located at the surface of the well borehole. The light emitted from the electromagnetic energy source  602  may be controlled (i.e. modulated) by the controller processor  614 ,  616 . The processor  614 ,  616  that receives the detector  610  output signal may also receive a signal from the controller  614 ,  616  modulating the electromagnetic energy source  602 . 
     In one embodiment, the electromagnetic energy source  602  may include one or more light-emitting semiconductors used as individual light sources  622 . For example, the electromagnetic energy source  602  may include nitride-based laser diodes as described above and shown in  FIG. 2 . The laser diodes may all be coupled to a single optical fiber  620 , and light from that fiber would interact with the fluid  608  (through transmission or through attenuated reflection) and afterwards be detected by the photodetector  610 . In other embodiments, the downhole spectrometer  600  may include arrayed electromagnetic energy sources  602  that are not all lasers, i.e. optical channels that have a wider bandpass (less resolution) than a laser. In these embodiments, an array of light-emitting diodes (LED) may be used. 
     Cooling one or more of these downhole components may be accomplished using a cooling device  618 . The cooling device  618  used may be any number of devices, examples of which include thermal-electric, thermo-tunneling, sorption cooling, evaporators, and Dewar. Cooling is optional where components selected are compatible with the downhole temperature environment. Cooling may be applied where a component operating temperature is lower than the downhole environment and/or were cooling may enhance performance of the component. In several embodiments, the electromagnetic energy source  602  is compatible with the downhole temperature environment. Cooling in some cases could improve photodetector signal-to-noise ratio and increase laser brightness. In one or more embodiments, the photodetector  610  comprises a nitride-based construction compatible with the downhole environment. In one or more embodiments, the photodetector  610  comprises a SiC construction, a material with a wider bandgap compared with GaN and therefore with a better high temperature performance. 
     Turning now to  FIG. 7 , a schematic diagram illustrates a Raman spectrometer  700  that may be used downhole for analyzing fluid withdrawn from a formation. Oil-based mud contamination in a formation fluid may be determined using Raman spectroscopy. UV Raman spectroscopy is one part of the spectrum that is of interest due to the large gap between the fluorescence emission and Raman emission. Wide band gap semiconductor materials such as GaN and SiC are excellent candidates for optical sources and detectors in the UV range. These materials possess superior thermal characteristics due to their wide band gap. For example photodetectors built in accordance with the present disclosure and using SiC materials have low leakage current at high temperature. Furthermore, laser diodes described herein, reduced thermal rolloff. In one or more embodiments, a downhole spectrometer may use a reflectance Raman measurement set-up. The Raman signal in the 250 nm case, for example, is only about 5 nm away from the pump and a Raman filter may be used to prevent the pump signal from reaching the photodetector. Such filters are commercially available. In one or more embodiments, a SiC photodetector or a GaN device can be used. 
     The Raman spectrometer  700  in  FIG. 7  includes a nitride-based electromagnetic energy source  710 . In one or more embodiments, the electromagnetic energy source  710  may include one or more nitride-based UV lasers used to induce or pump UV light  712  into a fluid  720  through a window  714  made into a wall of a fluid chamber  716 . The light path from the electromagnetic energy source  710  to the window  714  may be an optical fiber such as the fiber  620  described above and shown in  FIG. 6 . The electromagnetic energy source  710  of this example includes multiple lasers producing UV light within a relatively narrow wavelength band. Alternatively, the UV electromagnetic energy source  710  may produce multiple monochromatic (single wavelength) UV light from each of several lasers. The light  722  interacts with the fluid  720  and a portion of the light is reflected back to a detector  730 . 
     The detector  730  according to one or more embodiments may be a SiC photodetector or a nitride-based photodetector such as a GaN detector. The detector  730  produces a signal responsive to the light, which signal is received by a controller  750  for analysis. The controller  750  may further be used as a modulator for the electromagnetic energy source  710  to modulate the light emitted from the source  710 . 
     An advantage of a UV laser diode such as one made from GaN is that, because of its wide bandgap, the laser can operate better at high temperature in that there is less dimming and less chance of lasing cessation than when using a narrower bandgap laser at the high temperatures encountered downhole. Here, the wide bandgap may be extended for better temperature performance using modulation doping as described above with reference to  FIG. 2 . 
     Raman spectroscopy is based on the Raman Effect, which is the inelastic scattering of photons by molecules. In Raman scattering, the energies of the incident or pumped photons and the scattered photons are different. The energy of Raman scattered radiation can be less than the energy of incident radiation and have wavelengths longer than the incident photons (Stokes Lines) or the energy of the scattered radiation can be greater than the energies of the incident photons (anti-Stokes Lines) and have wavelengths shorter than the incident photons. Raman spectroscopy analyzes these Stokes and anti-Stokes lines. The spectral separation between the optical pump wavelength and the Raman scattered wavelengths form a spectral signature of the compound being analyzed. Oil-based mud filtrate often has a distinct spectral signature due to the presence of olefins and esters, which do not naturally occur in crude oils. In this way, Raman spectroscopy can be used to calculate the percentage of oil based mud filtrate contamination of crude oil samples as they are being collected downhole. One can continue withdrawing and discarding oil removed from the downhole formation until the contamination falls below a desired level, and then the clean sample may be diverted into a sample collection tank. However, fluorescence from aromatics in the fluid sample, often has much higher intensity, and can interfere or obscure certain Raman signals. By using source lights having a wavelength around 250 nm or less, the Raman spectrum is completed at wavelengths shorter than those at which fluorescence begins and, therefore, interference is eliminated. 
     Thus, in one aspect, the electromagnetic energy source  710  produces UV light at wavelengths near or below (shorter than) 250 nm. The detector  730  may be a SiC or GaN detector as described above that can detect spectra of the Raman scatters corresponding to the light emitted by the source  710 . 
     The light detected by the detector  730  passes to the controller  750 . The controller may include a processor  752 , and memory  754  for storing data and computer programs  756 . The controller  750  receives and processes the signals received from the detector  730 . In one aspect, the controller  750  may analyze or estimate the detected light and transmit a spectrum of the Raman scattered light to a surface controller using a transmitter  758 . In one aspect, the controller  750  may analyze or estimate one or more properties or characteristics of the fluid downhole and transmit the results of the estimation to a surface controller using the transmitter  758 . In another aspect, the controller  750  may process the signals received from the detector  730  to an extent and telemeter the processed data to a surface controller for producing a spectrum and for providing an in-situ estimate of a property of the fluid, including the contamination level of the mud in the formation fluid. 
       FIG. 8  is a non-limiting schematic diagram showing a portion of a surface-enhanced Raman spectrometer  800  for estimating a property of a fluid according to one embodiment of the disclosure. The exemplary spectrometer  800  shown includes a chamber  816  for holding a fluid  820  to be analyzed. The fluid  820  may be stationary or it may be passing through the chamber  816 . The chamber  816  includes a window  814  for allowing light to pass to the fluid  820 . The spectrometer  800  includes an electromagnetic energy source  810  that emits electromagnetic energy  812  having one or more selected wavelengths. In one or more embodiments, the electromagnetic energy source  810  may be a laser emitting several desired wavelengths or bands of wavelengths. A controller  750 , similar to the controller  750  described above and shown in  FIG. 7 , controls the operation of the electromagnetic energy source  810  to modulate the source output. The light path from the electromagnetic energy source  810  to the window  814  may be an optical fiber such as the fiber  620  described above and shown in  FIG. 6 . The incident energy  812  enters the chamber  816  through the window  814  at a selected angle. The Raman scattered light  824  from the fluid  820  leaves the window  814 . A semiconductor detector  830 , which may be a SiC detector or may be a nitride-based detector such a GaN detector described above and shown in  FIG. 4 , detects the Raman spectra. A processor  752  receives the signals from the detector  830  and processes the signals to estimate a property of the fluid  820 . The controller  750  may further include memory  754  and programs  756  for storing information and for controlling the tool. Likewise, a transmitter  758  may be used for communication with surface-located components. 
     Fluorescence can interfere with the Raman signals, so to increase the intensity of the Raman signal, an inside surface of the chamber  816  including the inside surface of the window  814  may be coated with conductive particles  826 . The conductive particles  826  may be placed in the form of scattered metallic particles, a lattice type structure, or in any other suitable form that will enhance the Raman scattered light. The conductive particles can enhance the Raman Effect due to Plasmon resonance, which consists of energy exchange between the Raman signals and a surface wave that exists in a conductive layer, such as the layer of particles  826 . The spectrometer  800  may be used downhole for in-situ analysis of a fluid, such as the fluid withdrawn from a formation or at the surface, to estimate one or more properties or characteristics of the fluid. 
     Having described above the several aspects of the disclosure, one skilled in the art will appreciate several particular embodiments useful in estimating one or more properties of a downhole fluid in-situ. 
     In one particular embodiment, an apparatus for estimating a property of a downhole fluid includes a carrier that is conveyed in a borehole, and a semiconductor electromagnetic energy source carried by the carrier, the semiconductor electromagnetic energy source having an active region that includes one or more nitride-based barrier layers that are modulation-doped using a nitride-based doped layer. 
     Another particular embodiment for estimating a property of a downhole fluid includes a semiconductor electromagnetic energy source that comprises a UV electromagnetic energy emitter. In one embodiment, the UV electromagnetic energy emitter includes a UV laser. 
     Another particular embodiment for estimating a property of a downhole fluid includes one or more nitride-based barrier layers that include at least one InGaN layer. The nitride-based doped layer may include a p-doped layer, and the p-doped layer may include AlGaN. 
     Another particular embodiment for estimating a property of a downhole fluid includes a nitride-based doped layer comprising a p-blocking layer that is disposed between two barrier layers. 
     In yet another particular embodiment for estimating a property of a downhole fluid, a detector responsive to an interaction between the emitted electromagnetic energy and the downhole fluid may be used, wherein the downhole fluid property is estimated at least in part using an output signal from the detector. In one or more embodiments, the detector comprises a SiC photodetector or a GaN photodetector. The output signal may be indicative of a downhole fluorescence property. 
     Another particular embodiment for estimating a property of a downhole fluid includes a semiconductor electromagnetic energy source that emits monochromatic electromagnetic energy. 
     Another particular embodiment for estimating a property of a downhole fluid includes a semiconductor electromagnetic energy source that includes one or more quantum wells, quantum wires, quantum dots and quantum dashes. 
     A method for estimating a property of a downhole fluid includes conveying a carrier in a borehole and carrying a semiconductor electromagnetic energy source in a borehole using the carrier, the semiconductor electromagnetic energy source having an active region that includes one or more nitride-based barrier layers that are modulation-doped using a nitride-based doped layer. The method may further include emitting electromagnetic energy from the emitter toward the downhole fluid in-situ and detecting an interaction between the emitted electromagnetic energy and the downhole fluid using a detector. The downhole fluid property is estimated at least in part using an output signal from the detector. 
     A particular method for estimating a property of a downhole fluid includes emitting electromagnetic energy in the form of monochromatic electromagnetic energy, narrow band electromagnetic energy, wide band electromagnetic energy or a combination thereof. 
     In one particular method for estimating a property of a downhole fluid, emitting electromagnetic energy includes emitting electromagnetic energy of at least one UV wavelength. 
     One particular method for estimating a property of a downhole fluid includes detecting an interaction between emitted electromagnetic energy and the downhole fluid using a semiconductor SiC photodetector. 
     Another particular method for estimating a property of a downhole fluid includes detecting a fluorescence property of the downhole fluid. 
     The present disclosure is to be taken as illustrative rather than as limiting the scope or nature of the claims below. Numerous modifications and variations will become apparent to those skilled in the art after studying the disclosure, including use of equivalent functional and/or structural substitutes for elements described herein, use of equivalent functional couplings for couplings described herein, and/or use of equivalent functional actions for actions described herein. Such insubstantial variations are to be considered within the scope of the claims below. 
     Given the above disclosure of general concepts and specific embodiments, the scope of protection is defined by the claims appended hereto. The issued claims are not to be taken as limiting Applicant&#39;s right to claim disclosed, but not yet literally claimed subject matter by way of one or more further applications including those filed pursuant to the laws of the United States and/or international treaty.